A novel isoform of δ-sarcoglycan is localized at the sarcoplasmic reticulum of mouse skeletal muscle

The sarcoglycan–sarcospan complex (α-, β-, γ-, δ-, ε-, and ζ-SG–SSPN), a component of the dystrophin-associated glycoprotein complex (DAGC), is located at the sarcolemma of muscle fibers where it contributes to maintain cell integrity during contraction–relaxation cycles; γ- and δ-SG are also expressed in the sarcoplasmic reticulum (SR). In this study, we report the identification of a novel isoform of murine δ-SG produced by alternative splicing that we named δ-SG3. This isoform is present at transcript level in several tissues, with its highest expression in skeletal and cardiac muscle. The δ-SG3 protein lacks the last 122 amino acids at the C-terminal, which are replaced by 10 new amino acids (EGFLNMQLAG). Interestingly, double immunofluorescence analysis for δ-SG3 and the dihydropyridine receptor (DHPR) shows a close localization of these two proteins. We propose the subcellular distribution of this novel δ-SG3 isoform at the SR and its involvement in intracellular calcium concentration regulation.