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The product of uncI gene in F1Fo-ATP synthase operon plays a chaperone-like role to assist c-ring assembly

Bacterial operons for F1Fo-ATP synthase typically include an uncI gene that encodes a function-unknown small hydrophobic protein. When we expressed a hybrid F1Fo (F1 from thermophilic Bacillus PS3 and Na+-translocating Fo from Propionigenium modestum) in Escherchia coli cells, we found that uncI derived from P. modestum was indispensable to produce active enzyme; without uncI, c-subunits in F1Fo existed as monomers but not as functional c11-ring. When uncI was expressed from another plasmid at the same time, active F1Fo with c11-ring was produced. A plasmid containing only uncI and c-subunit gene produced c11-ring, but a plasmid containing only c-subunit gene did not. Direct interaction of UncI protein with c-subunits was suggested from copurification of His-tagged UncI protein and c-subunits, both in the state of c11-ring and c-monomers. Na+ induced dissociation of His-tagged UncI protein from c11-ring but not from c-monomers. These results show that UncI is a chaperone-like protein that assists c11-ring assembly from c-monomers in the membrane.