MtGimC, a novel archaeal chaperone related to the eukaryotic chaperonin cofactor GimC/prefoldin

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Abstract
Group II chaperonins in the eukaryotic and archaeal cytosol assist in protein folding independently of the GroES‐like cofactors of eubacterial group I chaperonins. Recently, the eukaryotic chaperonin was shown to cooperate with the hetero‐oligomeric protein complex GimC (prefoldin) in folding actin and tubulins. Here we report the characterization of the first archaeal homologue of GimC, from Methanobacterium thermoautotrophicum . MtGimC is a hexamer of 87 kDa, consisting of two α and four β subunits of high α‐helical content that are predicted to contain extended coiled coils and represent two evolutionarily conserved classes of Gim subunits. Reconstitution experiments with MtGimC suggest that two subunits of the α class (archaeal Gimα and eukaryotic Gim2 and 5) form a dimer onto which four subunits of the β class (archaeal Gimβ and eukaryotic Gim1, 3, 4 and 6) assemble. MtGimα and β can form hetero‐complexes with yeast Gim subunits and MtGimβ partially complements yeast strains lacking Gim1 and 4. MtGimC is a molecular chaperone capable of stabilizing a range of non‐native proteins and releasing them for subsequent chaperonin‐assisted folding. In light of the absence of Hsp70 chaperones in many archaea, GimC may fulfil an ATP‐independent, Hsp70‐like function in archaeal de novo protein folding.