Drastic Changes in Accumulation and Synthesis of Plasma‐Membrane Proteins during Aggregation of Dictyostelium discoideum

Protein compositions of plasma membranes of Dictyostelium discoideum and their developmental changes during aggregation were analyzed by two‐dimensional polyacrylamide gel electrophoresis. More than 150 polypeptide species could be detected with the growth‐phase cells by staining. Most of them (about 120 spots) were conserved during aggregation, although their relative intensities of staining changes. On the initiation of cell differentiation, new polypeptides were accumulated on the plasma membrane and at least 100 new polypeptides were detected at the late‐aggregation stage. This accumulation of new components was completely inhibited when the growth‐phase cells were incubated in a liquid medium containing 5 mMEDTA. Studies on protein synthesis suggested that the accumulation and appearance of the new protein components was due to de novo synthesis. Pulse‐label experiments with [ 35 S]methionine showed that out of 200 proteins detected during aggregation, about 90 proteins showed various stage‐specific patterns of synthesis and the rest were synthesized consistently. Actin and discoidin present in the plasma membrane were identified by reference to purified actin and discoidin. Actin synthesized de novo accumulated progressively on the plasma membrane during aggregation. Discoidin also accumulated on the plasma membrane, but a large amount was also recovered in a cytoplasmic soluble fraction. Discoidin I was separated into two spots with different pI values on a two‐dimensional gel.