Incomplete contact site A glycoprotein in HL220, a modB mutant of Dictyostelium discoideum

Abstract

HL220, a modB mutant that lacks a modification of certain membrane proteins of Dictyostelium discoideum, has been shown to aggregate and to form EDTA-stable intercellular contacts typical of aggregating wild-type cells. A developmentally regulated glycoprotein of 80 X 10(3) apparent molecular weight has been identified as a target site of adhesion-blocking Fab and thought to be involved in EDTA-stable cell contact formation (Muller & Gerisch, 1978). In the HL220 mutant this glycoprotein is no longer recognized by a modB-specific antibody. Therefore, it has been suggested that the 80 X 10(3) Mr glycoprotein, or a modification on it, is not required for the EDTA-stable cell contact of aggregating cells. We show that HL220 synthesizes an equivalent of the 80 X 10(3) Mr glycoprotein with an apparent molecular weight of 68 X 10(3). The mutant product reacted with certain monoclonal antibodies highly specific for the 80 X 10(3) Mr glycoprotein in the wild type, and was developmentally regulated like the 80 X 10(3) Mr glycoprotein. These results indicate that the 68 X 10(3) Mr protein of the mutant lacks a modification, most likely an oligosaccharide residue, the absence of which causes the substantial shift of the apparent molecular weight from 80 X 10(3) to 68 X 10(3). Monoclonal antibodies that did not react with proteins of the mutant could be classified according to their reactions with different sub-sets of wild-type proteins. These results indicate that the proteins that reacted with either one or the other antibody were not modified by a uniform structure. The modification rather varies from one sub-set of cross-reacting proteins to another, suggesting differences between the glycosyl residues of the partially cross-reacting proteins. HL220 cells showed strongly reduced EDTA-stable contact formation under our conditions. EDTA-sensitive intercellular adhesion was undetectable in the mutant, whereas adhesion of the cells to the substratum appeared to be strengthened. The rear ends of the cells, in particular, were tightly attached to glass or Teflon surfaces. The mutant cells were capable of responding chemotactically. Propagated excitation waves like those known to be based on periodic cyclic AMP production and relay were clearly seen. Extracellular phosphodiesterase induction by cyclic AMP and phosphodiesterase inhibitor production were normal. These results indicate that the generation of chemotactic signals and the cellular responses to cyclic AMP are not severely affected by the mutation.