8.A.88  The Calciquestrin (CAQS) Family 

Calsequestrins (CSR1 and CSR2) are high-capacity, moderate affinity, calcium-binding proteins that act as internal calcium stores in the lumin of the muscle sarcoplasmic reticulum (SR). Calcium ions are bound by clusters of acidic residues at the protein surface, often at the interface between subunits. The protein can bind around 80 Ca2+ ions. It regulates the release of lumenal Ca2+ via the ryanodine calcium release channel RYR1 (TC# 1.A.3.1.2) which plays an important role in triggering muscle contraction (Sanchez et al. 2012). CSR also negatively regulates store-operated Ca2+ entry (SOCE) activity (Zhang et al. 2016).

This family belongs to the LysE Superfamily.



Chen, W. and M. Kudryashev. (2020). Structure of RyR1 in native membranes. EMBO Rep 21: e49891.

Fan, X.X., Z. Liu, H. Yu, X. Huang, C. Song, X. Guo, and S.Q. Wang. (2022). A non-transmembrane channel formed by Ca2+-bound calsequestrin-2. J Gen Physiol 154:.

Guerra, C. and M. Molinari. (2020). Thioredoxin-Related Transmembrane Proteins: TMX1 and Little Brothers TMX2, TMX3, TMX4 and TMX5. Cells 9:.

Haugstetter, J., T. Blicher, and L. Ellgaard. (2005). Identification and characterization of a novel thioredoxin-related transmembrane protein of the endoplasmic reticulum. J. Biol. Chem. 280: 8371-8380.

Ihara, M., S. Furutani, S. Shigetou, S. Shimada, K. Niki, Y. Komori, M. Kamiya, W. Koizumi, L. Magara, M. Hikida, A. Noguchi, D. Okuhara, Y. Yoshinari, S. Kondo, H. Tanimoto, R. Niwa, D.B. Sattelle, and K. Matsuda. (2020). Cofactor-enabled functional expression of fruit fly, honeybee, and bumblebee nicotinic receptors reveals picomolar neonicotinoid actions. Proc. Natl. Acad. Sci. USA 117: 16283-16291.

Lee, S., Y. Shin, K. Kim, Y. Song, Y. Kim, and S.W. Kang. (2020). Protein Translocation Acquires Substrate Selectivity Through ER Stress-Induced Reassembly of Translocon Auxiliary Components. Cells 9:.

Sanchez, E.J., K.M. Lewis, B.R. Danna, and C. Kang. (2012). High-capacity Ca2+ binding of human skeletal calsequestrin. J. Biol. Chem. 287: 11592-11601.

Zhang, L., L. Wang, S. Li, J. Xue, and D. Luo. (2016). Calsequestrin-1 Regulates Store-Operated Ca2+ Entry by Inhibiting STIM1 Aggregation. Cell Physiol Biochem 38: 2183-2193.


TC#NameOrganismal TypeExample

Calciquestrin (CASQ) of 396 aas and 1 TMS. Calsequestrin is a high-capacity (~80 Ca2+ ions), moderate affinity, calcium-binding protein and thus acts as an internal calcium store in muscle. Calcium ions are bound by clusters of acidic residues at the protein surface, often at the interface between subunits. Regulates the release of lumenal Ca2+ via the calcium release channel RYR1; thus playing a role in triggering muscle contraction (Sanchez et al. 2012).

Calciquestrin of Homo sapiens


Calciquestrin of 454 aas, a high-capacity, moderate affinity, calcium-binding protein that acts as an internal calcium store in muscle and interacts with a number of transporters.

Calciquestrin of Danio rerio (Zebrafish) (Brachydanio rerio)


Calsequestrin-2 of 484 aas

Calsequestring-2 of Ixodes ricinus (Common tick)


