Channel-Forming Peptides in Uniformly Aligned Multilayers of

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5 Channel-Forming Peptides in Uniformly Aligned Multilayers of Membranes Huey W. Huang Physics Department, Rice University, Houston, TX 77251

We describe one new method of ultraviolet circular dichroism (CD) and one improved method of X-ray lamellar diffraction for obtaining structural data of membrane proteins. Both methods employ samples of uniformly aligned multilayers of membranes. It was proved earlier that a CD band of α-helices is polarized along the helix axis. Because membrane proteins often contain α-helical sections, measurement of CD at the normal and oblique incident angles relative to the plane of the membrane reveals the orientation of the protein molecules. This method of oriented CD is used to study a long-standing problem of alamethicin; that is, how does the amphipathic helical peptide associ ate with a membrane? Our investigation led to the discovery of a new phenomenon of cooperative peptide insertion in bilayer lipid mem branes. We next describe a method of high-resolution lamellar diffrac tion that was used to reveal the location of the monovalent and divalent ion binding sites in the gramicidin channel.

Two

M E T H O D S FOR OBTAINING STRUCTURAL INFORMATION o n

membrane

proteins a n d applications o f these methods to the structural problems o f two channel-forming peptides are described i n this chapter. B o t h methods use u n i f o r m l y aligned multilayers o f membranes, a n d alamethicin a n d g r a m i c i d i n are the peptides that are investigated. W e w i l l show that t w o long-standing problems are resolved b y these methods: the location o f the i o n b i n d i n g sites i n the g r a m i c i d i n channel a n d the states o f alamethicin associated w i t h membranes. 0065-2393/94/0235-0083$08.72/00 © 1994 American Chemical Society

Blank and Vodyanoy; Biomembrane Electrochemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

84

BIOMEMBRANE ELECTROCHEMISTRY


U n i f o r m l y aligned multilayers o f membranes (containing proteins) pos sess one-dimensional structural order p e r p e n d i c u l a r to the plane o f the m e m b r a n e a n d preserve the orientational o r d e r o f proteins relative to the plane o f the m e m b r a n e . B o t h the one-dimensional structure a n d the orienta tion o f proteins can be measured w i t h the methods to be described. W e stress that proteins i n these samples are i n the m e m b r a n e active forms. W i t h time-resolved measurement, o u r methods can be extended to study the structural dynamics o f m e m b r a n e proteins.

Orientational Information M e m b r a n e proteins often contain α-helical sections. W e have developed a m e t h o d called oriented circular d i c h r o i s m ( O C D ; see reference 1), w h i c h can be used to determine the orientation o f α-helices w i t h respect to the plane o f the m e m b r a n e . T h i s m e t h o d is simple a n d easy to use c o m p a r e d w i t h , for example, the N M R m e t h o d , w h i c h requires isotope labeled samples. Indeed, it is the ease o f this m e t h o d that allowed us to examine alamethicin i n many different c h e m i c a l conditions a n d that resolved a controversial question about the n o n c o n d u c t i n g state o f alamethicin a n d subsequently l e d to the discovery o f a n e w p h e n o m e n o n o f a m p h i p h i h c helical peptides ( 2 ) .

One-Dimensional Structure X - r a y lamellar diffractions o f M a y e r membranes have b e e n studied since the late 1960s. T h e samples used i n most X - r a y measurements were either powders (not aligned) or partially aligned multilayers. W i t h u n i f o r m l y aligned multilayers, w e can p e r f o r m lamellar diffraction using the Θ-2Θ scanning geometry. I n this way w e have routinely obtained high-resolution diffraction data. F r o m these measurements, the ζ coordinate (normal to the plane o f the membrane) o f a label atom can be obtained f r o m difference electron density profiles. F o r example, the i o n distribution i n an i o n channel can be obtained b y subtracting the electron density profile o f a sample that contains no ions f r o m the electron density profile o f a sample that does contain ions. I n this way w e directly measured the monovalent a n d divalent cation b i n d i n g sites i n the gramicidin channel (3). M o r e recently, we also p e r f o r m e d X - r a y scatter i n g w i t h the m o m e n t u m transfer confined i n the xy plane (4). S u c h scatter i n g curves describe the lateral organizations o f proteins and peptides i n membranes. T h e in-plane scattering w i l l not be discussed here. I n the next section, u n i f o r m l y aligned multilayers o f membranes are described. T h e t h i r d section illustrates the theoretical basis a n d applications o f the m e t h o d of oriented circular d i c h r o i s m ( O C D ) . T h e O C D study o f alamethicin is discussed i n the f o u r t h section a n d the last section is devoted


5.

HUANG

Channel-Forming Peptides

85

to lamellar diffraction o f m e m b r a n e multilayers a n d the resolution o f the i o n b i n d i n g sites i n the g r a m i c i d i n channel.

Uniformly Aligned Multilayer Samples H y d r a t e d p e p t i d e - l i p i d mixtures can be

manually aligned between

two


parallel, flat surfaces. T h e procedures are described i n references 5 a n d 6. F o r O C D and neutron diffraction experiments, w e used two silica plates. F o r X - r a y diffraction experiments, w e used a p o l i s h e d b e r y l l i u m (Be) plate a n d a silica plate. T h e sample thickness ( 1 - 8 0

μιη) was c o n t r o l l e d b y a spacer

between two plates. A circular hole was made i n the spacer to provide a cavity to h o l d the sample. T h e thicker the sample, the smaller the area o f m o n o d o m a i n region w i l l be. T h e desirable radius o f the cavity is about 8 m m for thick (~ 80-μιη) samples. A sample was aligned b e t w e e n two plates b y h a n d using the procedure o f shearing a n d c o m p r e s s i o n - d i l a t i o n first de scribed i n reference 5. T h e c o n d i t i o n o f a multilayer sample, that is, its degree o f alignment (or mosaic) a n d its t h e r m o d y n a m i c phase, can be rigorously d e t e r m i n e d b y X - r a y diffraction. Fortunately, for the purpose o f aligning multilayers, the c o n d i t i o n can also be d e t e r m i n e d qualitatively by visual inspection w i t h a p o l a r i z e d microscope. This inspection is possible because the defect structures o f the l i q u i d crystalline L

