Activation of ERM proteins in vivo by Rho involves phosphatidyl-inositol 4-phosphate 5-kinase and not ROCK kinases.
When activated, ERM (ezrin, radixin, moesin) proteins are recruited to the plasma membrane, with concomitant carboxy-terminal threonine phosphorylation, where they crosslink actin filaments to the plasma membrane to form microvilli (reviewed in [1] [2] [3] [4] [5]). Here, we report that, when NIH3T3 or HeLa cells were transfected with a constitutively active mutant of the small GTPase RhoA (V14RhoA), microvilli were induced and the level of carboxy-terminal threonine-phosphorylated ERM proteins (CPERM) [6] [7] increased approximately 30-fold. This increase was not observed following transfection of constitutively active forms of two other Rho-family GTPases, Rac1 and Cdc42, or of a direct effector of Rho, Rho-kinase (also known as ROKalpha or ROCK-II) [8] [9] [10]. The V14RhoA-induced phosphorylation of ERM proteins was not suppressed by Y-27632, a specific inhibitor of ROCK kinases including Rho-kinase [11]. Overexpression of another direct effector of Rho, phosphatidylinositol 4-phosphate 5-kinase (PI4P5K) type Ialpha [12] [13] [14], but not a kinase-inactive mutant [15], increased approximately sixfold the level of CPERM, and induced microvilli. Together with the previous finding that the PI4P5K product phosphatidylinositol 4,5-bisphosphate (PIP(2)) activates ERM proteins in vitro [16], our data suggest that PIP(2), and not ROCK kinases, is involved in the RhoA-dependent activation of ERM proteins in vivo. The active state of ERM proteins is maintained through threonine phosphorylation by as yet undetermined kinases, leading to microvillus formation.
Matsui
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Department of Cell Biology, Faculty of Medicine, Kyoto University, Sakyo-ku, Kyoto, 606, Japan.
Yonemura
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Tsukita
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Tsukita
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D016475:3T3 Cells; D000818:Animals; D001798:Blood Proteins / Q000378:metabolism*; D003598:Cytoskeletal Proteins; D006367:HeLa Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins / Q000378:metabolism*; D051379:Mice; D008840:Microfilament Proteins / Q000378:metabolism*; D008871:Microvilli / Q000502:physiology; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D013912:Threonine / Q000378:metabolism; D014162:Transfection; D054460:rho-Associated Kinases; D020742:rhoA GTP-Binding Protein / Q000378:metabolism*; D000099149:Ezrin
D001798:Blood Proteins; D003598:Cytoskeletal Proteins; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D008840:Microfilament Proteins; D010750:Phosphoproteins; D017853:Phosphotransferases (Alcohol Group Acceptor); D017346:Protein Serine-Threonine Kinases; D013912:Threonine; D054460:rho-Associated Kinases; D020742:rhoA GTP-Binding Protein; D000099149:Ezrin; C071058:moesin; C071776:radixin; C020077:1-phosphatidylinositol-4-phosphate 5-kinase