Phosphorylation of alpha-syntrophin is responsible for its subcellular localization and interaction with dystrophin in muscle cells

Syntrophin is a well-known adaptor protein that links intracellular proteins with the dystrophin-glycoprotein complex (DGC) at the sarcolemma. However, little is known about the underlying mechanism that regulates the intracellular localization of alpha-syntrophin and its interaction with dystrophin. In this study, we demonstrate that alpha-syntrophin phosphorylation determines its intracellular localization and interaction with dystrophin in muscle cells. alpha-Syntrophin, a predominant isoform in skeletal muscles, directly interacts with ion channels, enzymes, receptors, and DGC proteins. Despite alpha-syntrophin being a potential signaling molecule, most studies focus on its function as a dystrophin-associated protein. However, we previously reported that alpha-syntrophin has a variety of DGC-independent functions to modulate cell migration, differentiation, survival, and protein stability. According to the results of the in vitro phosphorylation assays using subcellular fractions, the phosphorylated alpha-syntrophin accumulated only at the plasma membrane, and this event occurred regardless of dystrophin expression. However, the alpha-syntrophin interacting with dystrophin at the membrane was not in a phosphorylated state. We also identified that protein kinase C (PKC) was involved in the phosphorylation of alpha-syntrophin, which restricted alpha-syntrophin to interact with dystrophin. In conclusion, we demonstrate that the phosphorylation of alpha-syntrophin by PKC regulates its intracellular localization and interaction with dystrophin.