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

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.

Glycoprotein gp130 of dictyostelium discoideum influences macropinocytosis and adhesion

Glycoprotein gp130, found on the plasma membrane of Dictyostelium discoideum amoebae, was postulated previously to play a role in phagocytosis. The gene for gp130 was cloned and when translated, yielded a 768 amino acid preproprotein of 85.3 kDa. It had nearly 40% similarity to the 138 kDa family of glycoproteins implicated in sexual cell fusion during macrocyst formation in D. discoideum. The difference between the calculated size and observed M(r) of 130 kDa on protein gels likely was due to N-glycosylation that was confirmed by lectin blots. Consistent with its surface-exposure, an antibody raised against recombinant protein stained the plasma membrane of D. discoideum amoebae. Gp130 and its transcripts were high during axenic growth of cells, but relatively low during growth on bacteria. The gene for gp130 was disrupted and cell lines lacking the glycoprotein were efficient phagocytes, indicating that gp130 was dispensable for phagocytosis. Gp130-null cells were similar in size to parent DH1 cells, had enhanced macropinocytosis and grew faster to higher densities. They also exhibited weaker cell-substrate adhesion but displayed greater cell-cell cohesion. Collectively, the data indicated that gp130 influenced macropinocytosis and played a role in adhesion during vegetative growth.

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.

A site-specific DNA endonuclease specified by one of two ORFs encoded by a group I intron in Dictyostelium discoideum mitochondrial DNA

The second intron (DdOX1/2.2) of Dictyostelium discoideum cytochrome oxidase subunit 1/2 fused gene has two free-standing ORF genes (Dd ai2a and Dd ai2b) in a loop, which have similar amino acid sequences and are homologous to aI4 DNA endonuclease (I-SceII) of Saccharomyces cerevisiae. To elucidate the functions of these ORFs, we cloned the ORFs into an expression vector and introduced the composite vectors into E. coli. The expression of Dd ai2a in E. coli caused growth inhibition and degradation of the E. coli genomic DNA. To determine whether Dd ai2a protein is a homing type DNA endonuclease, the ability to cleave the homing site of its intron in vivo was examined. Dd ai2a cleaved only one strand of intronless DNA sequence at the site which coincides with the I-SceII cleavage recognition site. We suppose that Dd ai2a functions actually as a homing type DNA endonuclease in D. discoideum mitochondria by virtue of other factors. To obtain further information about the relationship between the existence of introns and the mating system, we carried out in vitro self-splicing assay and polymerase chain reaction analysis using 13 strains of the cellular slime mold.

Biochemical characterization of the phagocytic giant cell receptor for Dictyostelium discoideum amoebae: identification by cell blotting

A critical aspect of sexual development in Dictyostelium is the selection of "self" as a targeted food source by the phagocytic zygotic giant cell (ZGC). It has previously been demonstrated that phagocytic giant cells preferentially take up amoebae of the same species suggesting that the process is mediated by a specific receptor. By using tunicamycin, which inhibits N-linked glycosylation and mevastatin and mevinolin, which both inhibit HMG-CoA-reductase, we have been able to further characterize the glycoprotein(s) involved in the recognition process. We have utilized a "cell blotting" technique which lends itself well to identifying glycoproteins which are present on the ZGC at the phagocytic stage and interactive with the target amoebae. The cell blotting data, combined with pharmacological evidence, identifies these glycoproteins as the receptors for cannibalistic phagocytosis by the ZGC of D. discoideum.

A group-I intron in the mitochondrial large-subunit ribosomal RNA-encoding gene of Dictyostelium discoideum: same site localization in alga and in vitro self-splicing

A 547-bp group-I intron belonging to subgroup IA1 was found near the 3' end of the large subunit ribosomal RNA-encoding gene (LSUrRNA) in the mitochondrial (mt) DNA of the cellular slime mold Dictyostelium discoideum. This intron was inserted in a highly conserved stretch within the sequence that encodes the peptidyl transferase center domain V in the corresponding region of the Escherichia coli LSUrRNA. Interestingly, the insertion site of the intron is the same as that of the So.LSU.2 intron of the green alga, Scenedesmus obliquus, mt DNA and the Pw.LSU.2 intron of the colorless alga, Prototheca wickerhamii, mt DNA. The intron could self-splice in vitro at a concentration higher than 20 mM MgCl2. Polymerase chain reaction analysis showed the possible existence of an intron similar to that of D. discoideum LSUrRNA in another cellular slime mold, Polysphondylium pallidum (CK-8), but not in D. mucoroides (Dm7 and Dm11).

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

A glycoprotein, gp138, is implicated in the sexual cell fusion of Dictyostelium discoideum. We previously cloned and sequenced the two genes encoding the gp138 protein, GP138A and GP138B (Fang et al. (1993) Dev. Biol. 156, 201–208). Here, we have constructed a vector producing antisense RNA for the gp138 genes and have transformed the vector into Dictyostelium cells. The transformed cells showed a reduction in the amounts of gp138 mRNA and protein and their sexual cell fusion activity was considerably repressed.