The biosynthesis of glycoproteins during the development of Dictyostelium discoideum. The transfer of D-mannose in vegetative and aggregated cells.

Abstract

d-Mannose polysaccharide transferase is present in vegetative and aggregated cells of Dictyostelium discoideum. The enzyme together with the acceptor polymer for mannose are localized in subcellular fractions which were enriched from the microsomal and the 1500 ×g sediment fractions by sucrose density gradient centrifugation. The transfer of mannose into the acceptor depends on the development stage of the cells. During the aggregation period a steep rise in mannose transfer occurs. The fractions active in transferase consist of membraneous structures, ribosomal material being absent or at least extremely scarce. The enzymatic activity can be released partially by mild ultrasonic treatment, the acceptor function of the remaining particles cannot be destroyed by heat. The mannose transfer follows a two-step time course. During the initial phase of incubation a mannose containing intermediate is synthesized from GDP-Man. The linkage between the protein and the carbohydrate moieties of the acceptor consists predominantly of the O-glycosidic β-hydroxyamino acid type.