DELAYED ACTIVATION OF THE MANNOSE RECEPTOR FOLLOWING SYNTHESIS - REQUIREMENT FOR EXIT FROM THE ENDOPLASMIC-RETICULUM

The macrophage mannose receptor specifically recognizes proteins and p articles bearing mannose terminal oligosaccharide chains. In the prese nt study, we examined the ability of newly synthesized receptor to bin d ligand. Human monocyte-derived macrophages were pulse-labeled with [ S-35]Met and prepared for affinity chromatography on mannose-Sepharose . Mannose receptor in the flow-through and eluted fractions was detect ed by fluorography following immunoprecipitation and gel electrophores is, Labeled mannose receptor was found exclusively in the nonbinding f raction until 10 min of chase. Following a 60-min chase, 67-86% of new ly synthesized receptor was precipitated from the bound column fractio n. The half-time for development of receptor binding activity was dete rmined to be 35-40 min compared with a 45-min half-time for developmen t of endoglycosidase H resistance. Mannose receptor synthesized by cel ls incubated in brefeldin A required more than 120 min to acquire endo glycosidase H resistance and maximal binding activity. Inhibitors of N -linked oligosaccharide processing or of O-glycosylation had no effect on the development of mannose receptor binding activity. Monensin pre vented terminal sialylation of oligosaccharide side chains but did not inhibit receptor activation. Inclusion of aluminum fluoride in the ch ase media reversibly inhibited development of endoglycosidase H resist ance and mannose-binding activity. We conclude that the mannose recept or undergoes delayed activation following synthesis and suggest that t he activating event(s) occur following exit of the receptor from the e ndoplasmic reticulum and prior to its entry into the trans-Golgi.