SCIENCE & TECHNOLOGY
translocating substrate proteinsfromthe outside world toward the proteasome's active sites," which are located deep in the interior of the barrel-like structure. "The approach allows us to look at supramolecular machines and get quantitative information," Kay adds. "It sets up a Technique reveals motions of key residues framework for being able to address really in huge PROTEASOME complex neat questions about how these machines STU BORMAN, C&EN WASHINGTON work and the role of dynamics in their function." "This is an absolutely stunning piece sity and coworkers, made it possible for of work," says Ad Bax, chief of biophysical A NEW NUCLEAR magnetic resonance NMR spectroscopy at the National Instithem to obtain the high-resolution strucspectroscopy technique has made it posture of the 41-kDa maltodextrin-binding tutes of Health's Laboratory of Chemical sible to obtain information on the dynamprotein (Nature 2006,440,52). Physics in Bethesda, Md. He notes that the ics of key amino acid side chains in the Kay lab developed clever new methods The new technique is not a high-resoproteasome, a huge protein complex much lution structural approach. Instead, the for obtaining sharp resonances for methyl larger than other biomolecules generally NMR studies carried out by Sprangers and groups in large protein complexes such analyzed by NMR (Nature, DOI: 10.1038/ as the proteasome andfiguredout natureo55i2). how to attribute each resonance to The technique, developed by postDYNAMIC VIEW a specific methyl group. "They also doc Remco Sprangers and biochemisA new NMR technique measures the dynamics showthat quantitative measures for try professor Lewis E. Kay of the Uniof highly mobile amino acid residues (red) in the rates at which these groups move versity of Toronto, has two aspects. the proteasome cavity. These residues may help around within the proteasome parOne is an isotope-labeling scheme direct substrates toward proteasome active sites ticle can be obtained by NMR," Bax that involves protonation of methyl (yellow), at which protein degradation occurs. notes. groups in leucine, isoleucine, and valine residues in a protein that is othTHE ABILITY to determine the erwise highly deuterated. This scheme structure of molecular complexes as simplifies the spectrum by permitting large as the proteasome has generally the NMR spectrometer to focus only been considered to be the realm of on the labeled methyl groups and esX-ray crystallography until now, says sentially ignore everything else. Michele Vendruscolo of the UniverThis isotope-labeling scheme is sity of Cambridge. "This work shows combined with transverse relaxation that NMR spectroscopy represents a optimized spectroscopy (TROSY), uniquely powerful way to obtain dean approach that optimizes spectral tailed knowledge about the dynamics sensitivity and resolution by increasof such complexes—information that ing the time over which NMR signals is not readily accessible through crysfrom methyl groups can be detected. tallographic studies. Since dynamics A related TROSY method was develare often crucial for function, as illusoped earlier by biophysics professor trated for the proteasome by SprangKurt Wuthrich and coworkers at the ers and Kay, NMR is poised to play an Swiss Federal Institute of Technology, indispensable role in structural and Zurich. cell biology." The new technique makes it possible to overcome the molecular-weight NMR spectroscopist Dennis limitations that have traditionally Torchia of NIH's National Institute hampered quantitative NMR specof Dental 8c Craniofacial Research troscopy studies" of large biomoladds that the new technique should ecules, such as the 670-kilodalton be useful for obtaining information proteasome, Sprangers and Kay write. about internal biomolecular dyThe use of NMR to determine high-resoluKay analyze millisecond, nanosecond, and namics and interactions, not only in the picosecond dynamics of methyl groups, tion structures of biomolecules at atomic proteasome but in a wide range of other detail generally remains restricted to much which tend to move around a lot when the molecular machines as well. By combining proteasome does its job of catalyzing prosmaller systems, typically having masses this information with structural data, retein degradation. of 50 kDa or less. For example, stereo-array searchers will be able to achieve a deeper Sprangers says: "We found that the walls understanding than is currently possible isotope labeling (SAIL), a technique develof the chambers of the proteasome are very oped by chemistry professor Masatsune of the way these machines work on a momobile. This mobility can be important for Kainosho of Tokyo Metropolitan Univerlecular level, Torchia says. •
NMR OF LARGER BIOMOLECULES
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