The Role of PALS1 in Mammalian Epithelial Polarity.

Abstract

Polarity is an intrinsic feature of epithelial cells, and it is reflected by the differential distribution of proteins and lipids in the apical and basolateral surfaces. The apical and basolateral membranes are separated by tight junctions at the superior aspect of the lateral surface, which function as a physical barrier between these membrane domains. The adherens junctions lay basal to the tight junctions in mammalian epithelial cells and they mediate the adhesion between neighboring cells. Studies in Drosophila and mammalian cells have identified several evolutionarily conserved macromolecular complexes as polarity determinants, including two tight junction-associated complexes Crumbs(CRB)-PALS1-PATJ and PAR3-PAR6-aPKC. PALS1 is an adaptor protein with multiple protein-protein interaction domains, and it has been shown to play an important role in apical-basal polarity establishment and in tight junction biogenesis. However, the biochemical pathways that intersect PALS1 are not clear. In this thesis, the interaction between PALS1 and PAR6 was characterized. We found that the PALS1-PAR6 interaction, which links the CRB complex and the PAR complex together, is a dually-unconventional PDZ domain interaction, and the unusual features of this interaction may be essential for the coordinated function of the two complexes. On the other hand, a comprehensive PALS1 knockdown/rescue study in mammalian cells revealed that PALS1 not only regulates tight junction biogenesis, it is also involved in adherens junction formation and in the trafficking of E-cadherin, which is the major structural component of adherens junctions. Although the mechanism whereby tight junction-associated polarity proteins regulate adherens junction components is not clear, a similar process has been observed in Drosophila. Taken together, this work provides new functional information on PALS1 as a polarity regulator and increases our understanding of how the CRB complex and the PAR complex function in mammalian epithelial polarity.