Delayed activation of the mannose receptor following synthesis. Requirement for exit from the endoplasmic reticulum

The macrophage mannose receptor specifically recognizes proteins and particles bearing mannose terminal oligosaccharide chains. In the present study, we examined the ability of newly synthesized receptor to bind ligand. Human monocyte-derived macrophages were pulse-labeled with [35S]Met and prepared for affinity chromatography on mannose-Sepharose. Mannose receptor in the flow-through and eluted fractions was detected by fluorography following immunoprecipitation and gel electrophoresis. Labeled mannose receptor was found exclusively in the nonbinding fraction until 10 min of chase. Following a 60-min chase, 67-86% of newly synthesized receptor was precipitated from the bound column fraction. The half-time for development of receptor binding activity was determined to be 35-40 min compared with a 45-min half-time for development of endoglycosidase H resistance. Mannose receptor synthesized by cells incubated in brefeldin A required more than 120 min to acquire endoglycosidase 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 prevented terminal sialylation of oligosaccharide side chains but did not inhibit receptor activation. Inclusion of aluminum fluoride in the chase media reversibly inhibited development of endoglycosidase H resistance and mannose-binding activity. We conclude that the mannose receptor undergoes delayed activation following synthesis and suggest that the activating event(s) occur following exit of the receptor from the endoplasmic reticulum and prior to its entry into the trans-Golgi.