The product of uncl gene in [F.sub.1][F.sub.o]-ATP synthase operon plays a chaperone-like role to assist c-ring assembly

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