Temporal association of the N- and O-linked glycosylation events and theirimplication in the polarized sorting of intestinal brush border sucrase-isomaltase, aminopeptidase N, and dipeptidyl peptidase IV

The temporal association between O-glycosylation and processing of N-linked glycans in the Golgi apparatus as well as the implication of these events in the polarized sorting of three brush border proteins has been the subjec t of the current investigation. O-Glycosylation of pro-sucrase-isomaltase ( pro-SI), aminopeptidase N (ApN), and dipeptidyl peptidase IV (DPPIV) is dra stically reduced when processing of the mannose-rich N-linked glycans is bl ocked by deoxymannojirimycin, an inhibitor of the Golgi-located mannosidase I. By contrast, O-glycosylation is not affected in the presence of swainso nine, an inhibitor of Golgi mannosidase II. The results indicate that remov al of the outermost mannose residues by mannosidase I from the mannose-rich N-linked glycans is required before O-glycosylation can ensue. On the othe r hand, subsequent mannose residues in the core chain impose no sterical co nstraints on the progression of O-glycosylation. Reduction or modification of N- and O-glycosylation do not affect the transport of pro-SI, ApN, or DP PIV to the cell surface per se. However, the polarized sorting of two of th ese proteins, pro-SI and DPPIV, to the apical membrane is substantially alt ered when O-glycans are not completely processed, while the sorting of ApN is not affected. The processing of N-linked glycans, on the other hand, has no influence on sorting of all three proteins. The results indicate that O -linked carbohydrates are at least a part of the sorting mechanism of pro-S I and DPPIV, The sorting of ApN implicates neither O-linked nor N-linked gl ycans and is driven most likely by carbohydrate-independent mechanisms.