Disruption of the αB loop interface abrogates Vps1 function in vivo. (A) Plasmids containing C. thermophilum Vps1 harboring mutations in either the αB loop (Loop Mut 1 and 2) or in the αB helix (Control Mut 1 and 2) were expressed in Δvps1 cells. Growth was assessed on YPD plates at 30 or 37°C after 2 d. (B) Vacuolar morphology was assessed in Δvps1 cells expressing the loop and control mutants as in A. Vacuoles were visualized using FM 4–64. (C and D) As in A and B but using corresponding mutants in S. cerevisiae Vps1. Bars, 5 μm.