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

Bacterial operons for F₁F_o-ATP synthase typically include an uncI gene that encodes a function-unknown small hydrophobic protein. When we expressed a hybrid F₁F_o (F₁ from thermophilic Bacillus PS3 and Na⁺-translocating F_o 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 F₁F_o existed as monomers but not as functional c₁₁-ring. When uncI was expressed from another plasmid at the same time, active F₁F_o 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.