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Structure of the transition state for folding of the 129 aa protein CheY resembles that of a smaller protein, CI-2.

BACKGROUND
Protein engineering analysis has been used as a tool to determine the structure of the transition state of two different proteins: CI-2 and barnase. CI-2 belongs to the group of small, globular proteins with no disulphide bonds that fold via a two-state mechanism. Barnase is a larger protein (110 aa) and displays a folding intermediate. The structure of the transition state of both proteins is quite different. Whereas in CI-2 no region is fully native and it looks like an expanded form of the folded state, in barnase several regions are folded and the rate-limiting step seems to be the consolidation of the hydrophobic core. On the basis of these results, a unified scheme for the transition state of protein folding has been presented. We decided to characterize the folding pathway, or pathways, present in the alpha/beta parallel family of proteins using one of the smallest members, CheY (129 aa), as a model case.


RESULTS
The folding pathway of CheY contains, as does that of barnase, a kinetic intermediate. The picture obtained for CheY from the equilibrium and kinetic analyses of several mutations scattered throughout the whole protein is different from that found for barnase. On the basis of the experimental results and the structure of CheY, the protein can be divided into two subdomains (from beta-strand 1 to beta-strand 3 and from beta-strand 3 to the C terminus). Whereas the structure of the first subdomain in the transition state resembles that found for the CI-2 protein, the second subdomain is compact but unstructured. The packing of the first alpha-helix against beta-strands 1 and 2 seems to be the nucleus around which the rest of the protein folds.


CONCLUSIONS
Comparison of the transition state of barnase with those of CheY and CI-2 indicates that different proteins have different transition states, probably depending on the energetics and the position of the rate-limiting step in the folding pathway. CheY appears to fold through a nucleation/condensation mechanism as has been found for CI-2. The rate-determining step in some multimodular proteins could be the formation of a stable domain, with the less stable domains folding after the major rate-determining step.