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

后高尔基分泌和内吞途径中膜蛋白的选择性分选在很大程度上受被分选蛋白胞质尾部编码的分选信号调控。其中一种基于酪氨酸的分选信号，通过与网格蛋白包被囊泡（CCV）的包被组分AP复合物的μ亚基相互作用，选择性地将被分选的分子吸收到网格蛋白包被囊泡（CCV）中。AP复合物是一个蛋白质家族，以酪氨酸为基础的分选信号参与了许多细胞内分选途径，包括胞内吞噬和基底外分选。这就提出了一种可能性，即每个μ链特异性地识别基于酪氨酸的分选信号的子集，从而产生分选的特异性和多样性。我们利用酵母2杂交试验来证明这一点。我们克隆了一个新的μ同源物μ 1b，它与一个普遍表达的μ链μ 1a高度同源。然而，与μ1A相反，μ1B的表达仅限于上皮细胞。上皮细胞的质膜在物理上分为顶端和底外侧两个结构域，膜蛋白在这两个结构域上有选择性地进行分类。通过在缺乏μ1B表达的上皮细胞系中重建μ1B的表达，我们能够证明μ1B参与了上皮细胞中膜蛋白向基底外质膜的选择性分选。

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.