Characterization of human palladin, a microfilament-associated protein

Actin-containing microfilaments control cell shape, adhesion, and contracti on. In striated muscle, a-actinin and other Z-disk proteins coordinate the organization and functions of actin filaments. In smooth muscle and nonmusc le cells, periodic structures termed dense bodies and dense regions, respec tively, are thought to serve functions analogous to Z-discs. We describe he re identification and characterization of human palladin, a protein express ed mainly in smooth muscle and nonmuscle and distributed along microfilamen ts iri a periodic manner consistent with dense regions/bodies. Palladin con tains three Ig-domains most homologous to the sarcomeric Z-disk protein myo tilin. The N terminus includes an FPPPP motif recognized by the Ena-Vasp ho mology domain I domain in Ena/vasodilatator-stimulated phosphoprotein (VASP )/WiscottAldrich syndrome protein (WASP) protein family. Cytoskeletal prote ins with FPPPP motif target Ena/VASP/WASP proteins to sites of actin modula tion. We identified palladin in a yeast two-hybrid search as an ezrin-assoc iated protein. An interaction between palladin and ezrin was further verifi ed by affinity precipitation and blot overlay assays. The interaction was m ediated by the a-helical domain of ezrin and by Ig-domains 2-3 of palladin. Ezrin is typically a component of the cortical cytoskeleton, but in smooth muscle cells it is localized along microfilaments. These cells express pal ladin abundantly and thus palladin may be involved in the microfilament loc alization of ezrin. Palladin expression was up-regulated in differentiating dendritic cells (DCs), coinciding with major cytoskeletal and morphologica l alterations. In immature DCs, palladin localized in actin-containing podo somes and in mature DCs along actin filaments. The regulated expression and localization suggest a role for palladin in the assembly of DC cytoskeleto n.