The product of uncl gene in F1F0-ATP synthase operon plays a chaperone-like role to assist c-ring assembly

Bacterial operons for F1F0-ATP synthase typically include an uncl gene that encodes a function-unknown small hydrophobic protein. When we expressed a hybrid F1F0 (F-1 from thermophilic Bacillus PS3 and Na+-translocating F-o from Propionigenium modestum) in Escherchia coli cells, we found that uncl derived from A modestum was indispensable to produce active enzyme; without uncl, c-subunits in F1Fo existed as monomers but not as functional c(11)-ring. When uncl was expressed from another plasmid at the same time, active F1Fo with c(11)-ring was produced. A plasmid containing only uncl and c-subunit gene produced c(11)-ring, but a plasmid containing only C-subunit gene did not. Direct interaction of Uncl protein with c-subunits was suggested from copurification of His-tagged Uncl protein and c-subunits, both in the state of c(11)-ring and c-monomers. Na+ induced dissociation of His-tagged Uncl protein from c(11)-ring but not from c-monomers. These results show that Uncl is a chaperone-like protein that assists c(11)-ring assembly from c-monomers in the membrane.