Tyrosine Phosphorylation of Villin Regulates the Organization of the Actin Cytoskeleton

Abstract

We have previously shown that tyrosine phosphorylation of the actin-regulatory protein villin is accompanied by the redistribution of phosphorylated villin and a concomitant decrease in the F-actin content of intestinal epithelial cells. The temporal and spatial correlation of these two events suggested that tyrosine phosphorylation of villin may be involved in the rearrangement of the microvillar cytoskeleton. This hypothesis was investigated by analyzing the effects of tyrosine phosphorylation of villin on the kinetics of actin polymerization by reconstituting in vitro the tyrosine phosphorylation of villin and its association with actin. Full-length recombinant human villin was phosphorylated in vitro by expression in the TKX1-competent cells that carry an inducible tyrosine kinase gene. The actin-binding properties of villin were examined using a co-sedimentation assay. Phosphorylation of villin did not change the stoichiometry (1:2) but decreased the binding affinity (4.4 M for unphosphorylated versus 0.6 M for phosphorylated) of villin for actin. Using a pyrene-actin-based fluorescence assay, we demonstrated that tyrosine phosphorylation had a negative effect on actin nucleation by villin. In contrast, tyrosine phosphorylation enhanced actin severing by villin. Electron microscopic analysis showed complementary morphological changes. Phosphorylation inhibited the actin bundling and enhanced the actin severing functions of villin. Taken together our data show that tyrosine phosphorylation of villin decreases the amount of villin bound to actin filaments, inhibits the actin-polymerizing properties of villin, and promotes the actin-depolymerizing functions instead. These observations suggest a role for tyrosine phosphorylation in modulating the microvillar cytoskeleton in vivo by villin in response to specific physiological stimuli.Villin, an epithelial cell-specific protein, belongs to a family of actin-severing and -capping proteins, which includes gelsolin, severin, fragmin, and CapG among others. Villin is unique among this family of proteins in that it can also cross-link and bundle actin filaments. We have previously shown that villin is tyrosine-phosphorylated both in intestinal epithelial cells (1) and in vitro (2). Since our first demonstration of tyrosine phosphorylation of villin, other proteins of this family, including gelsolin, have been reported to be tyrosine-phosphorylated in vitro (3). Thus, tyrosine phosphorylation may also be a common feature of this family of proteins, and phosphorylation may play an important role in the organization of the actin network by these actin-binding proteins. Previous in vivo work from our laboratory shows that tyrosine phosphorylation of villin is accompanied by a decrease in the F-actin content of the cell (4). However, a causal relationship between tyrosine phosphorylation and changes in the distribution and/or kinetics of actin polymerization remains to be established.In addition to actin, villin interacts with several signaling molecules including phospha...

Association of Villin with Phosphatidylinositol 4,5-Bisphosphate Regulates the Actin Cytoskeleton

“…Overlapping and Identical Actin and PIP 2 -binding Sites in Villin-Two of the PIP 2 -binding sites identified in this study correspond to actin-binding sites in villin described previously (32,35,36). PB2 has been described as the site of F-actin binding to villin prior to severing, and PB5 has been identified as the F-actin-binding site involved in the bundling function of villin.…”

“…Electron Microscopic Analysis of Bundling Activity of Villin-The bundling activity of villin was analyzed essentially as described before (32). The divalent cation-free actin was polymerized together with villin constructs in polymerization buffer, pH 7.0, lacking MgCl 2 .…”

“…1). Ca 2ϩ (31), tyrosine phosphorylation (32), and PIP 2 (30) all regulate the actin modifying activities of villin.…”

Actin reorganization as the molecular basis for the regulation of apoptosis in gastrointestinal epithelial cells

“…Villin is tyrosine-phosphorylated both in vitro and in vivo and tyrosine phosphorylation of villin enhances its actin-severing function. 18 We have previously identified 10 phosphorylation sites in villin and demonstrated that mutation of these sites inhibits the actin severing activity of villin. 19 Further, we have demonstrated the absolute requirement of c-Src kinase for tyrosine phosphorylation of villin.…”

Regulation of Cell Motility by Tyrosine Phosphorylated Villin

“…However, studies done with the villin knockout mice suggest that in the absence of villin, the actin-bundling properties associated with villin can be substituted by other proteins in the microvilli (Pinson et al, 1998); on the other hand, the actin-severing activity of the microvilli is lost (Ferrary et al, 1999). In recent years, we have demonstrated that villin's actin-modifying functions can be regulated in vitro by tyrosine phosphorylation and phosphatidylinositol bisphosphate (PIP 2 ), suggesting that villin has the potential to function as a link between receptor activation and actin cytoskeleton reorganization even in the absence of high calcium (Arora and McCulloch, 1996;Zhai et al, 2001;. In addition, we have recently demonstrated that the autoinhibited conformation of villin can be released by tyrosine phosphorylation of villin (as opposed to high Ca 2ϩ ), allowing it to sever actin at physiological Ca 2ϩ concentrations .…”