Syntrophin is an actin-binding protein the cellular localization of which is regulated through cytoskeletal reorganization in skeletal muscle cells

We have characterized the interaction of syntrophin with F-actin. Subcellular fractionation of cardiac and skeletal muscle tissues showed that alpha-, beta1- and beta2-syntrophins were present in the soluble and the membrane fraction. Syntrophins are known to bind to the dystrophin-glycoprotein complex (DGC), but since the DGC is not present in the soluble fraction, it was concluded that some syntrophin did not associate with the DGC. Native syntrophins purified from the soluble fraction and recombinant syntrophins were both able to bind to F-actin, and binding occurred through several sites on syntrophin, including the second pleckstrin homology domain and the unique carboxyl-terminal domain. Syntrophin was also able to inhibit actin-activated myosin ATPase activity and actomyosin super-precipitation. alpha-Syntrophin co-localized with cortical F-actin fibers when expressed in Chinese hamster ovary cells, and deletion of the actin-binding region abolished co-localization. Most of exogenous or endogenous syntrophin also co-localized with stress fibers in endothelial and smooth muscle (A7r5) cells. However, syntrophins were mostly localized in the cytosol of serum-starved C2C12 or primary cultured skeletal muscle myotubes, and translocated to the membrane upon treatment with lysophosphatidic acid or the actin-stabilizing agent jasplakinolide. The actin-depolymerizing agent latrunculin-B abolished this syntrophin translocation. These findings suggest that syntrophin is an actin-binding protein the subcellular localization of which is regulated through cytoskeletal reorganization.