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

Group II chaperonins in the eukaryotic and archaeal cytosol assist in prote in folding independently of the GroES-like cofactors of eubacterial group I chaperonins, Recently, the eukaryotic chaperonin was shown to cooperate wi th the hetero-oligomeric protein complex GimC (prefoldin) in folding actin and tubulins. Here we report the characterization of the first archaeal hom ologue of GimC, from Methanobacterium thermoautotrophicum MtGimC is a hexam er of 87 kDa, consisting of two alpha and four beta subunits of high alpha- helical content that are predicted to contain extended coiled coils and rep resent two evolutionarily conserved classes of Gim subunits, Reconstitution experiments with MtGimC suggest that two subunits of the alpha class (arch aeal Gim alpha and eukaryotic Gim2 and 5) form a dimer onto which four subu nits of the beta class (archaeal Gim beta and eukaryotic Gim1, 3, 4 and 6) assemble. MtGim alpha and beta can form hetero-complexes with yeast Gim sub units and MtGim beta partially complements yeast strains lacking Gim1 and 4 , MtGimC is a molecular chaperone capable of stabilizing a range of non-nat ive proteins and releasing them for subsequent chaperonin-assisted folding. In light of the absence of Hsp70 chaperones in many archaea, GimC may fulf il an ATP-independent, Hsp70-like function in archaeal cie novo protein fol ding.