Cardiac Calsequestrin is a Physiological Dimer that Polymerizes through a Ca²⁺-Triggered Cooperative Switch

Cardiac Calsequestrin (CASQ2) polymerizes within the junctional sarcoplasmic reticulum to buffer Ca²⁺ and regulate ryanodine receptor 2 (RyR2) gating, yet the molecular mechanism governing this process remains poorly understood. Using an integrated set of complementary approaches spanning single-particle biophysics, bulk solution measurements, and polymer chemistry, we demonstrate that CASQ2 is an intrinsic dimer at nanomolar concentrations and under physiological ionic conditions, independently of Ca²⁺. In addition, Ca²⁺-dependent polymerization operates as a highly cooperative switch between a stable oligomeric phase and a high-order polymeric state. Physiological amounts of K⁺ ions modulate this switch through a biphasic electrostatic mechanism, supporting polymerization at low concentrations and inhibiting it beyond charge neutralization (∼194 mM). These findings redefine CASQ2 as an intrinsic dimer with polymerization-switch properties, and provide a mechanistic framework for understanding how catecholaminergic polymorphic ventricular tachycardia type 2 mutations, distributed evenly across the CASQ2 surface, cause disease through two distinct pathological trajectories.