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 W W1B, which is closely related to the previously described W W1A (79% amino acid sequence identity). Northern blotting and in situ hybridization analyses reveal the specific expression of W W1B mRNA in a subset of polarized epithelial and exocrine cells. Yeast two-hybrid analyses show that W W1B is capable of interacting with generic tyrosine-based sorting signals. These observations suggest that W W1B may be involved in protein sorting events specific to polarized cells.

Physical and Functional Links between Anion Exchanger-1 and Sodium Pump

“…Binding of m subunits, including AP-1B (m1b), to YXXØ motifs are able to mediate basolateral sorting of certain proteins in epithelial cells. [34][35][36][37] However, both kAE1 and the Na + ,K + -ATPase are sorted to the basolateral surface in the m1b-deficient cell line LLC-PK1, as is kAE1 in MDCK cells in which either m1b expression has been suppressed or Val 907 (Ø) in the YXXØ motif has been mutated. 5,6,38,39 AP-1A (m1a) was recently reported to interact with kAE1 via Y 904 DEV, and knockdown of endogenous m1a in HEK293T cells resulted in reduced membrane localization of kAE1.…”

Regulation of membrane trafficking in polarized epithelial cells

“…Typically, it is the medium subunit that interacts with the tyrosine-based sorting signals, whereas the di-leucine-based signals may be recognized by the γ/σ1 subunits of AP-1 or δ/σ3 of AP-3 [2,25]. In addition, polarized columnar epithelial cells express the epithelial cell-specific adaptor complex AP-1B [26], which differs from its close cousin AP-1A only in the incorporation of the medium subunits μ1B or μ1A, respectively. AP-1B plays important functions in basolateral sorting from recycling endosomes [27][28][29].…”

The Epidermal Growth Factor Receptor Juxtamembrane Domain Has Multiple Basolateral Plasma Membrane Localization Determinants, Including a Dominant Signal with a Polyproline Core

“…AP-1-regulated sequestration of membrane cargo with tyrosine-dependent sorting signals into clathrin-coated transport vesicles is thus far the only mechanism implicated in regulated sorting of basolateral proteins (23,24). However, the nature of the EGF receptor sorting signals, as well as the accuracy of EGF receptor sorting in LLCPK 1 cells lacking the AP-1 isoform necessary for polarized sorting (37), suggests that these motifs underlie a novel mechanism. Whether EGF receptors are recruited to clathrin-coated transport vesicles independent of AP-1, or undergo sorting mediated by an entirely different set of molecules, remains to be seen.…”

“…Residues Lys 652 through Ala 674 lack critical tyrosine residues (26) and do not overlap any of the EGF receptor sorting signals responsible for clathrin-mediated internalization located in the carboxyl terminus (34). In addition to MDCK cells, EGF receptors also exhibit a predominantly basolateral localization in LLC-PK 1 cells (35,36), which lack a novel epithelial cell-specific AP-1 subunit isoform necessary for correctly sorting basolateral membrane cargo with AP-dependent sorting signals (37). Hence, polarized EGF receptor sorting from internal compartments is likely mediated by novel proteinprotein interactions.…”