Pinocytosis in Dictyostelium discoideum cells. A possible implication of cytoskeletal actin for pinocytotic activity.

Pinocytosis in Dictyostelium discoideum axenic strain (Ax-2) cells in the growth phase is progressively inhibited at higher Ca2+ concentrations, the activity being maximal at submicromolar Ca2+ concentrations. The cytoskeletal actin content is also markedly reduced in the presence of 10 mM EGTA. This was confirmed by electronmicroscopy using intact cells and Triton X-100-insoluble cell cortices. Interestingly, the pinocytotic activity seemed to be somewhat increased in response to cytochalasin B (CB). Aggregation-competent Ax-2 cells which are usually devoid of pinocytotic activity can resume their activity considerably following treatment with 10 mM EGTA. Under these conditions, cytoskeletal actin declines markedly, as also was the case for growing Ax-2 cells. Our findings indicate a correlation between the pinocytotic activity and presence of cytoskeletal actin: reduced amounts of actin in the cell cortex seem to favour pinocytosis. Conceivably, membrane-associated actin filaments may function as a powerful anchor, restricting the flexibility of the cell membrane and thereby inhibiting the pinosome formation. Other properties of pinocytosis like a developmental change as well as the effects of pH and temperature are also described and were compared with the properties of wild-type strain, NC-4.