F-actin bundling sites are conserved in proteins with villin type headpiece domains.

Villin is a major actin bundling protein that assembles the brush border of intestinal and renal epithelial cells. The villin 'headpiece' domain and the actin binding residues within it regulate its actin bundling function. Substantial experimental and theoretical information about the three-dimensional structure of the isolated villin headpiece, including a description of the actin binding residues within the headpiece, is available. Despite that, the actin bundling site in the full-length villin protein remains unidentified. We used this existing villin headpiece NMR data and performed mutational analysis and functional assays, to identify the actin bundling site in full-length human villin protein. By careful evaluation of these conserved actin binding residues in human advillin protein, we demonstrate their functional significance in the over 30 proteins that contain a villin type headpiece domain. Our study is the first that combines the available structural data on villin headpiece with functional assays to identify the actin binding residues in full-length villin that regulate its filament bundling activity. Our findings could have wider implications for other actin bundling proteins that contain a villin type headpiece domain.