Abstract
In F-ATPases, ATP hydrolysis is coupled to translocation of ions through membranes by rotation of a ring of c subunits in the membrane. The ring is attached to a central shaft that penetrates the catalytic domain, which has pseudo-3-fold symmetry. The ion translocation pathway lies between the external circumference of the ring and another hydrophobic protein. The H+ or Na+:ATP ratio depends upon the number of ring protomers, each of which has an essential carboxylate involved directly in ion translocation. This number and the ratio differ according to the source, and 10, 11, and 14 protomers have been found in various enzymes, with corresponding calculated H+ or Na+:ATP ratios of 3.3, 3.7, and 4.7. V-ATPases are related in structure and function to F-ATPases. Oligomers of subunit K from the Na+-motive V-ATPase of Enterococcus hirae also form membrane rings but, as reported here, with 7-fold symmetry. Each protomer has one essential carboxylate. Thus, hydrolysis of one ATP provides energy to extrude 2.3 sodium ions. Symmetry mismatch between the catalytic and membrane domains appears to be an intrinsic feature of both V- and F-ATPases.
Highlights
IntroductionIn the V-ATPase from Saccharomyces cerevisiae, all three kinds of c subunits are essential [6]
As demonstrated in F-ATPases, the central stalk in the V-ATPases couples energy released by ATP hydrolysis in the globular catalytic sector (V1) to ion translocation across the membrane domain (Vo)
In the F-ATPases the rotary element consists of an ensemble made from the central stalk and a ring of hydrophobic c subunits in the Fo membrane domain [34, 38]
Summary
In the V-ATPase from Saccharomyces cerevisiae, all three kinds of c subunits are essential [6]. They assemble into Vo, where by analogy with Fo they may form a ring that rotates as an ensemble together with the central stalk [1]. NtpK is the homologue of eukaryotic V-type subunit c [13]. No isoforms of this subunit have been encountered in E. hirae. Similar to the FATPases, V-ATPases have a globular catalytic domain, V1, with 3-fold symmetry connected by central and peripheral stalks to an intrinsic membrane domain, Vo. It is likely that, as demonstrated in F-ATPases, the central stalk in the V-ATPases couples energy released by ATP hydrolysis in the globular catalytic sector (V1) to ion translocation across the membrane domain (Vo). The c and cЈ subunits are 16-kDa proteolipids that are probably folded into four membrane-spanning ␣-helices
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