3 research outputs found
Junctate is a key element in calcium entry induced by activation of InsP3 receptors and/or calcium store depletion
In many cell types agonist-receptor activation leads to a rapid and transient release of Ca2+ from intracellular stores via activation of inositol 1,4,5 trisphosphate (InsP3) receptors (InsP3Rs). Stimulated cells activate store- or receptor-operated calcium channels localized in the plasma membrane, allowing entry of extracellular calcium into the cytoplasm, and thus replenishment of intracellular calcium stores. Calcium entry must be finely regulated in order to prevent an excessive intracellular calcium increase. Junctate, an integral calcium binding protein of endo(sarco)plasmic reticulum membrane, (a) induces and/or stabilizes peripheral couplings between the ER and the plasma membrane, and (b) forms a supramolecular complex with the InsP3R and the canonical transient receptor potential protein (TRPC) 3 calcium entry channel. The full-length protein modulates both agonist-induced and store depletion–induced calcium entry, whereas its NH2 terminus affects receptor-activated calcium entry. RNA interference to deplete cells of endogenous junctate, knocked down both agonist-activated calcium release from intracellular stores and calcium entry via TRPC3. These results demonstrate that junctate is a new protein involved in calcium homeostasis in eukaryotic cells
Molecular cloning, expression, functional characterization, chromosomal localization, and gene structure of junctate, a novel integral calcium binding protein of sarco(endo)plasmic reticulum membrane
Screening a cDNA library from human skeletal muscle and cardiac muscle with a cDNA probe derived from junctin led to the isolation of two groups of cDNA clones. The first group displayed a deduced amino acid sequence that is 84% identical to that of dog heart junctin, whereas the second group had a single open reading frame that encoded a polypeptide with a predicted mass of 33 kDa, whose first 78 NH(2)-terminal residues are identical to junctin whereas its COOH terminus domain is identical to aspartyl beta-hydroxylase, a member of the alpha-ketoglutarate-dependent dioxygenase family. We named the latter amino acid sequence junctate. Northern blot analysis indicates that junctate is expressed in a variety of human tissues including heart, pancreas, brain, lung, liver, kidney, and skeletal muscle. Fluorescence in situ hybridization analysis revealed that the genetic loci of junctin and junctate map to the same cytogenetic band on human chromosome 8. Analysis of intron/exon boundaries of the genomic BAC clones demonstrate that junctin, junctate, and aspartyl beta-hydroxylase result from alternative splicing of the same gene. The predicted lumenal portion of junctate is enriched in negatively charged residues and is able to bind calcium. Scatchard analysis of equilibrium (45)Ca(2+) binding in the presence of a physiological concentration of KCl demonstrate that junctate binds 21.0 mol of Ca(2+)/mol protein with a k(D) of 217 +/- 20 microm (n = 5). Tagging recombinant junctate with green fluorescent protein and expressing the chimeric polypeptide in COS-7-transfected cells indicates that junctate is located in endoplasmic reticulum membranes and that its presence increases the peak amplitude and transient calcium released by activation of surface membrane receptors coupled to InsP(3) receptor activation. Our study shows that alternative splicing of the same gene generates the following functionally distinct proteins: an enzyme (aspartyl beta-hydroxylase), a structural protein of SR (junctin), and a membrane-bound calcium binding protein (junctate)