IMMUNOCHEMICAL, GENETIC AND MORPHOLOGICAL COMPARISON OF FUCOSYLATION MUTANTS OF DICTYOSTELIUM-DISCOIDEUM

Mutations in three loci in Dictyostelium discoideum which affect fucos ylation are described. Mutations in two of these loci resulted in the simultaneous loss of two separate carbohydrate epitopes. The GA-X epit ope, which was competed by L-fucose, was absent in strains carrying a modC354, modD352 or modE353 mutation. These strains exposed a new carb ohydrate epitope, competed by N-acetylglucosamine, and the size of sev eral glycoproteins was reduced. A second epitope (GA-XII) was also abs ent 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 affect ed a separate step in fucosylation. The lesions did not appear to inhi bit synthesis of the underlying carbohydrate, because detergent extrac ts of mutant vesicles were more active than normal vesicles at transfe rring [C-14]fucose from GDP-[C-14]fucose to endogenous acceptor specie s. The modD352 and modE353 mutant strains incorporated exogenous [H-3] fucose poorly, suggesting that lesions in the modD and modE genes inte rfere with the biosynthesis of fucoconjugates downstream from the prev iously described GDP-fucose synthesis defect of the modC mutation. Int act modE353 mutant vesicles were relatively inefficient in in vitro as says, suggesting a global fucosylation defect (which is consistent wit h the loss of both glycoantigens, GA-X and GA-XII, in this mutant). Fi nally, the modC354 mutation led to delayed accumulation of slime sheat h in vitro, The three genetic loci define a fucosylation pathway in D. discoideum comprising defined biochemical steps which contribute to m ulticellular morphogenesis in this organism.