Photoaffinity labeling of the cell surface adenosine 3':5'-monophosphate receptor of Dictyostelium discoideum and its modification in down-regulated cells.

The cAMP cell surface receptor of Dictyostelium discoideum amoebae was identified by the use of the photoaffinity analogue 8-N3-[32P]cAMP. Labeling by intact cells of one component, identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography, could be specifically inhibited by the presence of nonradioactive cAMP. The component, P45 (apparent molecular weight of 45,000), was not identified on vegetative cells but was labeled with increasing intensity as cells differentiated and increased their levels of surface cAMP binding sites. Developmental mutants, starved under conditions where they do not express significant levels of cAMP binding sites, did not incorporate radioactivity into this protein. These mutants did label P45 when starved under differentiation-inducing conditions such that their levels of surface cAMP binding sites increased. P45 co-purified with the plasma membrane fraction isolated from cells to which 8-N3-[32p]cAMP had been covalently bound. Down-regulated amoebae, which displayed approximately 25% of the binding activity of untreated cells, did not label P45. These cells did, however, label a new component with an apparent molecular weight of 47,000 (P47).l The appearance of this component represented the only discernible difference in labeling profile under these conditions. As in the case of P45, radioactive incorporation into P47 did not occur if the photoactivation of 8-N3-[32P]cAMP was performed in the presence of nonradioactive cAMP.