Involvement of glycosylation in the intracellular trafficking of glycoproteins in polarized epithelial cells.

The surface of epithelial cells is composed of apical and basolateral domains with distinct structure and function. This polarity is maintained by specific sorting mechanisms occurring in the Trans-Golgi Network. Peptidic signals are responsible for the trafficking via clathrin-coated vesicles by means of an interaction with an adaptor complex (AP). The basolateral targeting is mediated by AP-1B, which is specifically expressed in epithelial cells. In contrast, the apical targeting is proposed to occur via apical raft carriers. It is thought that apically targeted glycoproteins contain glycan signals that would be responsible for their association with rafts and for apical targeting. However, the difficulty in terms of acting specifically on a single step of glycosylation did not allow one to identify such a specific signal. The complete inhibition of the processing of N-glycans by tunicamycin often results in an intracellular accumulation of unfolded proteins in the Golgi. Similarly, inhibition of O-glycosylation can be obtained by competitive substrates which gave a complex pattern of inhibition. Therefore, it is still unknown if glycosylation acts in an indirect manner, i.e. by modifying the folding of the protein, or in a specific manner, such as an association with specific lectins.