Direct implication of surface mannosyl residues in cell adhesion of Dictyostelium discoideum

In the cell adhesion of aggregation-competent Dictyostelium cells, the requirement for the carbohydrate moiety of the glycoprotein appeared to be indirect in that it acts to protect the protein moiety from proteolytic degradation; however, the effect was limited to the tunicamycin (TM)-sensitive carbohydrate moiety (Hirano, T., et al. (1983) J. Biochem. 93, 1249-1257). In the present study, we showed that the EDTA-stable adhesion of aggregation-competent Dictyostelium cells was abolished by the treatment of intact cells with jack bean alpha-mannosidase, whereas neuraminidase, beta-galactosidase, beta-N-acetylhexosaminidase, or alpha-L-fucosidase had no effect. The EDTA-stable cohesiveness of TM-treated cells in the presence of leupeptin (TM/LP cells) was also abolished by the treatment of the cells with alpha-mannosidase. The effect of alpha-mannosidase was not prevented in the presence of LP. The N-glycoside-deficient contact site A (an adhesion-mediating glycoprotein) was obtained from TM/LP cells and was shown to have a molecular weight of 70,000. This protein (p 70) was shown to still have carbohydrates as detected by polyacrylamide gel electrophoresis (PAGE) in the presence of sodium dodecyl sulfate (SDS) and subsequent staining of the gel with periodic acid-silver stain. Moreover, p 70 reacted with anti-gp 68, which has a specificity against alpha-mannosyl residues of carbohydrate chains. However, p 70 treated with alpha-mannosidase showed decreased reactivity with anti-gp 68. The monovalent antibody fragment of anti-contact site A or anti-p 70 inhibited EDTA-stable cell adhesion of both control and TM/LP cells. These results indicated that TM-resistant mannosyl residues of contact site A are directly involved in EDTA-stable adhesion of aggregation-competent cells. This is the first report of the direct involvement of the carbohydrate moiety in cell adhesion of aggregation-competent Dictyostelium cells. A schematic model is presented of the role of the carbohydrate moiety in EDTA-stable cell adhesion, including the direct effect of carbohydrates.