Tyrosine-based Membrane Protein Sorting Signals Are Differentially Interpreted by Polarized Madin-Darby Canine Kidney and LLC-PK₁ Epithelial Cells*

Abstract

Tyrosine-dependent sequence motifs are implicated in sorting membrane proteins to the basolateral domain of Madin-Darby canine kidney (MDCK) cells. We find that these motifs are interpreted differentially in various polarized epithelial cell types. The H,K-ATPase β subunit, which contains a tyrosine-based motif in its cytoplasmic tail, was expressed in MDCK and LLC-PK₁ cells. This protein was restricted to the basolateral membrane in MDCK cells, but was localized to the apical membrane in LLC-PK₁ cells. Similarly, HA-Y543, a construct in which a tyrosine-based motif was introduced into the cytoplasmic tail of influenza hemagglutinin, was sorted to the basolateral membrane of MDCK cells and retained at the apical membrane of LLC-PK₁ cells. A chimera in which the cytoplasmic tail of the H,K-ATPase β subunit protein was replaced with the analogous region of the Na,K-ATPase β subunit polypeptide was localized to both surface domains of MDCK cells. Mutation of tyrosine-20 of the H,K-ATPase β subunit cytoplasmic sequence to an alanine was sufficient to disrupt basolateral localization of this polypeptide. In contrast, these constructs all remain localized to the apical membrane in LLC-PK₁ cells. The FcRII-B2 protein bears a di-leucine motif and is found at the basolateral membrane of both MDCK and LLC-PK₁ cells. These results demonstrate that polarized epithelia are able to discriminate between different classes of specifically defined membrane protein sorting signals.