Structural basis for a homodimeric ATPase subunit of an ECF transporter

The transition metal cobalt,an essential cofactor for many enzymes in prokaryotes,is taken up by several specifi c transport systems.The CbiMNQO protein complex be-longs to type-1 energy-coupling factor(ECF)transporters and is a widespread group of microbial cobalt transport-ers.CbiO is the ATPase subunit(A-component)of the cobalt transporting system in the gram-negative thermo-philic bacterium Thermoanaerobacter tengcongensis.Here we report the crystal structure of a nucleotide-free CbiO at a resolution of 2.3Å.CbiO contains an N-terminal canonical nucleotide-binding domain(NBD)and C-termi-nal helical domain.Structural and biochemical data show that CbiO forms a homodimer mediated by the NBD and the C-terminal domain.Interactions mainly via conserved hydrophobic amino acids between the two C-terminal do-mains result in formation of a four-helix bundle.Structural comparison with other ECF transporters suggests that non-conserved residues outside the T-component bind-ing groove in the A component likely act as a specifi city determinant for T components.Together,our data provide information on understanding of the structural organiza-tion and interaction of the CbiMNQO system.