The adaptor protein-1 μ1B subunit expands the repertoire of basolateral sorting signal recognition in epithelial
Xiaoli Guo1, Rafael Mattera, Xuefeng Ren
1Cell Biology and Metabolism Program, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
Abstract:
An outstanding question in protein sorting is why polarized epithelial cells express two isoforms of the micro1 subunit of the AP-1 clathrin adaptor complex: the ubiquitous micro1A and the epithelial-specific micro1B. Previous studies led to the notion that micro1A and micro1B mediate basolateral sorting predominantly from the trans-Golgi network (TGN) and recycling endosomes, respectively. Using improved analytical tools, however, we find that micro1A and micro1B largely colocalize with each other. They also colocalize to similar extents with TGN and recycling endosome markers, as well as with basolateral cargoes transiting biosynthetic and endocytic-recycling routes. Instead, the two isoforms differ in their signal-recognition specificity. In particular, micro1B preferentially binds a subset of signals from cargoes that are sorted basolaterally in a micro1B-dependent manner. We conclude that expression of distinct micro1 isoforms in epithelial cells expands the repertoire of signals recognized by AP-1 for sorting of a broader range of cargoes to the basolateral surface.
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