The emerging role of calsequestrin 2: from calcium sensor and modulator to arrhythmia driver

Fig. 1. Domain organization of calsequestrin 2 (CASQ2). Structural architecture of the CASQ2 protein, which is composed of three distinct acidic thioredoxin-like (TrxL) domains (Domain I, Domain II, Domain III) and an unstructured C-terminal tail. The domains are arranged in a compact, globular structure, with each domain characterized by a β-sheet flanked by α-helices. The figure highlights the flexibility of CASQ2, enabling it to bind Ca2+ and undergo polymerization, which is critical for Ca2+ buffering in cardiac muscle cells. The N-terminal and C-terminal regions, marked by their acidic residues, are key in the protein’s functionality, contributing to its role in Ca2+ storage and release within the jSR

Fig. 2. Calsequestrin 2 (CASQ2) higher order multimerization into filaments. Structure of several conformational states of CASQ2. The protein exists as a monomer and a wide range of higher order multimers. Cartoon as well as surface representation of CASQ2 monomer, dimer, tetramer and filament (PDBID: 6OWV). The CASQ2 monomer consists of three thioredoxin domains (domain I, II and III represented in green, pink and blue respectively). The higher multimer forming contacts exist between dimers and stacks along the screw axis to form the filament

Fig. 3. Ca2+-Dependent Regulation of RyR2 by CASQ2 within the Junctional Sarcoplasmic Reticulum (jSR) of Cardiac Muscle. At high luminal Ca²⁺ concentrations (≥ 5 mM), CASQ2 has the lowest affinity for RyR2, polymerizes in the SR lumen enhancing Ca²⁺ storage and has no inhibitory effect on RyR2. As luminal Ca²⁺ decreases (~ 250 µM − 1 mM), CASQ2 depolymerizes into small oligomers with intermediate affinity for RyR2 and minimal regulatory effect on the Ca²⁺ channel. When Ca²⁺ levels fall to ≤ 20 µM, monomeric CASQ2 has the highest affinity for the RyR2 and effectively inhibits RyR2 channel activity via interactions with triadin and junctin, preventing spontaneous Ca²⁺ release. This reversible mechanism ensures controlled Ca²⁺ dynamics critical for cardiac contraction and relaxation

Fig. 4. Location of disease associated mutations on the CASQ2 structure. CASQ2 monomeric crystal structure (PDBID: 6OWV) displaying the position of point mutations mentioned in Table 1. The locations of point mutations on CASQ2 domain I (left) are represented as spheres in green color. The locations of point mutations on CASQ2 domain II (middle) are shown as spheres in pink. Positions of domain III (right) mutations are shown as spheres in blue color. CASQ2 protein is colored domain-wise, domain I as green, domain II as pink and domain III as blue