μ1B, a novel adaptor medium chain expressed in polarized epithelial cells1

Abstract

The apical and basolateral plasma membrane domains of polarized epithelial cells contain distinct sets of integral membrane proteins. Biosynthetic targeting of proteins to the basolateral plasma membrane is mediated by cytosolic tail determinants, many of which resemble signals involved in the rapid endocytosis or lysosomal targeting. Since these signals are recognized by adaptor proteins, we hypothesized that there could be epithelial-specific adaptors involved in polarized sorting. Here, we report the identification of a novel member of the adaptor medium chain family, named μ1B, which is closely related to the previously described μ1A (79% amino acid sequence identity). Northern blotting and in situ hybridization analyses reveal the specific expression of μ1B mRNA in a subset of polarized epithelial and exocrine cells. Yeast two-hybrid analyses show that μ1B is capable of interacting with generic tyrosine-based sorting signals. These observations suggest that μ1B may be involved in protein sorting events specific to polarized cells.