Alternative splicing of dystrobrevin regulates the stoichiometry of syntrophin binding to the dystrophin protein complex

Dystrophin coordinates the assembly of a complex of structural and signalli ng proteins that is required for normal muscle function, A key component of the dystrophin-associated protein complex (DPC) is alpha -dystrobrevin, a dystrophin-related and -associated protein whose absence results in muscula r dystrophy and neuromuscular junction defects [1,2]. The current model of the DPC predicts that dystrophin and dystrobrevin each bind a single syntro phin molecule [3], The syntrophins are PDZ-domain-containing proteins that facilitate the recruitment of signalling proteins such as nNOS (neuronal ni tric oxide synthase) to the DPC [4], Here we show, using yeast two-hybrid a nalysis and biochemical binding studies, that alpha -dystrobrevin in fact c ontains two independent syntrophin-binding sites in tandem. The previously undescribed binding site is situated within an alternatively spliced exon o f alpha -dystrobrevin, termed the variable region-3 (vr3) sequence, which i s specifically expressed in skeletal and cardiac muscle [5,6]. Analysis of the syntrophin-binding region of dystrobrevin reveals a tandem pair of pred icted a helices with significant sequence similarity. These a helices, each termed a syntrophin-binding motif, are also highly conserved in dystrophin and utrophin, Together these data show that there are four potential syntr ophin-binding sites per dystrophin complex in skeletal muscle: two on dystr obrevin and two on dystrophin or utrophin, Furthermore, alternative splicin g of dystrobrevin provides a mechanism for regulating the stoichiometry of syntrophin association with the DPc, This is likely to have important conse quences for the recruitment of specific signalling molecules to the DPc and ultimately for its function.