Mechanism of regulation of selective intracellular protein sorting by the family of Adaptor (-like) protein complexes

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Selective sorting of membrane proteins in the post-Golgi secretory and endocytic pathways are largely regulated by sorting signals encoded within the cytoplasmic tails of sorted proteins. One of the sorting signals, the tyrosine-based sorting signal, works for selective intake of the sorted molecules into the clathrin-coated vesicles (CCV), by interacting with the μ subunits of AP complexes, a coat component of the CCV. AP complexes consist a protein family, and the tyrosine-based sorting signals are known to be involved in many intracellular sorting pathways including endocytosis and basolateral sorting. This raised the possibility that each μ chain specifically recognizes a subset of the tyrosine-based sorting signals to produce specificity and diversity of the sorting. We utilized a yeast 2-hybrid assay to show this is the case.We cloned a new μ homologue, μ1B, which is highly homologous to one of the ubiquitously expressed μ chains, μ1A. In contrast to μ1A, however, the expression of μ1B is restricted to epithelial cells. Plasma membrane of epithelial cells are physically divided into two domains, apical and basolateral, and membrane proteins are sorted selectively to these two domains. By reconstituting the expression of μ1B into an epithelial cell line lacking the μ1B expression, we were able to show that μ1B is involved in the selective sorting of membrane proteins to the basolateral plasma membrane in epithelial cells.