Calsequestrin 2, CSR2, CASQ2 of 399 aas and possibly one N-terminal TMS. While CSR1 (TC# 8.A.88.1.1) is found in the sarcoplasmic reticulum (SR) lumen of skeletal muscle, and possibly in the SR lumen of cardiac muscle (Chen and Kudryashev 2020). CSQ2) is a Ca2+-binding protein suppling Ca2+ for ryanodine receptor Ca2+ release during the excitation-contraction coupling in cardiomyocytes. CSQ2 mutations causes catecholaminergic polymorphic ventricular tachycardia (CPVT2), suggesting that it may function beyond that of a Ca2+ buffer. Fan et al. 2022 identified a non-transmembrane channel in Ca2+-enriched CSQ2 dimers, which facilitates fast Ca2+ mobilization. Using crystallography, they solved the high-resolution structure of Ca2+-bound CSQ2 and discovered that the negatively charged residues at the dimer interface encompassed a tubular channel-like structure, dubbed "tunnel," in which approximately 15 Ca2+ ions aligned across the approximately 5 nm tunnel path. To determine the potential tunnel conductance, the authors developed a graphene-based nanoelectronic technology to connect a CSQ2 dimer into a nanocircuit. In the Tyrode solution containing 1 mM Ca2+, a CSQ2 dimer exhibited a conductance one order of magnitude higher than the background level. This conductance was Ca2+ dependent, and was largely suppressed by the single-residue mutation D309N at the bottleneck region of the tunnel path, indicating that the tunnel conducted Ca2+ fluxes. When the D309N mutant CSQ2 was expressed in wild-type rat cardiomyocytes, isoproterenol treatment induced chaotic Ca2+ waves, delayed after-depolarizations and trigged activities resembling those occurring in CPVT2 models. This dominant negative effect of the CSQ2 mutation agreed well with the structural observation that CSQ2 tunnels were interconnected to form a tunnel network. Thus, CSQ2 builds a nano-highway network for energy-efficient Ca2+ mobilization in the SR. Factors that block the Ca2+ highway may lead to arrhythmogenesis (Fan et al. 2022).

CSR2 of Homo sapiens


TC#NameOrganismal TypeExample

Protein disulfide-isomerase A6 of 441 aas and 1 N-terminal TMS. This protein has an internal duplication.

PDI of Larimichthys crocea


Endoplasmic reticulum resident protein of 406 aas and one N-terminal TMS.

ERR protein of Anas platyrhynchos (Mallard) (Anas boschas)


Protein disulfide-isomerase, TMX3, of 454 aas and 2 TMSs, N- and C-terminal. It probably participates in the folding of proteins containing disulfide bonds as a dithiol oxidoreductase because it catalyzes the rearrangement of -S-S- bonds in proteins. (Haugstetter et al. 2005).

TMX3 of Homo sapiens


Uncharacterized probable protein disulfide oxidoreductase of 455 aas and 1 - 3 TMSs. This protein has an internal duplication.

UP of Ustilaginoidea virens


Thioredoxin-related transmembrane protein 1, TMX1, TMX, TXNDC, TXNDC1, UNQ235/PRO268, of 280 aas and 2 - 4 TMSs, one N-terminal, one C-terminal, and possibly 1 or 2 inbetween (Guerra and Molinari 2020). The ER of mammals contains more than 20 members of the Protein Disulfide Isomerase (PDI) family. These enzymes regulate formation, isomerization and disassembly of covalent bonds between cysteine residues. As such, PDIs ensure protein folding, which is required to attain functional and transport-competent structure, and protein unfolding, which facilitates dislocation of defective gene products across the ER membrane for ER-associated degradation (ERAD). The PDI family includes over a dozen soluble members and few membrane-bound ones. Among the latter are five PDIs grouped in the thioredoxin-related transmembrane (TMX) protein family (Guerra and Molinari 2020).

TMX1 of Homo sapiens


Thioredoxin-related transmembrane protein 3 (TMX3) is essential to enable robust expression in Xenopus laevis oocytes of honeybee (Apis mellifera) and bumblebee (Bombus terrestris) as well as fruit fly (Drosophila melanogaster) nAChR heteromers targeted by neonicotinoids and not hitherto robustly expressed. This has enabled the characterization of picomolar target site actions of neonicotinoids, findings important in understanding their toxicity (Ihara et al. 2020).



Protein disulfide-isomerase A3, PDIA3, ERP57, ERP60, GRP58, of 505 aas and 1 N-terminal TMS. Protein translocation in the ER via the general secretory pathway (3.A.5) acquires substrate selectivity through ER stress-induced reassembly of translocon auxiliary components including calnexin and ERP57 (Lee et al. 2020).

ERP57 of Homo sapiens