Assembly and activation of the AAA+ motor of the multi-subunit cobaltochelatase
Cobalamin is a complex yet essential cobalt-containing cofactor synthesized exclusively by prokaryotes and archaea. Aerobic cobalamin biosynthesis requires the CobNST multi-subunit class I cobaltochelatase for cobalt incorporation. Cobalt chelation is dependent on the energy transfer from the CobST AAA+/VWA motor unit to the substrate-binding catalytic unit (CobN). However, the mechanism for assembly and function of CobNST cobaltochelatase remains poorly understood. Here, we used cryo-EM to solve the structures of the AAA+ motor CobS, in the absence and presence of ATP and its VWA-partner protein CobT. Our data reveal that the AAA+ CobS assembles into various conformations, including hexameric planar rings and different spiral left-handed assemblies: hexamers, double rings and filaments. In the absence of nucleotide, the CobT N-terminal domain interacts with CobS left-handed oligomers, remaining in a half-assembled, nonfunctional motor unit. In the CobST complex, ATP induces conformational rearrangement of CobS from left-handed helical oligomers to a right-handed closed hexamer. The simultaneous repositioning of the CobT VWA domain to the AAA+ central pore shifts the motor unit to a functional state, ready-to-drive CobN-catalyzed chelation reaction. These findings define the mechanism for assembly and activation of the class I cobaltochelatase CobST motor.