Villin and the structurally-related proteins gelsolin, fragmin and severin, all regulate the framework and assembly of actin. Villin is unique among these proteins in its ability to cross-link actin filaments into bundles, a process observed only at low Ca2+ concentration. Villin is composed of three domains. The first two domains are homologous and the third domain is called the headpiece. This headpiece region is located at the C-terminus. Villin is mainly produced by epithelial cells that develop a brush border. Cells producing villin are reported to be found either in the epithelial cells of the intestinal mucosa and gallbladder, or in epithelial cells of the kidney proximal tubules and ductuli efferentes of the testis. However, villin is also reported to be found in some epithelia which lack a brush border but which are derived from embryonic gut such as duct cells of the exocrine pancreas and biliary cells of the liver. In these cell types, villin is concentrated in the apical cytoplasm. Epithelial cells of the intestinal mucosa are continually being renewed and this involves a migration of these cell types from the intestinal crypts to the tips of the villi, gradually acquiring their differentiated phenotype as they do so. The maximum production of villin occurs at the base of the villus. Villin, therefore, shows tissue-specific expression being restricted to certain epithelia and their apical domains, thus indicating their polarity. The morphological loss of polarity of colonic epithelial cells is reported to be one of the most significant indicators of dysplasia or neoplasia.
Villin is recommended for the detection of specific antigens of interest in normal and neoplastic tissues, as an adjunct to conventional histopathology using non-immunologic histochemical stains.