Arrangement of subunits in the proteolipid ring of the V-ATPase

The vacuolar ATPases (V-ATPases) are multisubunit complexes containing two domains. The V-1 domain (subunits A-H) is peripheral and carries out ATP hydrolysis. The V-0 domain (subunits a, c, c', c '', d, and e) is membrane-integral and carries out proton transport. In yeast, there are three proteolipid subunits as follows: subunit c (Vma3p), subunit c' (Vma11p), and subunit c '' (Vma16p). The proteolipid subunits form a six-membered ring containing single copies of subunits c' and c '' and four copies of subunit c. To determine the possible arrangements of proteolipid subunits in V-0 that give rise to a functional V-ATPase complex, a series of gene fusions was constructed to constrain the arrangement of pairs of subunits in the ring. Fusions containing c '' employed a truncated version of this protein lacking the first putative transmembrane helix (which we have shown previously to be functional), to ensure that the N and C termini of all subunits were located on the luminal side of the membrane. Fusion constructs were expressed in strains disrupted in c', c '', or both but containing a wild copy of c to ensure the presence of the required number of copies of subunit c. The c-c ''(Delta TM1), c ''(Delta TM1)-c', and c'-c constructs all complemented the vma(-) phenotype and gave rise to complexes possessing greater than 25% of wild-type levels of activity. By contrast, neither the c-c', the c'-c ''(Delta TM1), nor the c ''(Delta TM1)-c constructs complemented the vma(-) phenotype. These results suggest that functionally assembled V-ATPase complexes contain the proteolipid subunits arranged in a unique order in the ring.