The importance of a mobile loop in regulating chaperonin/co-chaperonin interaction - Humans versus Escherichia coli

Chaperonins are universally conserved proteins that nonspecifically facilit ate the folding of a wide spectrum of proteins. While bacterial GroEL is fu nctionally promiscuous with various co-chaperonin partners, its human homol ogue, Hsp60 functions specifically with its co-chaperonin partner, Hsp10, a nd not with other cochaperonins, such as the bacterial GroES or bacteriopha ge T4-encoded Gp31. Co-chaperonin interaction with chaperonin is mediated b y the co-chaperonin mobile loop that folds into a beta -hairpin conformatio n upon binding to the chaperonin, A delicate balance of flexibility and con formational preferences of the mobile loop determines co-chaperonin affinit y for chaperonin, Here, me show that the ability of Hsp10, but not GroES, t o interact specifically with Hsp60 lies within the mobile loop sequence. Us ing mutational analysis, we show that three substitutions in the GroES mobi le loop are necessary and sufficient to acquire Hsp10-like specificity. Two of these substitutions are predicted to preorganize the beta -hairpin turn and one to increase the hydrophobicity of the GroEL-binding site. Together , they result in a GroES that binds chaperonins with higher affinity, It se ems likely that the single ring mitochondrial Hsp60 exhibits intrinsically lower affinity for the co-chaperonin that can be compensated for by a highe r affinity mobile loop.