INTERACTION OF MUSCLE AND BRAIN SODIUM-CHANNELS WITH MULTIPLE MEMBERSOF THE SYNTROPHIN FAMILY OF DYSTROPHIN-ASSOCIATED PROTEINS

Syntrophins are cytoplasmic peripheral membrane proteins of the dystro phin-associated protein complex (DAPC). Three syntrophin isoforms, alp ha 1, beta 1, and beta 2, are encoded by distinct genes. Each contains two pleckstrin homology (PH) domains, a syntrophin-unique (SU) domain , and a PDZ domain. The name PDZ comes from the first three proteins f ound to contain repeats of this domain (PSD-95, Drosophila discs large protein, and the zona occludens protein 1). PDZ domains in other prot eins bind to the C termini of ion channels and neurotransmitter recept ors containing the consensus sequence (S/T)XV-COOH and mediate the clu stering or synaptic localization of these proteins. Two voltage-gated sodium channels (NaChs), SkM1 and SkM2, of skeletal and cardiac muscle , respectively, have this consensus sequence. Because NaChs are sarcol emmal components like syntrophins, we have investigated possible inter actions between these proteins. NaChs copurify with syntrophin and dys trophin from extracts of skeletal and cardiac muscle. Peptides corresp onding to the C-terminal 10 amino acids of SkM1 and SkM2 are sufficien t to bind detergent-solubilized muscle syntrophins, to inhibit the bin ding of native NaChs to syntrophin PDZ domain fusion proteins, and to bind specifically to PDZ domains from alpha 1-, beta 1-, and beta 2-sy ntrophin. These peptides also inhibit binding of the syntrophin PDZ do main to the PDZ domain of neuronal nitric oxide synthase, an interacti on that is not mediated by C-terminal sequences. Brain NaChs, which la ck the (S/T)XV consensus sequence, also co-purify with syntrophin and dystrophin, an interaction that does not appear to be mediated by the PDZ domain of syntrophin. Collectively, our data suggest that syntroph ins link NaChs to the actin cytoskeleton and the extracellular matrix via dystrophin and the DAPC.