Activation of ERM proteins in vivo by Rho involves phosphatidylinositol 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 phosphorylatio n, where they crosslink actin filaments to the plasma membrane to form micr ovilli (reviewed in [1-5]). Here, we report that, when NIH3T3 or HeLa cells were transfected with a constitutively active mutant of the small GTPase R hoA (V14RhoA), microvilli were induced and the level of carboxy-terminal th reonine-phosphorylated ERM proteins (CPERM) [6,7] increased similar to 30-f old. This increase was not observed following transfection of constitutivel y active forms of two other Rho family GTPases, Rad and Cdc42, or of a dire ct effector of Rho, Rho-kinase (also known as ROK alpha or ROCK-II) [8-10]. The V14RhoA-induced phosphorylation of ERM proteins was not suppressed by Y-27832, a specific inhibitor of ROCK kinases including Rho-kinase [11], Ov erexpression of another direct effector of Rho, phosphatidylinositol 4-phos phate 5-kinase (PI4P5K) type I alpha [12-14], but not a kinase-inactive mut ant [15], increased similar to sixfold the level of CPERM, and induced micr ovilli, Together with the previous finding that the PI4P5K product phosphat idylinositol 4,5-bisphosphate (PIP2) activates ERM proteins in vitro [16], our data suggest that PIP2, and not ROCK kinases, is involved in the RhoA-d ependent activation of ERM proteins in vivo. The active state of ERM protei ns is maintained through threonine phosphorylation by as yet undetermined k inases, leading to microvillus formation. (C) 1999 Elsevier Science Ltd. Al l rights reserved.