Possible existence of a light-inducible protein that inhibits sexual cell fusion in Dictyostelium discoideum.

Sexual cell fusion is an initial step of macrocyst formation in Dictyostelium discoideum and requires environmental conditions such as darkness, plenty of water and the presence of calcium ions. We have been analyzing the mechanism of sexual cell fusion between HM1 and NC4, heterothallic strains in D. discoideum. Cells of these strains have been shown to be fusion competent when cultured in a liquid medium in darkness, but not so when cultured on agar plates or in a liquid medium in the light. Two cell-surface proteins, gp70 and gp138, have been identified as target molecules for fusion-blocking antibodies and therefore as relevant to sexual cell fusion. In the present study, gp70 was shown to be present in HM1 cells cultured in the light, and fusion incompetent. Intact HM1 cells cultured in the light were unable to absorb the fusion-blocking activity of antibodies against membrane components of fusion-competent HM1 cells, whose activity had been shown to be absorbed by gp70, but they did so after separation of proteins in the SDS-PAGE. In addition, fusion-competent HM1 cells were found to lose their fusion competence by subsequent cultivation in the light. This loss of competence was cycloheximide sensitive, indicating that de novo synthesis of proteins was necessary for this inhibition. From these results, we presume that light induces a protein that hinders the interaction of gp70 in HM1 cells with its receptor on the NC4 cell surface and thereby inhibits the sexual process between these strains.