Maturation of asparagine-linked oligosaccharides in Dictyostelium discoideum analyzed with modification-specific...

Lysosomal enzymes in Dictyostelium discoideum contain high mannose oligosaccharides that contain mannose 6-phosphate and several unusual structures. The synthesis and distribution of these post-translational modifications were studied using probes for different carbohydrate groups. These probes include lectin-like antibodies directed to two distinct sulfated and one nonsulfated N-linked determinants, the lectin Con A, and the mammalian 215-kDa phosphomannosyl receptor. Only Con A binds to newly synthesized alpha-mannosidase present in the rough endoplasmic reticulum. The other modifications are acquired at different rates and are first detected on protein in light density Golgi-like membranes. Mutations which prevent protein transport to Golgi membranes block synthesis of these moieties, but inhibitors which prevent later transport steps have no effect. The majority of modified proteins are in lysosomes but significant amounts are delivered to nonlysosomal destinations. Different lysosomal proteins contain unequal amounts of each modification.