a

or smectic A phase of lipids have b e e n classified a n d

studied b y polarization microscopy (5, 7-9). L

a

I n d e e d i f a l i p i d sample is i n the

phase, it is most conveniently ascertained b y the appearance o f smectic

defects. T h e defects that are most disruptive to multilayer alignment are oily streaks. T h e most effective way to align a sample is to p u s h (rather than dissolve) the oily streaks to the periphery. I f polygonal array defects appear, either the heating a n d cooling process described i n reference 4 can be used or the sample can simply be left at the L

a

phase temperature for several

days, i n w h i c h case the defects usually anneal away. F o r X - r a y diffraction samples, the alignment is examined f r o m the silica side b y using a reflection p o l a r i z i n g microscope. X - r a y diffraction was mea sured f r o m the B e side. F o r electric field experiments, the silica plate is coated w i t h i n d i u m t i n oxide ( I T O ) to f o r m a t h i n transparent electrode o n the inside (10). F o r circular d i c h r o i s m ( C D ) experiments, it is important to remove any possible stress i n the silica plates. Stress removal can be accom p l i s h e d b y temperature annealing at 1150 °C for 6 h , f o l l o w e d b y slow c o o l i n g at a rate o f 10 ° C / h d o w n to 900 °C a n d subsequent c o o l i n g at a rate o f 100 ° C / h u n t i l r o o m temperature is reached. T h u s far, w e have aligned the f o l l o w i n g lipids: dilauroyl-, dimyristoyl-, dipalmitoyl-, diphytanoyl-, a n d dioleoylphosphatidylcholine ( D L P C ,

DMPC,

D P P C , D P h P C , a n d D O P C , respectively), dimyristoylphosphatidylglycerol (DMPG),

dipalmitoylphosphatidylethanolamine

(DPPE),

L-a-phosphati-


86

BIOMEMBRANE

ELECTROCHEMISTRY

dylcholine f r o m bovine b r a i n ( B B P C ) , a n d D M P C - c h o l e s t e r o l mixtures. T h e peptides a n d proteins incorporated i n multilayer samples i n c l u d e alamethicin, melittin, g r a m i c i d i n , magainins, a n d their synthetic analogues. It has b e e n shown ( 2 , 3, 11, 12) that b o t h the M a y e r structure a n d the state o f a peptide i n a multilayer preparation are the same as i n a vesicle


sample p r e p a r e d f r o m the same materials.

Method of Oriented Circular Dichroism Theory.

Ultraviolet circular d i c h r o i s m ( C D ) spectroscopy has

been

u s e d routinely i n the analysis o f the secondary structures of proteins. T h e p r i n c i p l e o f this diagnostic m e t h o d is based o n the experimental fact that the C D spectra (below 250 n m ) o f polypeptides a n d proteins are d o m i n a t e d b y the electronic transitions o f the peptide units a n d are relatively independent of side chains. T h e asymmetric a n d periodic arrangements o f a polypeptide give rise to characteristic C D spectra. I n particular, the α-helix conformation has a highly distinctive spectrum. A c c o r d i n g to the exciton theory o f M o f f i t t (13)

(see

review o f the theory i n reference 6), the peptide I T - i r * transition

i n an α-helix is split into components w i t h polarization either perpendicular o r parallel to the helix axis. T h i s important theory is difficult to prove experimentally, because it is difficult to align a sample o f α-helices. T h e use o f l o n g polypeptides i n an electric field l e d to conflicting results (see

review

i n reference 10), because the b e n d i n g o f l o n g polypeptides was not taken into account. T h e theory was finally demonstrated experimentally b y the use o f membrane-spanning

α-helices aligned i n l i p i d multilayers. I n particular, it

was shown that the C D b a n d o f helices at 205 n m is p o l a r i z e d along the axis (6,

JO). T h u s w e may summarize the ultraviolet C D spectra of α-helical

peptides as follows (2, 14): B e t w e e n 185 and 240 n m the peptide spectra are d o m i n a t e d b y the

υ-IT*

and η - τ τ *

transitions. T h e

η-ττ*

transition is

characterized b y a magnetic dipole transition m o m e n t d i r e c t e d along the carbonyl b o n d , w h i c h i n a helix gives rise to a negative C D b a n d near 224 n m that is approximately Gaussian. T h e ττ-ττ*

transition i n a helix splits into

three. O n e transition has its electric transition dipole p o l a r i z e d parallel to the helical axis a n d gives rise to a negative Gaussian b a n d near 205 n m . T h e other two transitions have their electric transition dipoles p o l a r i z e d perpendicular to the helical axis, and their amplitudes strongly d e p e n d o n the

angle

between the direction o f the p r o b i n g light a n d the helical axis. W h e n the angle is 0°, these two transitions c o m b i n e to have the shape o f the derivative o f a Gaussian centered near 190 n m w i t h the positive amplitude o n the l o n g wavelength side, called the helix b a n d (15). O n the other hand, w h e n the angle is 90°, the two transitions are degenerate, b o t h positive Gaussians, a n d centered near 190 n m . T h u s i f the incident fight is parallel to the helix axis,


5.

HUANG

87


its C D , G y , is given b y two components (denoted b y Θ): G ( = e ^ „ * ( g , 1 9 0 n m , II) + θ „ ^ „ * ( - g , 224 n m , II) w

H

I f the incident light is perpendicular to the helix axis, its C D , G

±

(1)

is given b y

G ± = e ^ * ( + g , 1 9 0 n m , ± ) + θ , _ , * ( - g , 205 n m , ± ) w

+ e ^ *(-g,224nm, ± ) Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0235.ch005

n

(2)

w

where, i n the parentheses, the symbol g o r g indicates that the b a n d is a Gaussian o r the helix f o r m , respectively, a n d the sign i n front o f g stands f o r positive o r negative amplitude; the second entry is the wavelength; a n d the t h i r d entry denotes whether the helix is parallel o r perpendicular to the light. T h e polarization o f the 2 0 5 - n m b a n d (the M o f f i t t theory) can b e used to determine the orientation o f α-helices i n a membrane. T h i s determination is accomplished b y the m e t h o d o f O C D ; that is, the C D spectra o f a multilayer sample are measured w i t h light incident at various angles w i t h respect to the n o r m a l o f the m e m b r a n e planes ( J ) . L e t the n o r m a l to the plane o f the m e m b r a n e b e η (the sign o f η is immaterial, as w e w i l l see), the direction o f the p r o b i n g light b e k, a n d the angle between t h e m b e a . O C D is the C D spectrum as a function o f α , θ ( α ) . T h e f o l l o w i n g equation is a general property o f C D spectra ( 1 6 ) : H

