ISOFORMS OF GP138, A CELL-FUSION RELATED PROTEIN IN DICTYOSTELIUM-DISCOIDEUM

Sexual development of Dictyostelium discoideum is a unique and useful system for the study of sexual phenomena. We have been studying molecu lar mechanisms of sexual cell fusion in D. discoideum and have identif ied several relevant cell-surface proteins. One of the proteins, gp138 , was identified as a target molecule for fusion-blocking antibodies, and two genes for gp138, GP138A and GP138B, were cloned, The participa tion of gp138 in the sexual cell fusion was confirmed by antisense RNA mutagenesis, but it is unclear which of the genes encodes gp138, More over, the presence of a third gene for gp138 was indicated by gene dis ruption. In the present study, we generated strains of D. discoideum o verexpressing either GP138A or GP138B to investigate the products of t hese genes. The transformants overexpressing GP138A and GP138B overpro duced glycoproteins with molecular masses of 135 and 130 kDa, respecti vely. Although their molecular masses were different from that of gp13 8, the results of peptide mapping and amino acid sequencing showed tha t they are related proteins, suggesting that the proteins encoded by G P138A and GP138B are isoforms of gp138 protein.