Distribution of gp138, a cell surface protein responsible for sexual cell fusion, among cellular slime moulds

Sexual cell fusion occurs between NC4 and HM1, heterothallic strains of Dictyostelium discoideum. Several cell surface proteins relevant to the process have been identified. One of them, gp138, exists in fusion-competent cells of both NC4 and HM1, and is considered to be more concerned with membrane fusion than gamete recognition. In this study, we raised monoclonal antibodies against gp138 and examined gp138 distribution among strains and species of cellular slime moulds to confirm its importance in sexual cell fusion. All heterothallic and bisexual D. discoideum strains examined were found to possess gp138, while asexual and homothallic strains lacked it. The anti-gp138 monoclonal antibody detected several distinct proteins in homothallic strains and one in an asexual strain. Some of the former proteins appeared together with the increase in binucleated cells. Cells of Dictyostelium mucoroides and Polysphondylium pallidum did not possess proteins reactive to the monoclonal antibody. These results indicate that gp138 is common among, but restricted to, cross-matable strains of D. discoideum. Our results also support previously published molecular phylogenetic studies which suggest that homothallic and asexual strains of D. discoideum are remote from other strains of D. discoideum but are less distantly related to them than other species are.