Molecular cloning and characterization of two genes encoding gp138, a cell surface glycoprotein involved in the sexual cell fusion of Dictyostelium discoideum.

The sexual development of cellular slime molds is initiated by acquisition of sexual fusion competence of myxamoeboid cells. During the acquisition of fusion competence in NC4 and HM1 strain cells of Dictyostelium discoideum, opposite in mating types, a glycoprotein gp138, relevant to sexual cell fusion, is expressed on the cell surfaces of both the strains. To investigate the mechanisms controlling cell fusion, gp138 was purified and its N-terminal amino acid sequence was determined. An oligonucleotide with sequence predicted from the amino acid sequence was synthesized to isolate the gene encoding gp138. Two genes, referred to as GP138A and GP138B, were then cloned. They contain the region which encodes the N-terminal amino acid sequence of the gp138 protein and their structure is very similar on the whole. The C-terminal portion of the predictive polypeptides produced by the genes is highly hydrophobic and proline rich. It has homology with a portion of gp80, a glycoprotein for cell-cell adhesion, and PsA, a glycoprotein specific for prespore cells, reported previously in D. discoideum. The expression of GP138A is greater than that of GP138B. The mRNA of GP138A is expressed at the time of acquisition of fusion competence of cells during cultivation but the mRNA of GP138B is not.