θ ( α ) = θ(0°) cos α + θ(90°) sin α 2

2

(3)

T h u s , θ(0°) a n d θ(90°) can b e obtained b y measuring θ ( α ) at two different α angles. Suppose that helices are e m b e d d e d i n bilayers w i t h their axes i n c l i n e d at a n angle φ w i t h respect to the n o r m a l η a n d u n i f o r m l y o r r a n d o m l y distributed i n the azimuthal angles a r o u n d n. U s i n g the general property e q 3, w e c a n show θ(0°) = G y cos φ + G 2

sin φ

θ ( 9 0 ° ) = |G|, s i n φ + σ 2

(4)

2

±

±

(1 - f s i n φ ) 2

(5)

Equations 3 - 5 are the basis o f O C D analysis.

Applications.

W e w i l l discuss two possible applications o f O C D .

Nonhelical Molecules. I f there are n o theoretical restrictions o n G | and G , O C D c a n b e used to distinguish a rotation o f a molecule f r o m a conformational change. ( I n this case G , , a n d G are, respectively, t h e C D parallel a n d perpendicular to a molecular axis.) Suppose that the same protein molecules are p r e p a r e d under two different conditions A a n d B , such (

±

±


88


that the i n c l i n a t i o n angle φ is changed f r o m φ

to φ . T h e θ(0°) a n d 6(90°)

Α

Β

for states A a n d Β are given b y Θ (0°) = G„ ο ο 8 φ 2

Α

Θ ( 9 0 ° ) = \G „ s i n φ 2

Α

Θ (0°) = G υ cos φ 2


Β

Α

2

Β

5ίη φ 2

X

+ G + G

Β

Θ ( 9 0 ° ) = |G„ s i n φ Β

+ G

Α

(1 -

x

sin φ 2

x

+ G

x

(1 -

(6)

Α

\ sin φ 2

)

Α

(7) (8)

Β

\ s i n φ) 2

Β

(9)

T h e s e four equations are not independent (even i f one makes use o f two or m o r e C D bands at different wavelengths). T h e f o u r t h equation is consistent w i t h the other three. T h i s is, o f course, o n l y true i f the states A a n d Β are related b y a rotation. I n other w o r d s , i f states A a n d Β represent two different conformations ( G

M A

Φ G|

) B

, etc.), the f o u r t h equation w i l l not be consistent

w i t h the other three. T o test i f states A a n d Β are related b y a rotation, one may assume an arbitrary value for φ G,| , G

±

Α

a n d use three equations to solve for

, a n d φ . T h e n the f o u r t h equation is used to check for consistency. Β

A positive result can be used as a p r o o f that states A a n d Β differ only b y a rotation o f the molecular axis. O n the other h a n d , a negative result w o u l d indicate that a conformation change occurs b e t w e e n states A a n d B . A n o t h e r case o f interest is that there are two (and only two) possible orientations for the molecular axis, a n d states A a n d Β represent two different mixtures o f the molecules i n these two orientations. I n this case, O C D w i l l satisfy eqs 6 - 9 . O C D cannot distinguish a two-orientation p r o b l e m f r o m a rotation p r o b l e m ( I ) . Helices. OCD

Because the 2 0 5 - n m b a n d is p o l a r i z e d along the helix axis,

can be u s e d to determine the orientation o f helices. A s s u m e that the

190- a n d 2 2 4 - n m bands make negligible contributions to the C D a m p l i t u d e at 205 n m . T h e n , f r o m eqs 4 a n d 5, w e obtain the approximate relation θ(0°)

2 0 5

n m

/ θ(90°)

2 0 5 n m

« s i n φ / (1 2

I sin φ) 2

(10)

f r o m w h i c h the value for the i n c l i n a t i o n angle φ can be estimated. F o r a m o r e exact analysis, decompose the spectra into the bands described i n eqs 1 and 2. A n example o f such an analysis is given later for the c h a n n e l - f o r m i n g peptide, alamethicin. C D of alamethicin associated w i t h phospholipids shows a typical a - h e l i cal f o r m . F o r example, C D o f alamethicin w i t h dilauroylphosphatidylcholine ( D L P C ) vesicles is shown i n F i g u r e 1 ( I ) . I f w e assume that the m e a n residue eUipticity o f α-helices compare

the

is i n d e p e n d e n t o f the length o f the p e p t i d e a n d

C D o f alamethicin w i t h that

o f a standard α-helix (e.g.,



5.

HUANG

89


h

ι

Ι

180

ι

ι

l . i

ι

i t

t

ι

ι

i i i ι

ι

ι

ι

ι .ι

200 220 240 WAVELENGTH (nm)

I n

260

Figure 1. CD spectrum of alamethicin in DLPC vesicles. (Reproduced with permission from reference 6. Copyright 1988 American Institute of Physics.) poly-7-methyl-L-glutamate i n hexafluoro-2-propanol; see reference 10), w e conclude that about 4 0 % of the residues o f alamethicin are helical. H o w e v e r , the m e a n residue elliptieity depends o n the length o f the peptide w h e n it is short. F o r example, synthetic α-helical peptides o f 21 amino acids show a m e a n residue elliptieity of about —2 Χ 1 0 deg c m / d at 224 n m ( 1 7 ) , w h i c h is only about 6 0 % o f the standard value for l o n g α helices. Therefore, i n fact as m u c h as 6 0 - 7 0 % o f alamethicin residues c o u l d be α helical. T h e nonhelical part o f alamethicin apparently contributes little to the total C D (18). T h u s the O C D o f alamethicin i n the m e m b r a n e w i l l reflect the orientation of its α-helical section. 4

2

F i g u r e 2 shows two sets o f O C D (I a n d S) for a multilayer sample o f alamethicin i n D P h P C w i t h two different hydration conditions. (The tech nique o f O C D measurement is described i n reference 1.) Moffitt's theory predicts that the negative C D b a n d near 205 n m consists o f G components only; that is, G y = 0 for this b a n d . Consequently, w e expect the C D a m p l i tude near 205 n m to obey the α-dependence ±

