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Book of Abstracts: Albany 2011

category image Albany 2011
Conversation 17
June 14-18 2011
©Adenine Press (2010)

Linking Allostery In Chaperonins To Protein Folding

Chaperonins consist of two back-to-back stacked oligomeric rings with a cavity at each end in which protein folding can take place under confining conditions (1). They are molecular machines that assist protein folding by undergoing large-scale ATP-driven allosteric transitions between protein substrate binding and release states (1, 2). The intra-ring conformational changes in chaperonins were found to be concerted in the case of the homo-oligomeric prokaryotic chaperonin GroEL and sequential in the case of the hetero-oligomeric eukaryotic chaperonin CCT (2). Previously (3), we hypothesized that a sequential allosteric mechanism might be more beneficial for eukaryotic proteins that tend to be larger and multi-domain as it may enable one domain to detach from the chaperonin and start folding while the other domain(s) is still bound, thereby mimicking co-translational folding. By contrast, we reasoned that a concerted mechanism is likely to be more beneficial for prokaryotic proteins that tend to be smaller and single-domain and, thus, may need to be released in an all-or-none fashion in order to fold efficiently. Support for this hypothesis was first obtained from lattice model simulations of single- and double-domain protein folding in chaperonin cages that undergo concerted or sequential allosteric transitions (4). We then took advantage of a GroEL mutant (D155A) that undergoes sequential intra-ring allosteric transitions (5) to test our hypothesis experimentally. In one test, we used a chimeric fluorescent protein substrate, CyPet-Ypet, for which it was possible to determine the folding yield of each domain from its intrinsic fluorescence and that of the entire chimera by measuring FRET between the two domains. Hence, it was possible to determine whether release (and thus also folding) of one domain is accompanied by release of the other domain (concerted mechanism) or if their release is not coupled. Our results showed that the chimera’s release tends to be concerted when its folding is assisted by wild-type GroEL but not when it is assisted by the D155A mutant that undergoes a sequential allosteric switch (6). In a second test, we used chimeras in which a substrate whose release requires the co-chaperonin GroES is fused to a GroES-independent substrate (7). In the case of the D155A mutant, release and folding of the GroES-dependent substrate was found to take place in a step-wise fashion upon addition of ATP whereas, in the case of wild-type GroEL, substrate release was found to have a sigmoidal dependence on ATP concentration. Our results, therefore, demonstrate that changes in the allosteric mechanisms of chaperonins can impact their folding function.

References

  1. A. L. Horwich, W. A. Fenton, E. Chapman, G. W. Farr, Annu. Rev. Cell. Dev. Biol. 23, 115-145 (2007).
  2. A. Horovitz, K. R. Willison, Curr. Opin. Struct. Biol. 15, 646-651 (2005).
  3. D. Rivenzon-Segal, S. G. Wolf, L. Shimon, K. R. Willison, A. Horovitz, Nat. Struct. Mol. Biol. 12, 233-237 (2005).
  4. E. Jacob, A. Horovitz, R. Unger, Bioinformatics 23, i240-i248 (2007).
  5. O. Danziger, D. Rivenzon-Segal, S. G. Wolf, A. Horovitz, Proc. Natl. Acad. Sci. USA 100, 13797-13802 (2003).
  6. N. Papo, Y. Kipnis, G. Haran, A. Horovitz, J. Mol. Biol. 380, 717-725 (2008).
  7. Y. Kipnis, N. Papo, G. Haran, A. Horovitz, Proc. Natl. Acad. Sci. USA 104, 3119-3126 (2007).

Amnon Horovitz

Dept. of Structural Biology, Weizmann Institute Rehovot 76100, Israel

Phone: 972-8-9343399
Fax: 972-8-9344188

Amnon.Horovitz@weizmann.ac.il