The C-terminal tails of GroEL and its mitochondrial and chloroplastic homologs adopt polyproline II helices.

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The C-terminal tails of GroEL and its mitochondrial and chloroplastic homologs adopt polyproline II helices.

Authors

Segura Rodriguez, C.; Lopez-Sanchez, R.; Laurents, D. V.

Abstract

The chaperonin GroEL and its mitochondrial and chloroplastic homologs mHsp60 and Cpn60 are large barrel-like oligomeric proteins. Chaperonins facilitate folding by isolating nascent chains in their hollow interior and undergoing conformational transitions driven by ATP hydrolysis. Due to their vital importance, the structure of GroEL and its homologs have been extensively studied by X-ray crystallography and CryoEM, revealing one or two rings each of which contains seven subunits. Each subunit has three folded domains and a twenty-four residue C-terminal extension. Whereas this C-terminal tail has been reported to bind and stimulate the client protein folding, it appears to be invisible or poorly resolved, which suggests that it is disordered. The objective here is to characterize conformational preferences in the C-terminal tails of GroEL, mHsp60 and representative Cpn60s using circular dichroism and nuclear magnetic resonance spectroscopy methods. The tails of GroEL and mHsp60 consist of two segments. The first is rich in residues common in intrinsically disordered proteins, i.e. charged, proline, small like Ala, and polar. By contrast, the second segment is consists exclusively (GroEL) or almost entirely (mHsp60) of Gly and Met residues. The spectroscopic results evince that these C-terminal extensions, especially their second segments, are not wholly disordered but adopt high populations of PPII conformations. Regarding the plant cytoplastic chaperonins, whereas the C-terminal segments of Cpn60s are Gly-poor, they are rich in proline and also adopt PPII helix conformations. These results provide insight into the biological activities of the C-terminal tails.

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