Mannose analog 1-deoxymannojirimycin inhibits the Golgi-mediated processing of bean storage glycoproteins

The asparagine-linked oligosaccharide chains of glycoproteins can be processed to form a wide variety of structures. The Golgi complex is the main compartment involved in this processing. In mammalian cells the first enzyme acting along the Golgi processing pathway is mannosidase I, whose action is a prerequisite for any further processing and which is inhibited by the mannose analog 1-deoxymannojirimycin (dMM). To have insights into the processing pathway in plant cells, we have studied the in vivo effect of dMM on the processing of the bean (Phaseolus vulgaris) storage proteins phaseolin and phytohemagglutinin, two well characterized plant glycoproteins. Cotyledons obtained from developing seeds were labeled with radioactive leucine, glucosamine, or fucose in the presence or absence of dMM. Treatment with dMM fully inhibited the acquisition of resistance to endo-beta-N-acetylglucosaminidase H by phaseolin and phytohemagglutinin and the incorporation of fucose into protein. Furthermore, the apparent molecular weight of the polypeptides of phaseolin and phytohemagglutinin synthesized in dMM-treated cotyledons was consistent with the exclusive presence of oligommanose oligosaccharide chains which had not been processed in the Golgi complex. The inhibition of processing did not prevent exit from the Golgi complex, and most probably the storage proteins were correctly targeted to the protein bodies as indicated by the post-translational polypeptide cleavage of phaseolin. These results indicate that the action of a mannosidase is the first obligatory step of Golgi-mediated processing also in a plant cell and, together with data obtained in other laboratories on the in vitro specificity of glycosidases and glycosyltransferases present in the Golgi complex of plant cells, support the hypothesis that the key early reactions in Golgi-mediated processing are similar if not identical in plants and mammals.