Swiprosin-1 is a novel actin bundling protein that regulates cell spreading and migration.

Protein functions are often revealed by their localization to specialized cellular sites. Recent reports demonstrated that swiprosin-1 is found together with actin and actin-binding proteins in the cytoskeleton fraction of human mast cells and NK-like cells. However, direct evidence of whether swiprosin-1 regulates actin dynamics is currently lacking. We found that swiprosin-1 localizes to microvilli-like membrane protrusions and lamellipodia and exhibits actin-binding activity. Overexpression of swiprosin-1 enhanced lamellipodia formation and cell spreading. In contrast, swiprosin-1 knockdown showed reduced cell spreading and migration. Swiprosin-1 induced actin bundling in the presence of Ca(2+), and deletion of the EF-hand motifs partially reduced bundling activity. Swiprosin-1 dimerized in the presence of Ca(2+) via its coiled-coil domain, and a lysine (Lys)-rich region in the coiled-coil domain was essential for regulation of actin bundling. Consistent with these observations, mutations of the EF-hand motifs and coiled-coil region significantly reduced cell spreading and lamellipodia formation. We provide new evidence of how swiprosin-1 influences cytoskeleton reorganization and cell spreading.