Phosphorylation of the contact site A glycoprotein (gp80) of Dictyostelium discoideum

Developmentally regulated cohesion of Dictyostelium discoideum can be blocked by the Fab fragment of antiserum prepared against a glycoprotein of about 80,000 daltons, gp80, purified from the membranes of developing amoebae. Immunoprecipitation of gp80 with this serum, from ³²P-labeled cell extracts of aggregating D. discoideum amoebae showed it to be a phosphoprotein. Serine phosphate was found in the molecule. All multiple isoelectric forms of gp80 were phosphorylated. Synthesis of the phosphorylated form of gp80 was found to be limited to the period of aggregation and coincided with the period of incorporation of [³⁵S]methionine into gp80. Phosphorylation could be rapidly inhibited by cycloheximide suggesting that phosphorylation occurs only on newly made gp80. No unphosphorylated gp80 could be detected in cell extracts.