In simple epithelia, specialized vectorial functions such as transport and secretion are made possible by the segregation of proteins and lipids into opposite surface domains. This polarized distribution results from selective delivery to and retention at the appropriate domain. In the case of direct delivery, the sorting site for apical and basolateral proteins is the trans-Golgi network (TGN) where they are incorporated into distinct apical and basolateral vesicles that are targeted to the respective surfaces. The machinery that controls this simple process is in fact rather complicated. It involves many different steps from the recognition event (between 'sorting signal(s)' and 'sorting receptor(s)' to the formation of the vesicles, their budding, and the docking to the specialized plasma membrane domain. Here we summarize the latest developments in the sorting of apical and basolateral proteins, focusing in particular on the signals that are involved in this process and the current hypotheses about the mechanisms responsible for it, in both epithelia and in non-polarized cells.
