A 130-kDa Plasma Membrane Glycoprotein Involved inDictyosteliumPhagocytosis

Abstract

Phagocytosis involves interactions between cell-surface receptors and the actin-based cytoskeleton. Plasma membrane glycoproteins cosedimenting with detergent-insoluble cytoskeletons were postulated to be phagocytosis receptor candidates of the unicellular slime moldDictyostelium discoideum.A 130-kDa glycoprotein (gp130) was associated with cytoskeletons of bacterially but not axenically grown cells, suggesting a cytoskeletal interaction that depended on nutrient conditions. Labeling of gp130 with a membrane-impermeant biotinylating reagent showed it was surface-exposed and provided a tag that was used to monitor gp130. Biotin-labeled gp130 was an integral protein and found to be a single species by two-dimensional gel electrophoresis. An antibody was raised against a synthetic octapeptide corresponding to internal amino acid sequence of biotin-labeled gp130 enriched through avidin affinity chromatography. The affinity-purified antibody was monospecific, reacting with both axenic and bacterial forms of gp130 on immunoblots. There was less gp130 in plasma membranes of bacterially grown cells than in plasma membranes of axenically grown cells, which was consistent with the idea that as a receptor, gp130 would be internalized during phagocytosis and fewer molecules would be on the cell surface of actively feeding cells. This suggestion was supported by the observation that there was a reduced amount of surface (biotin)-labeled gp130 on bacterially grown cells relative to axenically grown cells. gp130 also was implicated in phagocytosis through immunoblotting analyses that revealed smaller versions of gp130 in plasma membranes of phagocytosis mutant HV29 (Vogelet al. J. Cell Biol.86: 456, 1980). Taken together, these biochemical and immunological data support the idea that gp130 plays a role in the phagocytosis process.