Calsequestrin and the calcium release channel of skeletal and cardiac muscle

Abstract

Calsequestrin is by far the most abundant Ca(2+)-binding protein in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. It allows the Ca2+ required for contraction to be stored at total concentrations of up to 20mM, while the free Ca2+ concentration remains at approximately 1mM. This storage capacity confers upon muscle the ability to contract frequently with minimal run-down in tension. Calsequestrin is highly acidic, containing up to 50 Ca(2+)-binding sites, which are formed simply by clustering of two or more acidic residues. The Kd for Ca2+ binding is between 1 and 100 microM, depending on the isoform, species and the presence of other cations. Calsequestrin monomers have a molecular mass of approximately 40 kDa and contain approximately 400 residues. The monomer contains three domains each with a compact alpha-helical/beta-sheet thioredoxin fold which is stable in the presence of Ca2+. The protein polymerises when Ca2+ concentrations approach 1mM. The polymer is anchored at one end to ryanodine receptor (RyR) Ca2+ release channels either via the intrinsic membrane proteins triadin and junctin or by binding directly to the RyR. It is becoming clear that calsequestrin has several functions in the lumen of the SR in addition to its well-recognised role as a Ca2+ buffer. Firstly, it is a luminal regulator of RyR activity. When triadin and junctin are present, calsequestrin maximally inhibits the Ca2+ release channel when the free Ca2+ concentration in the SR lumen is 1mM. The inhibition is relieved when the Ca2+ concentration alters, either because of small changes in the conformation of calsequestrin or its dissociation from the junctional face membrane. These changes in calsequestrin's association with the RyR amplify the direct effects of luminal Ca2+ concentration on RyR activity. In addition, calsequestrin activates purified RyRs lacking triadin and junctin. Further roles for calsequestrin are indicated by the kinase activity of the protein, its thioredoxin-like structure and its influence over store operated Ca2+ entry. Clearly, calsequestrin plays a major role in calcium homeostasis that extends well beyond its ability to buffer Ca2+ ions.

T-tubule disorganization and defective excitation-contraction coupling in muscle fibers lacking myotubularin lipid phosphatase

“…We also examined the expression level of key regulators of the E-C coupling process in TA muscle. By using quantitative RT-PCR, we measured the mRNA level of the ␣ 1 , ␤ 1 , and ␥ 1 subunits of the voltage-sensing DHPR (Cacna1s, Cacnb1, and Cacng1), the SR Ca 2ϩ release channel type 1 ryanodine receptor (RyR1), type 1 and 2 SERCA pumps (Atp2a1, Atp2a2), and calsequestrin 1 and 2 (Casq1, Casq2), which encode the major Ca 2ϩ -buffering proteins in the lumen of striated muscle SR (23). In 5-week-old-mice, we found that Cacnb1 and Cacng1 mRNAs were significantly increased by 1.3-and 3-fold, respectively, whereas transcript level of Casq2 was decreased by about 30% (Fig.…”

Transcriptional Analysis of the Human Cardiac Calsequestrin Gene in Cardiac and Skeletal Myocytes

“…The SR is the main storage site of intracellular calcium, storing Ca 2ϩ up to 20 mM while maintaining the free SR Ca 2ϩ concentration at ϳ1 mM (2,3). The SR storage capacity enables the muscle cell to continuously contract without diminishing the Ca 2ϩ available for each contraction-relaxation cycle.…”

“…It is a highly acidic protein with over 50 Ca 2ϩ binding sites, which are formed by the clustering of two or more negatively charged residues. The molecular weight of the CASQ monomer is 40 kDa; and is capable of polymerizing in response to increasing Ca 2ϩ concentrations (Ͼ1 mM) (3). CASQ locates near the ryanodine receptor and attaches to it, via direct interaction or through anchoring by triadin and junctin.…”

Role of Junctin Protein Interactions in Cellular Dynamics of Calsequestrin Polymer upon Calcium Perturbation

“…Sarcoplasmic reticulum (SR) 3 is a major Ca 2ϩ storage reservoir in muscle. The SR Ca 2ϩ release channel, ryanodine receptor (RyR), forms a large macromolecular complex with transmembrane, luminal, and cytoplasmic components (1).…”