Glycans in post-Golgi apical targeting: sorting signals or structural props?

A recent model proposed that N-glycans serve as apical targeting signals for soluble and membrane proteins in epithelial cells and neurons by interacting with lectin sorters in the trans-Golgi network. However, we believe that a number of experimental observations support an alternative hypothesis, that N-glycans play a facilitative role, by providing structural support or preventing aggregation of the proteins for example, thereby allowing interaction of proteinaceous apical sorting signals with the sorting machinery. This article discusses the experimental data currently available and how they relate to the proposed models.