Immunochemical, genetic and morphological comparison of fucosylation mutants of Dictyostelium discoideum.

Mutations in three loci in Dictyostelium discoideum which affect fucosylation are described. Mutations in two of these loci resulted in the simultaneous loss of two separate carbohydrate epitopes. The GA-X epitope, which was competed by L-fucose, was absent in strains carrying a modC354, modD352 or modE353 mutation. These strains exposed a new carbohydrate epitope, competed by N-acetylglucosamine, and the size of several glycoproteins was reduced. A second epitope (GA-XII) was also absent in strains carrying the modC354 or modE353 mutations, reducing the size of the glycoprotein which normally expresses it. Fucose content was reduced in the three mutants, suggesting that each mutation affected a separate step in fucosylation. The lesions did not appear to inhibit synthesis of the underlying carbohydrate, because detergent extracts of mutant vesicles were more active than normal vesicles at transferring [14C]fucose from GDP-[14C]fucose to endogenous acceptor species. The modD352 and modE353 mutant strains incorporated exogenous [3H]fucose poorly, suggesting that lesions in the modD and modE genes interfere with the biosynthesis of fucoconjugates downstream from the previously described GDP-fucose synthesis defect of the modC mutation. Intact modE353 mutant vesicles were relatively inefficient in in vitro assays, suggesting a global fucosylation defect (which is consistent with the loss of both glycoantigens, GA-X and GA-XII, in this mutant). Finally, the modC354 mutation led to delayed accumulation of slime sheath in vitro. The three genetic loci define a fucosylation pathway in D. discoideum comprising defined biochemical steps which contribute to multicellular morphogenesis in this organism.