θ(α) = G

L

[sin φ + ( l 2

f sin φ) sin a] 2

2

(11)

I n particular, for helices parallel to the m e m b r a n e n o r m a l ( φ = 0°), θ ( α ) increases w i t h s i n a , whereas for helices p e r p e n d i c u l a r to the m e m b r a n e n o r m a l ( φ = 90°), θ ( α ) decreases w i t h s i n a . T h u s a visual inspection o f the O C D ( F i g u r e 2) is sufficient to conclude that the inclination angle φ must be close to 0° for the spectra I a n d close to 90° for the spectra S. 2

2

A nonlinear least-squares p r o g r a m was w r i t t e n to fit the p h e n o m e n o i o g i cal expressions eqs 1 a n d 2 to the spectra o f n o r m a l incidence ( a = 0°) i n


90


π—ι—ι—r

π—ι—ι—Γ

40 h-


•rt

20

-20 _Ι

I

I

L·

180

-J

200

I

I

I

1

_Ι

L·

I

I

L.

220 240 wavelength (nm)

260

Figure 2. OCD of alamethicin in aligned multilayers of DPhPC (L:P = 50:1) when the sample is in equilibrium with 100% relative humidity (RH) (spectra I) and with 50% RH (spectra S). CD was measured with light incident at an angle a relative to the normal to the planes of hilayers. The a dependence of spectra I indicates that the helical parts of alamethicin molecules are perpendicular to the plane of the hilayer, whereas the a dependence of spectra S indicates that the helices are parallel to the plane of the hilayer. The solid lines for the a = 0° spectra are the least-squares fits; the solid lines for the spectra of oblique angles are theoretical constructions from the a = 0° spectra. (Reproduced with permission from reference 1. Copyright 1990.)

F i g u r e 2. E a c h Gaussian b a n d is assumed to have the f o r m g=Aexp[-(X-X ) /A ] 0

w i t h three parameters:

2

(12)

2

amplitude A , peak position λ , a n d b a n d w i d t h Δ . 0

T h e helix b a n d has the f o r m (12) g

H

=

Α[2(λ-λ )(λ /Δ ) + ΐ]βχρ[-(λ-λ ) /Δ ] 0

0

2

0

2

2

(13)

also w i t h three parameters. T h e 0 ° spectrum o f the S state fits very w e l l (see F i g u r e 3) w i t h three Gaussian bands as p r e s c r i b e d b y the theory ( e q 2) f o r helices perpendicular to the light. O n the other band, the 0° spectrum o f the


5.

91


HUANG

I state w o u l d not fit w e l l w i t h a combination o f Gaussian bands; instead it fits a combination o f a Gaussian a n d a helix b a n d (see F i g u r e 4), exactly as p r e d i c t e d b y the M o f f i t t theory ( e q 1) f o r helices parallel to the light. T h e b a n d parameters obtained f r o m o u r fit (Table I) are i n g o o d agreement w i t h the calculated values given b y reference 19. T h u s w e showed that the spectrum I, 0° a n d spectrum S, 0° are i n d e e d G n a n d G , respectively. ±

W e c o m p l e t e d the experimental p r o o f f o r G a n d G b y constructing the right-hand side o f e q 3 using eqs 4 a n d 5 w i t h φ = 0°, α = 2 7 ° a n d 4 0 ° , and c o m p a r i n g it w i t h spectrum I, α = 2 7 ° a n d 4 0 ° i n F i g u r e 2, respectively; similarly w i t h φ = 9 0 ° , α = 2 7 ° a n d 4 0 ° was c o m p a r e d w i t h spectrum S, α = 2 7 ° a n d 4 0 ° , respectively. W e see that they are a l l i n g o o d agreement. T h u s w e have demonstrated the following two points:


|(

x

1. T h e α-helical section is p e r p e n d i c u l a r to the plane o f the membrane i n state I a n d parallel to the plane o f the m e m b r a n e i n state S. 2. T h e r e is n o change i n the secondary structure between states I a n d S because their spectra are related to each other b y rotation. F i n a l l y , w e c a n also compare the O C D o f a multilayer sample w i t h the C D o f a vesicle sample. Because the orientations o f helices are isotropically

Τ

α

« i

Ο

-20 180

200

220 240 260 wavelength (nm)

280

300

Figure 3. Spectrum S, a = 0° of Figure 2 is fitted with eq 2. The hand parameters are given in Table I. (Reproduced with permission from reference 1. Copyright 1990.)



92

BlOMEMBRANE ELECTROCHEMISTRY

180

200

220

240

260

280

300

wavelength (nm) Figure 4. Spectrum I, a = 0° of Figure 2 is fitted with eq 1. The band parameters are given in Table I. (Reproduced with permission from reference 1. Copyright 1990.) Table I. Band Parameters for C D of α Helix

CD Band

b

η -> I T * , helix helix IT -> I T * , helix ΊΓ -> I T * , helix helix

X k (g) \\kU) _L k ( g ) _L k ( g ) II k ( g ) H

Band Center λ (nm)

Bandwidth Δ (nm)

Amplitude of Alamethicin A(10~ deg cm /dmolj

222.3 224.9 204.8 190.4 188.5

12.9 11.9 7.4 6.4 10.1

-20.37 -4.04 -18.65 36.73 -0.95

3

2

Amplitude of a Helix A(10~ deg cm /dmol) a

3

2

-51 -10 -46 92 -2.4

The mean residue elliptieity poly-7-methyl-L-glutamate in hexafluoro-2-propanol solution is used as the standard C D for isotropically distributed α-heliees. The C D of alamethicin in vesicles is compared to the standard, from which we estimate that 40% of the residues of alamethicin are helical. The numbers in the fifth column are the numbers in the fourth column divided by 40%. g denotes a Gaussian band; g denotes a helix band. SOURCE: Reproduced with permission from reference 1. Copyright 1990. H

distributed i n a vesicle sample, w e can take, for example, φ = 0° i n eqs 4 a n d 5 ( w h i c h give θ(0°) = G y a n d θ(90°) = G ) a n d average e q 3 over the solid angle 2 T T G ? ( C O S a ) to obtain the C D for vesicles ( θ ) : ±

ν

e = |G„+|G v

±


(14)

5.

