Characteristics and structural requirements of apical sorting of the rat growth hormone through the O-glycosylated stalk region of intestinal sucrase-isomaltase.

The apical sorting of the small intestinal membrane glycoprotein sucrase-isomaltase (SI) depends on the presence of O-linked glycans and the transmembrane domain. Here, we investigate the role of O-glycans carried by the Ser/Thr-rich stalk region of SI as an apical sorting signal and evaluate the spatial requirements for an efficient recognition of this signal. Several hybrid proteins are generated comprising the unsorted and unglycosylated protein, the rat growth hormone (rGH), fused to either the transmembrane domain of SI (GH-SI(TM)), or the transmembrane and the stalk domains (GH-SI(SR/TM)). Both constructs are randomly distributed over the apical and basolateral membranes of MDCK cells indicating that neither the transmembrane domain nor the O-glycans are sufficient per se for an apical delivery. Only when a polyglycine spacer is inserted between the stalk region of SI and the luminal part of rGH in the GH-SI(Gly/SR/TM) fusion protein does efficient apical sorting of an O-glycosylated protein as well as a time-dependent association with detergent-insoluble lipid microdomains occur. Obviously, the polyglycine spacer facilitates the accessibility of the O-glycans in GH-SI(Gly/SR/TM) to a putative sorting receptor, whereas these glycans are inadequately recognized in GH-SI(SR/TM). We conclude that the O-glycans in the stalk region of SI act as an apical sorting signal within a sorting machinery that comprises at least a carbohydrate-binding protein and fulfills specific spatial requirements provided, for example by a polyglycine spacer in the context of rGH or the P-domain within the SI enzyme complex.