pubs.acs.org/Langmuir © 2010 American Chemical Society
Targeting Proteins to Liquid-Ordered Domains in Lipid Membranes† Jeanne C. Stachowiak, Carl C. Hayden, Mari Angelica A. Sanchez, Julia Wang, Bruce C. Bunker, James A. Voigt, and Darryl Y. Sasaki* Sandia National Laboratories, P.O. Box 969, Livermore, California 94551, United States, and P.O. Box 5800, Albuquerque, New Mexico, United States Received October 15, 2010. Revised Manuscript Received November 18, 2010 We demonstrate the construction of novel protein-lipid assemblies through the design of a lipid-like molecule, DPIDA, endowed with tail-driven affinity for specific lipid membrane phases and head-driven affinity for specific proteins. In studies performed on giant unilamellar vesicles (GUVs) with varying mole fractions of dipalymitoylphosphatidylcholine (DPPC), cholesterol, and diphytanoylphosphatidyl choline (DPhPC), DPIDA selectively partitioned into the more ordered phases, either solid or liquid-ordered (Lo) depending on membrane composition. Fluorescence imaging established the phase behavior of the resulting quaternary lipid system. Fluorescence correlation spectroscopy confirmed the fluidity of the Lo phase containing DPIDA. In the presence of CuCl2, the iminodiacetic acid (IDA) headgroup of DPIDA forms the Cu(II)-IDA complex that exhibits a high affinity for histidine residues. His-tagged proteins were bound specifically to domains enriched in DPIDA, demonstrating the capacity to target protein binding selectively to both solid and Lo phases. Steric pressure from the crowding of surface-bound proteins transformed the domains into tubules with persistence lengths that depended on the phase state of the lipid domains.
Introduction One of the goals of supramolecular chemistry is to understand biochemical processes through bottom-up reconstruction from minimal molecular components.1 The cell membrane, a biomolecular assembly of lipids and proteins, contains supramolecular structures that range in size from clusters of a few molecules to large, micrometer-scale assemblies depending on their function. Researchers have used lipid bilayers as platforms to model molecular recognition events at the membrane interface and to create multivalent receptors and sensor materials.2-5 These studies have provided valuable insights into supramolecular assembly in the lipid bilayer, highlighting the rapid and specific organizational and structural transformations necessary for membrane functions such as trans-membrane signaling and endocytosis. It is widely recognized that membrane-associated molecules may be assisted in assembly by partitioning into phase-separated structures in the membrane called microdomains or lipid rafts. Although controversial, the existence of such assemblies could explain how the vast array of functions occurring on the surfaces of membranes are spatially and temporally organized. Lipid rafts are thought to be dynamic nanoscale structures that are enriched in cholesterol and a select group of proteins and lipids, giving them a liquid-ordered (Lo) structure.6-8 Receptor molecules are believed to partition into lipid rafts on the basis of the packing preferences of their hydrophobic anchors.9 To explore the range of behaviors that can arise when domains confine interactions between lipids and proteins and to borrow these concepts to Part of the Supramolecular Chemistry at Interfaces special issue. *Corresponding author. Fax: (þ1) 925-294-3020. E-mail: dysasak@ sandia.gov. †
(1) Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4763. (2) Voskuhl, J.; Ravoo, B. J. Chem. Soc. Rev. 2009, 38, 495. (3) Huskens, J. Curr. Opin. Chem. Biol. 2006, 10, 537. (4) Menger, F. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4818. (5) Sasaki, D. Y. Cell Biochem. Biophys. 2003, 39, 145. (6) Lingwood, D.; Simons, K. Science 2010, 327, 46. (7) Binder, W. H.; Barragan, V.; Menger, F. M. Angew. Chem., Int. Ed. 2003, 42, 5802. (8) Silvius, J. R. Biochim. Biophys. Acta, Biomembr. 2003, 1610, 174. (9) Levental, I.; Grzybek, M.; Simons, K. Biochemistry 2010, 49, 6305.
Langmuir 2011, 27(4), 1457–1462
construct synthetic supramolecular materials, we endeavor to build a minimal molecular model of protein attachment to phaseseparated lipid membranes. To approximate the expected interactions within a raft-like structure, a model system must meet at least two requirements: (i) the existence of Lo phases in a liquid-disordered (Ld) membrane and (ii) the presence of select proteins targeted to these phases.10,11 The Lo phase is the state where lipid components are in condensed, structurally ordered assemblies yet attain high lateral mobility approaching diffusion rates similar to that of the Ld phase. Several groups have observed and precisely defined the behavior of Lo-phase separation in Ld-phase membranes by studying mixtures (e.g., sphingomyelin (SM) or dipalmitoylphosphatidyl choline (DPPC)) with cholesterol and a lipid of low phase-transition temperature (Tg
