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Using a biochemical/immunological approach to analyse the protein constituents of skeletal-muscle junctional-face membrane (JFM), we identified a 45kDa protein. Its N-terminal amino acid was blocked, but the amino acid sequence obtained from several peptides after proteolytic treatment did not significantly match that of any protein present in the SwissProt and NCBI (National Center for Biotechnology Information) databases. We synthesized a peptide whose sequence matched that of one of the peptides obtained after CNBr cleavage of the 45kDa protein; the peptide was conjugated to a carrier and used to raise antibodies. The antiserum was used to study in more detail the biochemical characteristics of the novel 45kDa protein. Analysis of the proteins present in different subcellular membrane fractions show that the novel 45kDa polypeptide: (i) is an integral membrane constituent present both in neonatal and adult skeletal-muscle sarcoplasmic reticulum; (ii) is selectively localized in the JFM; (iii) is not present in microsomes obtained from rabbit heart, liver or kidney. Immunoprecitation with anti-(45kDa protein) antibody indicates that the 45kDa protein is part of a complex which can be phosphorylated in vitro by the catalytic subunit of protein kinase A.

DNA analysis to detect mutations which cosegregate with MH as well as biochemical assays on cultured lymphocytes obtained from blood can serve as useful diagnostic tools for MH susceptibility and genotype-phenotype correlations.

We investigated the functional role of JP-45, a recently discovered protein of the junctional face membrane (JFM) of skeletal muscle. For this purpose, we expressed JP-45 C-terminally tagged with the fluorescent protein DsRed2 by nuclear microinjection in myotubes derived from the C2C12 skeletal muscle cell line and performed whole-cell voltage-clamp experiments. We recorded in parallel cell membrane currents and Ca 2+ signals using fura-2 during step depolarization.

JP-45 is a novel integral protein constituent of the skeletal muscle sarcoplasmic reticulum junctional face membrane. We identified its primary structure from a cDNA clone isolated from a mouse skeletal muscle cDNA library. Mouse skeletal muscle JP-45 displays over 86 and 50% identity with two hypothetical NCBI data base protein sequences from mouse tongue and human muscle, respectively. JP-45 is predicted to have a cytoplasmic domain, a single transmembrane segment followed by an intralumenal domain enriched in positively charged amino acids. Northern and Western blot analyses reveal that the protein is mainly expressed in skeletal muscle. The mRNA encoding JP-45 appears in 17-day-old mouse embryos; expression of the protein peaks during the second month of postnatal development and then decreases ϳ3-fold during aging. Double immunofluorescence of adult skeletal muscle fibers demonstrates that JP-45 co-localizes with the sarcoplasmic reticulum calcium release channel. Co-immunoprecipitation experiments with a monoclonal antibody against JP-45 show that JP-45 interacts with the ␣1.1 subunit voltage-gated calcium channel and calsequestrin. These results are consistent with the localization of JP-45 in the junctional sarcoplasmic reticulum and with its involvement in the molecular mechanism underlying skeletal muscle excitation-contraction coupling.

The impact of calcium signaling on many cellular functions is reflected by the tight regulation of the intracellular Ca(2+) concentration that is ensured by diverse pumps, channels, transporters and Ca(2+) binding proteins. In this review, we present recently identified novel sarco(endo)plasmic reticulum proteins that may have a potential involvement in the regulation of Ca(2+) homeostasis in striated muscles.