HUANG

93


T h e spectrum constructed f r o m G | a n d G agrees w e l l w i t h the measured vesicle spectrum ( F i g u r e 5) a n d proves that the secondary structure o f alamethicin is essentially the same, w h e t h e r the membranes are vesicles o r multilayers. (

±


How Does Alamethicin Associate with a Membrane? A l a m e t h i c i n is a natural peptide o f 20 amino acids ( p r o d u c e d b y the fungus Trichoderma viride). Since the late 1960s, the channels f o r m e d b y ala m e t h i c i n i n l i p i d bilayers have b e e n studied as a m o d e l f o r voltage-gated i o n channels ( 2 0 ; review i n reference 21; see a fairly complete list o f references i n reference 2). Recently the alamethicin channels have b e e n shown to b e tension dependent ( 2 2 ) . A l t h o u g h alamethicin is one o f the best character i z e d channels, there is a long-standing controversy about w h i c h m o d e l best describes the experimental data. Since the early 1970s, most investigators agree that alamethicin monomers f o r m a water-filled c o n d u c t i n g pore like the staves o f a barrel, a n d this assumption is consistent w i t h most i o n c o n d u c t i o n data ( 2 3 - 2 5 ) . H o w e v e r , conduction experiments p r o v i d e d n o clues for the n o n c o n d u c t i o n state, either

180

200

220 240 260 wavelength (nm)

280

300

Figure 5. CD of alamethicin in DPhPC vesicles. The solid line is the construction (eq 14) from the OCD spectra of the multilayer sample (Figure 2). The dashed and dotted lines are the measured spectra of vesicles with 100:1 and 50:1 lipid-peptide ratios, respectively. (Reproduced with permission from reference 1. Copyright 1990)


94


about the location o f the molecule (relative to the m e m b r a n e ) o r its configu ration. I n the last 10 years, numerous spectroscopic a n d other methods have b e e n used to study the n o n c o n d u c t i n g state. Such methods i n c l u d e R a m a n spectroscopy (26-28); H, H , a n d Ρ nuclear magnetic resonance ( N M R ) studies (27); i n f r a r e d attenuated total reflection spectroscopy ( 2 9 ) ; alam e t h i c i n - p h o s p h o l i p i d cross-finking studies (22); titration a n d stopped-flow analyses using circular d i c h r o i s m ( C D ) a n d fluorescence to m o n i t o r the a l a m e t h i c i n - l i p i d interactions (30); capacitance studies (31); a n d studies o f synthetic analogues (31-33). These studies l e d to many different conclusions. In particular, there are conflicting conclusions as to whether, i n the absence o f a transmembrane electric field, alamethicin partitions into the apolar region o f a l i p i d bilayer o r adsorbs to the l i p i d - w a t e r interface. E v i d e n c e for b o t h interfacial interactions a n d alamethicin insertion into bilayers was f o u n d . Recent theoretical models (30, 32-35) have avoided the surface state o f alamethicin.


l

2

3 1

W e investigated this p r o b l e m b y O C D ( 2 ) . A l a m e t h i c i n - D P h P C m u l t i layer samples at various p e p t i d e - l i p i d molar ratios (1:10-1:140) were pre p a r e d as previously described. T h e aligned multilayer samples were p l a c e d i n a series o f h u m i d i t y chambers to b e e q u i l i b r a t e d at chosen relative humidities ( R H ) at r o o m temperature. Samples obviously exchanged water w i t h the environment through the gap between the t w o silica plates. D e p e n d e n t o n initial a n d final R H , the equilibration t i m e f o r a sample varied f r o m 4 to 20 days. T h e e q u i l i b r i u m states o f the samples were examined b y O C D . I n most cases the samples exhibited spectra either like spectra I o r like spectra S ( F i g u r e 2). Because spectra I represent helices p e r p e n d i c u l a r to the plane o f the m e m b r a n e , w e call such a state the inserted (I) state. O n the other h a n d , spectra S w i l l b e called the surface (S) state because they represent helices parallel to the plane o f the m e m b r a n e . Samples were either i n the surface state o r i n the inserted state as shown i n the R H versus L : P (the l i p i d - p e p t i d e molar ratio) phase diagram ( F i g u r e 6), unless a sample was near the phase boundary. N e a r the phase b o u n d a r y the sample spectrum was a linear superposition o f spectra I a n d S. Phase change reversibility was examined b y exchanging t w o samples o f the same L : P between difference h u m i d i t y chambers across the phase boundary. D u r i n g the transition, the spectra were linear superpositions o f I a n d S, b u t once the samples reached e q u i l i b r i u m , the initial spectra were reversed. W h e n a sample changed its state (i.e., underwent a phase transition), it took a longer time ( u p to 2 0 days) to reach e q u i f i b r i u m , i f the final point was closer to the phase boundary. A n e q u i l i b r i u m spectrum r e m a i n e d u n c h a n g e d i n t i m e as long as the sample was kept i n the same h u m i d i t y chamber. T h e samples w e r e examined u n d e r a p o l a r i z e d microscope each time a C D measurement was made. A l l the data points o n the phase diagram are i n the L phase o f the l i p i d . E x c e p t for L : P less than 10:1, the l i p i d changes to the g e l phase f o r R H b e l o w 8 9 % . a


5.

