Fig. 6. Visualization of proton pumps in contractile vacuole membranes of VatMpr and DH1 cells. Cells grown on a suspension of K. aerogenes for 2 days were washed free of bacteria and plated on glass. The cells were ruptured and freeze-dried, and platinum replicas were prepared as described (Heuser et al., 1993). The proton pumps appear as 15 nm diameter `studs' on the cisternal and tubular elements of the contractile vacuole system; the density of the studs is reduced in VatMpr cells.