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

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 folding of the client protein, it appears to be invisible or blurry, which suggests disorder. The objective of this study is to characterize conformational preferences in the C-terminal tails of GroEL, mHsp60 and representative Cpn60s using circular dichroism and nuclear magnetic resonance spectroscopies. The tails of GroEL and mHsp60 consist of two segments. The first is rich in residues typical of intrinsically disordered proteins and the second segment consists exclusively (GroEL) or almost entirely (mHsp60) of Gly and Met residues. The spectroscopic results evince that these C-terminal extensions are not wholly disordered but adopt polyproline II helices whose populations are higher in the second Gly/Met-rich segment. Whereas the C-terminal segments of chloroplastic chaperonins are Gly-poor, they are rich in proline and also adopt polyproline II helix conformations. These results provide insight into the function of chaperonin C-terminal tails.