Differential association of syntrophin pairs with the dystrophin complex

The syntrophins are a multigene family of intracellular dystrophin-associated proteins comprising three isoforms, alpha 1, beta 1, and beta 2. Based on their domain organization and association with neuronal nitric oxide synthase, syntrophins are thought to function as modular adapters that recruit signaling proteins to the membrane via association with the dystrophin complex. Using sequences derived from a new mouse beta 1-syntrophin cDNA, and previously isolated cDNAs for alpha 1- and beta 2-syntrophins, we prepared isoform-specific antibodies to study the expression, skeletal muscle localization, and dystrophin family association of all three syntrophins. Most tissues express multiple syntrophin isoforms. In mouse gastrocnemius skeletal muscle, alpha 1- and beta 2-syntrophin are concentrated at the neuromuscular junction but are also present on the extrasynaptic sarcolemma. beta 1-syntrophin is restricted to fast-twitch muscle fibers, the first fibers to degenerate in Duchenne muscular dystrophy. beta 2-syntrophin is largely restricted to the neuromuscular junction. The sarcolemmal distribution of alpha 1- and beta 1-syntrophins suggests association with dystrophin and dystrobrevin, whereas all three syntrophins could potentially associate with utrophin at the neuromuscular junction. Utrophin complexes immunoisolated from skeletal muscle are highly enriched in beta 1- and beta 2-syntrophins, while dystrophin complexes contain mostly alpha 1- and beta 1-syntrophins. Dystrobrevin complexes contain dystrophin and alpha 1- and beta 1-syntrophins. From these results, me propose a model in which a dystrophin-dystrobrevin complex is associated with two syntrophins. Since individual syntrophins do not have intrinsic binding specificity for dystrophin, dystrobrevin, or utrophin, the observed preferential pairing of syntrophins must depend on extrinsic regulatory mechanisms.