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100%

96%


ε

92%

4-*

88%

84%

0

40

80

120

160

DPhPC/alamethicin molar ratio Figure 6. The phase diagram for alamethicin in DPhPC on the plane of relative humidity (RH) versus the lipid-peptide molar ratio (L:P). A multilayer sample of a certain L:P was in turn equilibrated in humidity chambers of various RH; in each equilibrium state, the OCD spectrum was measured. If the OCD spectrum is spectrum I (Figure 2), indicating that alamethicin is in the inserted state, a black circle is shown at the corresponding L:P and RH. If the OCD spectra are spectra S, indicating that alamethicin is in the surface state, an open circle is shown. A gray circle implies that the OCD spectra are linear superpositions of spectra I and S, which indicates that the state of alamethicin is a coexistent state. The shaded area for L:P = 10:1 indicates that the sample at RH below 89% turned into the gel phase. In all other data points, the samples were in the L phase. We define a critical L:P value L:P*. For L:P greater than L . P * , the majority of alamethicin molecules are in the surface state; for L:P smaller than L . P * , the majority of alamethicin molecules are in the inserted state if the sample is in equilibrium at 100% RH. (Reproduced with permission from reference 2. Copyright 1991.) a

T h u s w e discovered a n e w p h e n o m e n o n o f phase transition for ala m e t h i c i n i n a m e m b r a n e . T h e phase diagram ( F i g u r e 6) is characterized b y a phase b o u n d a i y that ends at a critical lipid:peptide molar ratio L : P * at 1 0 0 % R H . I f the l i p i d - p e p t i d e ratio is greater than L : P * (that is, at a l o w peptide concentration), alamethicin is always o n the m e m b r a n e surface i n e q u i l i b r i u m . F o r L : P smaller than L : P * , alamethicin is always inserted i n the membrane

i n e q u i l i b r i u m at 1 0 0 % R H , b u t i f R H is b e l o w the phase

boundary, alamethicin is again o n the m e m b r a n e surface. Starting f r o m the critical L : P * , the phase boundary decreases to lower R H f o r lower L : P .


96


T h i s surprising property o f alamethicin is not u n i q u e to the l i p i d D P h P C . A l a m e t h i c i n behaves similarly i n dioleoylphosphatidylcholine ( D O P C ) ( F i g ure 7). A l a m e t h i c i n i n L - a - p h o s p h a t i d y l c h o l i n e f r o m bovine b r a i n ( B B P C ) at L : P ~ 50:1 also shows an I

DOPC/alamethicin molar ratio Figure 7. The phase diagram of alamethicin in DOPC. See Figure 6 for explanations. (Reproduced with permission from reference 2. Copyright 1991.)


5.

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97

peptide dipole w i t h the electric field (2). T h i s mechanism of voltage gating was i n fact proposed a l o n g t i m e ago b y B a u m a n , M u e l l e r , a n d B o h e i m (21).


Location of Ion Binding Sites in the Gramicidin Channel by High-Resolution Lamellar Diffraction T h e r e are two frequently used methods for X - r a y diffraction o f m e m b r a n e systems. I n one m e t h o d , diffraction o f an aligned, flat sample is p e r f o r m e d w i t h an oscillating camera. I n another m e t h o d , the membrane multilayers are aligned o n a c u r v e d surface to produce p o w d e r patterns. T h e diffraction patterns obtained b y these methods are sometimes intermediate between the pattern o f a p o w d e r a n d that o f a single crystal; consequently the L o r e n t z corrections for such patterns are ambiguous. A l s o , i n the second m e t h o d , it is difficult to make corrections for the sample absorption (this correction is necessary for the first two B r a g g orders). W e believe that an alternative m e t h o d o f single crystal diffraction (i.e., Θ-2Θ scan) that uses well-aligned samples is advantageous for two reasons: (1) the above-mentioned difficulties are avoided a n d (2) high-resolution data are always p r o d u c e d . W e used this single crystal diffraction m e t h o d to solve a long-standing p r o b l e m o f the g r a m i c i d i n channel (3), namely, where are its i o n b i n d i n g sites? G r a m i c i d i n , a linear pentadecapeptide, is b y far the most extensively studied membrane-active peptide that forms a transmembrane i o n channel. T h e g r a m i c i d i n channel is a cylindrical pore f o r m e d b y two monomers, each a single-stranded β helix, that are h y d r o g e n - b o n d e d head-to-head at their Ν t e r m i n i (36, 3 7 ) . T h e pore selectively facilitates the diffusion o f monovalent cations across bilayer membranes, but does not transmit anions a n d divalent cations (38). Extensive kinetic data describe the effect o n the channel conductivities o f a great n u m b e r o f variables i n c l u d i n g amino acid variation, membrane variation, i o n valence variation, and cation variation (38-40). The relatively simple structure a n d the wealth o f experimental data o n its i o n conductions make g r a m i c i d i n an ideal m o d e l system to study the principles governing i o n transport across l i p i d membranes. M a n y molecular dynamics computations a n d simulations have b e e n p e r f o r m e d i n an attempt to under stand the detailed properties o f the channel, such as the free energy profiles o f ions, the hydrogen-bonding pattern o f water, a n d the i o n a n d water motions (41-43). D e s p i t e these extensive studies, there were no direct structural measurements o n the g r a m i c i d i n channel. It o c c u r r e d to us that X - r a y diffraction o f well-aligned gramicidin-containing m e m b r a n e multilayers might resolve the location o f the i o n b i n d i n g sites i n the channel (3). That the channel has two monovalent cation b i n d i n g sites is consistent w i t h k n o w n experimental data (36, 38). Indeed, the b i n d i n g constants for the first a n d second bindings o f alkali metal cations have b e e n estimated b y T l - 2 0 5 chemical shift studies (44) and N M R relaxation methods (36). T h e 6 3

+


98


bindings o f divalent cations were first i n f e r r e d f r o m the ability of the cations to reduce the fluxes o f monovalent ions (38); further evidence was obtained f r o m C a n d C a N M R relaxation studies (36, 45). H o w e v e r , the locations o f all these b i n d i n g sites were u n k n o w n . A n earlier study o n i o n - i n d u c e d chemical shifts o f carbonyl carbon N M R resonances b y U r r y and his collabo rators c o n c l u d e d that two symmetric b i n d i n g sites for monovalent cations are each localized between the T i p - 1 1 a n d T i p - 1 3 carbonyls o f one m o n o m e r (36). H o w e v e r , this same evidence was also used to conclude that the gramicidin helix is l e f t - h a n d e d — a conclusion contradicted b y m o r e recent N M R studies (37, 46).


