ABSTRACT
Small GTPases of the Rho family (RhoA, Rac1, and Cdc42) and the Ras family GTPase Rap1 are essential for the assembly and function of epithelial cell-cell junctions. Through their downstream effectors, small GTPases modulate junction formation and stability, primarily by orchestrating the polymerization and contractility of the actomyosin cytoskeleton. The major upstream regulators of small GTPases are guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). Several GEFs and a few GAPs have been localized at epithelial junctions, and bind to specific junctional proteins. Thus, junctional proteins can regulate small GTPases at junctions, through their interactions with GEFs and GAPs. Here we review the current knowledge about the mechanisms of regulation of small GTPases by junctional proteins. Understanding these mechanisms will help to clarify at the molecular level how small GTPases control the morphogenesis and physiology of epithelial tissues, and how they are disregulated in disease.
Subject(s)
Epithelial Cells/metabolism , Monomeric GTP-Binding Proteins/metabolism , Tight Junctions/enzymology , Actomyosin/genetics , Actomyosin/metabolism , Adherens Junctions/enzymology , Adherens Junctions/genetics , Animals , Cytoskeleton/genetics , Cytoskeleton/metabolism , Desmosomes/enzymology , Desmosomes/genetics , GTPase-Activating Proteins/genetics , GTPase-Activating Proteins/metabolism , Guanine Nucleotide Exchange Factors/genetics , Guanine Nucleotide Exchange Factors/metabolism , Humans , Monomeric GTP-Binding Proteins/genetics , Morphogenesis/genetics , Tight Junctions/genetics , Vertebrates , rho GTP-Binding Proteins/genetics , rho GTP-Binding Proteins/metabolismABSTRACT
The pleckstrin-homology-domain-containing protein PLEKHA7 was recently identified as a protein linking the E-cadherin-p120 ctn complex to the microtubule cytoskeleton. Here we characterize the expression, tissue distribution and subcellular localization of PLEKHA7 by immunoblotting, immunofluorescence microscopy, immunoelectron microscopy, and northern blotting in mammalian tissues. Anti-PLEKHA7 antibodies label the junctional regions of cultured kidney epithelial cells by immunofluorescence microscopy, and major polypeptides of M(r) approximately 135 kDa and approximately 145 kDa by immunoblotting of lysates of cells and tissues. Two PLEKHA7 transcripts ( approximately 5.5 kb and approximately 6.5 kb) are detected in epithelial tissues. PLEKHA7 is detected at epithelial junctions in sections of kidney, liver, pancreas, intestine, retina, and cornea, and its tissue distribution and subcellular localization are distinct from ZO-1. For example, PLEKHA7 is not detected within kidney glomeruli. Similarly to E-cadherin, p120 ctn, beta-catenin and alpha-catenin, PLEKHA7 is concentrated in the apical junctional belt, but unlike these adherens junction markers, and similarly to afadin, PLEKHA7 is not localized along the lateral region of polarized epithelial cells. Immunoelectron microscopy definitively establishes that PLEKHA7 is localized at the adherens junctions in colonic epithelial cells, at a mean distance of 28 nm from the plasma membrane. In summary, we show that PLEKHA7 is a cytoplasmic component of the epithelial adherens junction belt, with a subcellular localization and tissue distribution that is distinct from that of ZO-1 and most AJ proteins, and we provide the first description of its distribution and localization in several tissues.
