MITSUGUMIN29, A NOVEL SYNAPTOPHYSIN FAMILY MEMBER FROM THE TRIAD JUNCTION IN SKELETAL-MUSCLE

In skeletal muscle, excitation-contraction (E-C) coupling requires the conversion of the depolarization signal of the invaginated surface me mbrane, namely the transverse (T-) tubule, to Ca2+ release from the sa rcoplasmic reticulum (SR). Signal transduction occurs at the junctiona l complex between the T-tubule and SR, designated as the triad junctio n, which contains two components essential for E-C coupling, namely th e dihydropyridine receptor as the T-tubular voltage sensor and the rya nodine receptor as the SR Ca2+-release channel. However, functional ex pression of the two receptors seemed to constitute neither the signal- transduction system nor the junction between the surface and intracell ular membranes in cultured cells, suggesting that some as-yet-unidenti fied molecules participate in both the machinery. In addition, the mol ecular basis of the formation of the triad junction is totally unknown . It is therefore important to examine the components localized to the triad junction. Here we report the identification using monoclonal an tibody and primary structure by cDNA cloning of mitsugumin29, a novel transmembrane protein from the triad junction in skeletal :muscle. Thi s protein is homologous in amino acid sequence and shares characterist ic structural features with the members of the synaptophysin family. T he subcellular distribution and protein structure suggest that mitsugu min29 is involved in communication between the T-tubular and junctiona l SR membranes.