PONTICULIN IS THE MAJOR HIGH-AFFINITY LINK BETWEEN THE PLASMA-MEMBRANE AND THE CORTICAL ACTIN NETWORK IN DICTYOSTELIUM

Interactions between the plasma membrane and underlying actin-based co rtex have been implicated in membrane organization and stability, the control of cell shape, and various motile processes. To ascertain the function of high affinity actin-membrane associations, we have disrupt ed by homologous recombination the gene encoding ponticulin, the major high affinity actin-membrane link in Dictyostelium discoideum amoebae . Cells lacking detectable amounts of ponticulin message and protein a lso are deficient in high affinity actin-membrane binding by several c riteria. First, only 10-13% as much endogenous actin cosediments throu gh sucrose and crude plasma membranes from ponticulin-minus cells, as compared with membranes from the parental strain. Second, purified pla sma membranes exhibit little or no binding or nucleation of exogenous actin in vitro. Finally only 10-30% as much endogenous actin partition s with plasma membranes from ponticulin-minus cells after these cells are mechanically unroofed with polylysine-coated coverslips. The loss of the cell's major actin-binding membrane protein appears to be surpr isingly benign under laboratory conditions. Ponticulin-minus cells gro w normally in axenic culture and pinocytose FITC-dextran at the same r ate as do parental cells. The rate of phagocytosis of particles by pon ticulin-minus cells in growth media also is unaffected. By contrast, a fter initiation of development, cells lacking ponticulin aggregate fas ter than the parental cells. Subsequent morphogenesis proceeds asynchr onously, but viable spores can form. These results indicate that ponti culin is not required for cellular translocation, but apparently plays a role in cell patterning during development.