Antisense RNA inactivation of gp138 gene expression results in repression of sexual cell fusion in Dictyostelium discoideum

Glycopeptidome of a heavily N-glycosylated cell surface glycoprotein of Dictyostelium implicated in cell adhesion

Genetic analysis has implicated the cell surface glycoprotein gp130 in cell interactions of the social amoeba Dictyostelium, and information about the utilization of the 18 N-glycosylation sequons present in gp130 is needed to identify critical molecular determinants of its activity. Various glycomics strategies, including mass spectrometry of native and derivatized glycans, monosaccharide analysis, exoglycosidase digestion, and antibody binding, were applied to characterize a nonanchored version secreted from Dictyostelium. s-gp130 is modified by a predominant Man(8)GlcNAc(4) species containing bisecting and intersecting GlcNAc residues and additional high-mannose N-glycans substituted with sulfate, methyl-phosphate, and/or core alpha 3-fucose. Site mapping confirmed the occupancy of 15 sequons, some variably, and glycopeptide analysis confirmed 14 sites and revealed extensive heterogeneity at most sites. Glycopeptide glycoforms ranged from Man(6) to Man(9), GlcNAc(0-2) (peripheral), Fuc(0-2) (including core alpha 3 and peripheral), (SO(4))(0-1), and (MePO(4))(0-1), which represented elements of virtually the entire known cellular N-glycome as inferred from prior metabolic labeling and mass spectrometry studies. gp130, and a family of 14 related predicted glycoproteins whose polypeptide sequences are rapidly diverging in the Dictyostelium lineage, may contribute a functionally important shroud of high-mannose N-glycans at the interface of the amoebae with each other, their predators and prey, and the soil environment.

Genes involved in Dictyostelium discoideum sexual reproduction

Macrocyst formation in the cellular slime moulds is a sexual process induced under dark and humid conditions. Normal development life cycle in these organisms involves proliferation by cell division and, upon starvation, formation of multicellular aggregates and fruiting bodies, consisting of spores and stalk cells. Macrocyst formation, cell division by binary fission and spore formation are thus three alternative modes of reproduction, for which it is of interest to understand how a choice is made. The genetic basis of asexual development and fruiting body formation is well known, by contrast information on the genetic control of sexual reproduction during macrocyst formation is scarce. In Dictyostelium discoideum, the most widely used species, several cell-surface proteins relevant to sexual cell fusion have been identified using cell fusion-blocking antibodies, but isolation of the relevant genes has been unsuccessful. Analysis of sexually deficient mutants, some of which are normal for asexual development, has shown that sexual reproduction is regulated by both specific genes and genes that are also involved in asexual development. Reverse genetic analysis of 24 genes highly enriched in a gamete-specific subtraction library has revealed four genes involved in the regulation of sexual cell interactions. One of them was found to be a novel regulator of the cAMP signalling pathway specific to sexual development. Studies on the molecular genetic control of the sexual cycle will be reviewed and their contribution to our understanding of the organization and function of the D. discoideum genome as a whole discussed.

Total tetra knockout of GP138 multigene family implicated in cell interactions in Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum reproduces sexually under submerged and dark conditions. A cell surface glycoprotein gp138 has been identified as a target molecule for cell fusion-blocking antibodies, and is considered to be indispensable for the sexual cell fusion in this organism. Currently, four isoforms of gp138, DdFRP1alpha, DdFRP1beta, DdFRP2, and DdFRP3, are known. Genes encoding the latter three isoforms, GP138C, GP138A, and GP138B, have been isolated, comprising a GP138 multigene family. Here we isolated the fourth GP138 gene, GP138D, encoding DdFRP1alpha. These GP138 genes were found to cluster in a tandem array on chromosome 5, being bordered by two GP138-like sequences highly homologous to them but truncated. To clarify functional relationships among the GP138 family members, the entire GP138 region was deleted by a single knockout. Northern hybridization and western immuno-blotting analyses confirmed complete losses of GP138 mRNA and DdFRPs in the knockout strains, indicating that there are no more GP138 genes. Unexpectedly, however, the GP138-null mutants were fully potent for both sexual cell fusion and subsequent development. In addition, the original fusion-blocking antibodies detected a cell surface protein of close electrophoretic mobility to gp138 in the knockouts, suggesting the possibility that the actual target molecule of the fusion-blocking antibodies was not DdFRPs but this unidentified component. Since GP138-null mutants exhibited no obvious defects either in growth or asexual development, the real function of the GP138 family is unknown. Nevertheless, the expression levels of other developmental genes such as acaA, csaA, cotA-C, and spiA appeared to be altered in the GP138-null mutants. Therefore, it seems to have a non-critical but some role(s) during asexual development.

A new member of the GP138 multigene family implicated in cell interactions in Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum reproduces sexually under submerged and dark conditions. Its mating system is polymorphic and particularly interesting with respect to mechanisms of cell recognition. The cell-surface glycoprotein gp138 has been implicated in sexual cell interactions, as it was identified as a target molecule for the antibodies that block sexual cell fusion in D. discoideum. Two mutually homologous genes, GP138A and GP138B, have been cloned, but gene disruption experiments to clarify their functional relationships suggested that there is at least one more gene for gp138. Further protein analysis including peptide mapping also revealed that gp138 exists as three isoforms, DdFRP1, DdFRP2, and DdFRP3. GP138A encodes DdFRP2 and GP138B, DdFRP3, and the presence of a third gp138 gene encoding DdFRP1 was suggested. Here, we isolated and characterized a third GP138 gene, GP138C. Although the deduced amino acid sequences of GP138C matched completely with those of peptide fragments of DdFRP1 in the N-terminal half, the rest did not give complete matches. Overexpression of GP138C caused an increase in the intensity of DdFRP1, but disruption of this gene did not diminish DdFRP1. Our results indicate that GP138C encodes a protein very similar to but distinct from DdFRP1. The GP138 multigene family is thus composed of more members than previously expected, and their functional relationships are of special interest.