1 3

4 3

T h e direct measurement o f i o n locations was p e r f o r m e d b y difference X - r a y diffraction o f g r a m i c i d i n samples w i t h various ions. U n l i k e the C D experiment, w h e r e the multilayer samples can be as t h i n as 1 |xm or less, X - r a y diffraction samples must be at least 10 μχη. T o align thick multilayers, it is essential to use two substrates. W e k n o w o f no example o f a thick ( > 1 0 |xm) a n d large ( > 1 0 m m ) single-domain multilayer preparation o n one flat substrate without the h e l p o f a second surface. F u r t h e r m o r e , i f multilayers are f o r m e d b y evaporation o r centrifugation o f a vesicle solution (either m e t h o d requires only one substrate surface), it w o u l d be difficult to control the i o n concentration i n the multilayers. O n the other hand, finding a substrate suitable for alignment o f multilayers as w e l l as for X - r a y transmis sion is not trivial either. T h e p r o b l e m is accentuated b y the fact that the first Bragg peak o f m e m b r a n e diffraction is about 1°, w h i c h means that the X - r a y path length t h r o u g h the substrate w i l l be 60 times the thickness. T h i s p r o b l e m shows w h y it is necessary to use a polished b e r y l l i u m plate. 2

D L P C membranes were chosen for convenience because they are i n the L phase at r o o m temperature. G r a m i c i d i n - D L P C multilayer samples aligned between a p o l i s h e d B e plate a n d a silica plate p r o d u c e d h i g h quality diffraction patterns. T y p i c a l l y eight B r a g g orders were recorded ( F i g u r e 8), w h i c h is a very h i g h resolution for D L P C membranes i n the L l i q u i d - c r y s talline phase. B y the same method, 13 B r a g g orders were recorded for dimyristoylphosphatidylcholine ( D M P C ) - c h o l e s t e r o l multilayers i n the L phase (47), w h i c h represents the highest resolution ever r e c o r d e d for such systems (11). D a t a reduction i n c l u d e d (1) b a c k g r o u n d subtraction; (2) correc tions for polarization, the L o r e n t z factor, scattering v o l u m e , B e a n d specimen absorption, the second h a r m o n i c ( w h i c h becomes significant due to the absorption by the B e plate) a n d the atomic scattering factors; a n d (3) the detector vertical slit correction for b e a m divergence a n d sample mosaic ( 0 . 3 - 0 . 5 ° ) (48). a

a

a

T h e phases o f Bragg reflections were d e t e r m i n e d b y the swelling m e t h o d . T h e reciprocal space is sampled b y changing the lamellar spacing (or repeat distance) o f the multilayers through the hydration variation. I f the bilayer structure is unchanging or slowly changing w i t h water content, the scattering amplitudes (whose phases are either 0 or I T due to the centrosymmetry o f


5.

HUANG

99



A

. \

V 1

0

1

2'

3

1

1

1

1

4 5 6 θ (degree)

7

8

9

Β

.

-j Q1

\

I 01

'

'

•

2

ι

•

ι

3 4 5 6 θ (degree)

1

7

1

8

J 9

Figure 8. Our typical X-ray diffraction patterns of gramicidin-DLPC multilayers with thallium acetate, A, and without salt, B. Eight Bragg orders were recorded.


100


bilayer structures) w i l l fall o n a single smooth curve. Examples are given i n F i g u r e 9. O n c e the phases are d e t e r m i n e d , the structure factors are F o u r i e r transformed to obtain the scattering density profiles. T h e scattering density profile can b e n o r m a l i z e d to the true electron density, p, i f w e k n o w the composition o f the sample a n d the molecular areas (along the plane o f the membrane) o f l i p i d a n d g r a m i c i d i n (see the details i n reference 3). F i g u r e 10 shows the n o r m a l i z e d electron density profiles o f g r a m i c i d i n - D L P C bilayers w i t h T l ( p e p t i d e - l i p i d - i o n molar ratios 1:10:1), K Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0235.ch005

+

+

(1:10:1.5), a n d without

salt (1:10:0). A difference electron density profile is obtained f r o m t w o ρ profiles that have the same lamellar spacing D . T h e top two curves i n F i g u r e 11 are t w o examples o f the difference profile p ( T l s a m p l e ) — p(salt-free sample) ob +

tained f r o m the data o f 42.4- a n d 43.4-Â lamellar spacings. E a c h profile represents a measurement o f the electron density distribution o f T l . T h e +

distributions indicate that the majority o f T l channels;

+

ions are b o u n d inside the

each (dimeric) channel binds two T l

ions according to the

+

i o n - g r a m i c i d i n ratio. T h e peak positions indicate the locations o f the T l

+

b i n d i n g sites; the w i d t h o f the peak represents the resolution o f diffraction, ~ 5 Â. W e also calculated p ( T l

+

sample) — p ( K sample) w h i c h is shown as +

the bottom two curves i n F i g u r e 11. Because K b i n d i n g constant (44), excessive amounts o f K

+

+

has a relatively small

i n the sample are considered

necessary to ensure that the majority o f the b i n d i n g sites are o c c u p i e d . A s a result, about o n e - t h i r d o r m o r e o f K

+

ions are outside the channel. C o n s e

quently the apparent peak positions o f T l p(K

+

+

obtained f r o m p ( T l

+

sample) —

sample) are shifted slightly toward the center. D u e to its unfavorable

signal-to-noise ratio, the K

+

distribution profile was not analyzed further. T h e

location o f T l

+

profiles, p ( T l

sample) - p(salt-free sample).

+

b i n d i n g sites was d e t e r m i n e d solely f r o m the difference

T h e locations o f the t w o b i n d i n g sites are symmetric w i t h respect to the center. T h i s is p r o v e n b y the fact that, although b o t h b i n d i n g sites are occupied, there is only one sharp peak o n each side o f the center. (Because o f the manner o f sample preparation, i f two sites were asymmetric, two peaks w o u l d appear o n each side.) W e measured the peak positions o f the differ ence profiles p ( T l different T l

+

+

sample) — p(salt-free sample) obtained f r o m using four

samples a n d three different salt-free samples, evaluated at

various lamellar spacings. F r o m the average, T l

+

b i n d i n g sites are deter

m i n e d to be at 9.6 ± 0.3 Â f r o m the m i d p o i n t o f the channel. F i g u r e 12 shows the profiles o f the g r a m i c i d i n - D L P C bilayers w i t h B a ( p e p t i d e - l i p i d - i o n molar ratios 1:10:1) a n d M g

2 +

2 +

(1:10:1) at 42.8-Â lamellar

spacing. T h e central regions o f the two profiles are essentially the same, b u t they are substantially different f r o m the central regions o f the monovalent cation samples a n d the salt-free sample, w h i c h are essentially the same i n the central region ( F i g u r e 10). F o r this reason, w e subtract the M g

2 +

profile, b u t


5.

