Role of the Vtc proteins in V-ATPase stability and membrane trafficking

Abstract

Vtc proteins have genetic and physical relations with the vacuolar H+-ATPase (V-ATPase), influence vacuolar H+ uptake and, like the V-ATPase V0 sectors, are important factors in vacuolar membrane fusion. Vacuoles from vtc1Δ and vtc4Δ mutants had slightly reduced H+-uptake activity. These defects could be separated from Vtc function in vacuole fusion, demonstrating that Vtc proteins have a direct role in membrane fusion. We analyzed their involvement in other membrane trafficking steps and in V-ATPase dynamics. Deletion of VTC genes did not impede endocytic trafficking to the vacuole. However, ER to Golgi trafficking and further transport to the vacuole was delayed in Δvtc3 cells. In accordance with that, Δvtc3 cells showed a reduced growth rate. Vtc mutations did not interfere with regulated assembly and disassembly of the V-ATPase, but they affected the number of peripheral V1 subunits associated with the vacuoles. Δvtc3 vacuoles carried significantly more V1 subunits, whereas Δvtc1, Δvtc2 and Δvtc4 had significantly less. The proteolytic sensitivity of the V0 subunit Vph1p was different in Δvtc and wild-type cells in vivo, corroborating the physical interaction of Vtc proteins with the V-ATPase observed in vitro. We suggest that Vtc proteins affect the conformation of V0. They might thereby influence the stability of the V-ATPase holoenzyme and support the function of its V0 sector in vacuolar membrane fusion.