FIGURE 2 | Superimposing protein sorting demands on the generic trafficking pathway.

In polarized cells, such as epithelia and neurons, protein processing occurs along the generic pathway between the endoplasmic reticulum (ER) and the trans-Golgi network (TGN; see also Fig. 1). However, proteins in the TGN might be sorted into various different vesicles through the recognition of different intrinsic sorting motifs (Table 1) and cytoplasmic adaptor complexes. These vesicles are then targeted either directly or indirectly, through an endosome (E), to different plasma membrane domains (designated as domains I and II) along cytoskeletal elements. These cytoskeletal elements might have different orientations (or polarity) relative to the different membrane domains. Vesicle delivery to each plasma membrane domain is mediated by different vesicle-tethering and SNARE complexes. Some proteins are internalized through another set of adaptors and delivered to an endosome, from which they might be recycled back to the original plasma membrane domain, or to the other domain by trancytosis, depending on the presence (or activation or inactivation) of specific protein-sorting motifs. AP, adaptor protein; CL, clathrin; COP, coatomer protein complex.

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