HUANG

101


0.25 h1- 1

0.00

1 1

J

1 1 1 1

J1

I1 I

J

Μ

Γ I

J

-

—


-8-0.25

-

a

1-0.50

ι

—

τ

-

t

-0.75

I I I I

Α

-

1

: ί -1.00

Μ

0.00

1 I I I I I I Iι ι ! ι ! ι ! ι ι ι ι ! 0.25 0.50 0.75 1.00 1.25 4wsinô/X ( Â ) I

I I

- 1

0.00

0.25 0.50 0.75 1.00 4πείη0/λ (Ι )

1.25

-1

Figure 9. Structure factors obtained from H 0 swelling experiments for four different Tl samples, A, and three different salt-free samples, B. Phases are chosen so that the data points fall on a single smooth curve. (Reproduced with permission from reference 3. Copyright 1991.) 2

+


102


τ—ι—ι—ι—Γ"

τ—ι—ι—ι—ι—ι—ι—ι—Γ

^

0.40 h

°< \


fi 0.35 Ο


103


0.20 -20

-10 0 10 distance (K)

20

Figure 12. Normalized electron density profiles of gramicidin-DLPC bilayers with Ba (dotted line) and with M g (solid line), obtained from the structure factors of 42.8-A lamellar spacing. (Reproduced with permission from reference 3. Copynght 1991.) 2

+

not the salt-free profile, f r o m the B a p(Tl

+

sample) — p ( K

equal numbers o f B a

+

2 +

2 +

profile. H o w e v e r , u n l i k e the case o f

sample), w h e r e there are excessive K and M g

2 +

+

ions, w e have

ions i n the respective samples.

F i g u r e 13 shows two examples o f the difference profile p ( B a - p(Mg

2 +

2 +

ions are w e l l localized. W e

measured the peak positions o f the difference profiles p ( B a 2 +

sample)

sample) obtained f r o m the data o f 42.8- a n d 44.4-Â lamellar

spacings. T h e sharp peaks indicate that B a p(Mg

2 +

sample) obtained f r o m using four different B a

different M g

2 +

2 +

2 +

sample) —

samples a n d three

samples evaluated at various lamellar spacings. F r o m the

average, two symmetric B a

2 +

b i n d i n g sites are d e t e r m i n e d to be at 13.0 ±

0.2

Â f r o m the channel m i d p o i n t . Based o n the β

6 , 3

helical structure o f the g r a m i c i d i n channel, it is quite

natural to suggest that the i o n b i n d i n g site is o n the first t u r n o f the helix f r o m the m o u t h (38).

A t the m o u t h of the channel, the last six carbonyls are

h y d r o g e n - b o n d e d only to one neighbor. T h r e e u n b o n d e d carbonyl oxygens are p o i n t i n g toward the outside o f the channel, as is the hydroxy! group o f the ethanol amine tail. T h i s cluster o f negative charges seems to provide a b i n d i n g site for cations. T h e surprising finding o f our experiment is that the Tl

+

b i n d i n g site, at 9.6 ± 0.3 Â f r o m the channel m i d p o i n t , is either near the

b o t t o m of or b e l o w the first t u r n o f the helix On

the other h a n d , B a

2 +

(49).

ions, at 13.0 ± 0.2 Â f r o m the

channel

midpoint, apparently b i n d to the channel near the ends. T h i s location is


104


I '

1

1

1

I


0.015 h

< 0.000 -0.005

-10

0

10

d i s t a n c e (Κ) Figure 13. Difference electron density profiles p(harium sample) — p(magnesium sample) at 42.8- and 44.4-Â lamellar spacing. (Reproduced with permission from reference 3. Copyright 1991.)

consistent w i t h the experimental observation that divalent cations d o not permeate b u t block t h e c h a n n e l ( 5 0 ) . T h u s w e suggest that the separation between t w o opposite B a

2 +

b i n d i n g sites (i.e., 26.0 + 0.4 Â ) is a g o o d

measure f o r the length o f the g r a m i c i d i n channel. T h e molecular basis for the selectivity against divalent cations is probably straightforward. T h e g r a m i c i d i n channel is a pore o f 4 Â i n diameter separated f r o m t h e hydrophobic dielectric m e d i u m only b y a single layer o f polypeptide backbone. A cation entering t h e c h a n n e l must overcome t h e c h a n n e l dehydration energy (42) and encounters an image potential (51). B o t h the dehydration energy a n d the image potential are greater for divalent cations than for monovalent cations. O u r diffraction experiment revealed some other interesting properties o f m e m b r a n e - g r a m i c i d i n interactions. It is w e l l - k n o w n that a p u r e l i p i d bilayer changes its thickness w i t h hydration (47). H o w e v e r , i f the M a y e r s contain cholesterol at a sufficiently high concentration, this effect is absent (47 a n d references cited therein)

a n d there

is a tendency f o r t h e hydrocarbon

thickness o f the bilayer to match the thickness o f a pair o f cholesterol molecules (11). F o r this reason, cholesterol is called a m e m b r a n e thickness buffer. G r a m i c i d i n is another m e m b r a n e thickness b u f f e r . I n all o u r samples the phosphate peak-to-peak distances across the D L P C - g r a m i c i d i n bilayers are virtually identical at 32.1 Â, irrespective o f the degree o f hydration. T h e assumption that t h e local hydrocarbon thickness o f a l i p i d bilayer tends to match that o f an e m b e d d e d g r a m i c i d i n channel was t h e basis o f a previous study o n the effect o f m e m b r a n e thickness o n the g r a m i c i d i n channel lifetime (38, 52, 53).


5.

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105

Acknowledgments T h e research reported here was supported i n part b y the Office o f N a v a l Research G r a n t N00014-90-J-1020, the R o b e r t A . W e l c h F o u n d a t i o n , a n d the N a t i o n a l Institutes o f H e a l t h Biophysics T r a i n i n g G r a n t G M 08280.


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RECEIVED for review January 29, 1991. ACCEPTED revised manuscript January 28, 1993.


Channel-Forming Peptides in Uniformly Aligned Multilayers of

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