THE HUMAN CARDIAC-MUSCLE RYANODINE RECEPTOR-CALCIUM RELEASE CHANNEL -IDENTIFICATION, PRIMARY STRUCTURE AND TOPOLOGICAL ANALYSIS

Rapid Ca2+ efflux from intracellular stores during cardiac muscle exci tation-contraction coupling is mediated by the ryanodine-sensitive cal cium-release channel, a large homotetrameric complex present in the sa rcoplasmic reticulum. We report here the identification, primary struc ture and topological analysis of the ryanodine receptor-calcium releas e channel from human cardiac muscle (hRyR-2). Consistent with sediment ation and immunoblotting studies an the hRyR-2 protein, sequence analy sis of ten overlapping cDNA clones reveals an open reading frame of 14 901 nucleotides encoding a protein of 4967 amino acid residues with a predicted molecular mass of 564 569 Da for hRyR-2. In-frame insertions corresponding to eight and ten amino acid residues were found in two of the ten cDNAs isolated, suggesting that novel, alternatively splice d transcripts of the hRyR-2 gene might exist. Six hydrophobic stretche s, which are present within the hRyR-, C-terminal 500 amino acids and are conserved in all RyR sequences, may be involved in forming the tra nsmembrane domain that constitutes the Ca2+-conducting pathway, in agr eement with competitive ELISA studies with a RyR-2-specific antibody. Sequence alignment of hRyR-2 with other RyR isoforms indicates a high level of overall identity within the RyR family, with the exception of two important regions that exhibit substantial variability. Phylogene tic analysis suggests that the RyR-2 isoform diverged from a single an cestral gene before the RyR-1 and RyR-3 isoforms to form a distinct br anch of the RyR family tree.