Structural organization of the yeast vacuolar H+-ATPase The yeast vacuolar proton-translocating ATPase

The yeast vacuolar proton-translocating ATPase (VATPase) is structurally very similar to the V-ATPases of other fungi, plants and animals (for reviews, see Stevens and Forgac, 1997; Forgac, 1999; Nelson and Harvey, 1999). In all these systems, the V-ATPase is composed of a complex of peripheral membrane proteins containing the ATP binding sites attached to a complex of integral membrane proteins that makes up the proton pore. A structural model of the yeast V-ATPase is shown in Fig. 1. The yeast 69 kDa A subunit, which appears to be present in three copies per complex, contains the catalytic sites for ATP hydrolysis. The 60 kDa B subunit is also present in multiple copies and may contain regulatory sites for nucleotide binding. Three structurally related proteolipid subunits (c, c′ and c′′) form all or part of the proton pore. These five subunits appear to constitute the core of the catalytic machinery of V-ATPases (for reviews, see Stevens and Forgac, 1997; Forgac, 1999). The other eight subunits are also essential for activity of the enzyme, since genetic deletion of any of these subunits in yeast results in a set of growth defects characteristic of loss of V-ATPase activity (the Vma− phenotype), and each of the subunits is part of the final assembled V-ATPase structure (Stevens and Forgac, 1997). Their individual functions, however, are not clear. Some of these subunits must be involved in the conformational coupling of ATP hydrolysis and proton transport. Others may be essential for regulation of V-ATPases. Structural and sequence similarities between the V-ATPases and F-ATPases have been examined extensively (for reviews, see Margolles-Clark et al., 1999; Forgac, 1999). In both enzyme classes, nucleotide-dependent conformational changes are communicated between the peripheral complex and the proton pore via a number of smaller subunits that form a stalk. There are readily identifiable sequence similarities between the A, B and proteolipid subunits of the V-ATPases, and the β, α and c subunits of the F-ATPases, suggesting a fundamental similarity between the catalytic mechanisms of the two enzymes. Structural and functional parallels between the other subunits of the two enzyme classes have proved to be more elusive (Margolles-Clark et al., 1999). For the purposes of this review, it is also important to emphasize certain differences between the Vand F-ATPases. 81 The Journal of Experimental Biology 203, 81–87 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2332