TARGETED DISRUPTION OF GENES FOR GP138, A CELL-FUSION-RELATED PROTEININ DICTYOSTELIUM-DISCOIDEUM, REVEALED THE EXISTENCE OF A 3RD GENE

The sexual cycle of the cellular slime mold, Dictyostelium discoideum, offers a suitable experimental system to analyze sexual cell interact ions. We have been analyzing molecular mechanisms involved in sexual c ell fusion using complementary heterothallic strains in D. discoideum and have identified several cell surface proteins involved in the proc ess. One of them, gp138 is present in strains of both mating types and considered to be responsible for membrane fusion itself. Two genes wi th high mutual homology, GP138A and GP138B, have been identified so fa r as encoding this protein. Expression of antisense RNA for GP138B has been shown to suppress sexual cell fusion, confirming the critical im portance of these genes in sexual cell fusion. However, neither the fu nctional relationship of the two gp138 genes nor the possibility of th e existence of more genes that encode gp138 has been determined yet. I n the present study, GP138A and GP138B were disrupted by homologous re combination in an effort to clarify these points. Analysis of the doub le knock-out mutants suggested the presence of a third gene for gp138.