High-capacity Ca2+ Binding of Human Skeletal Calsequestrin

Calsequestrin, the major calcium storage protein in both cardiac and skeletal muscle, binds large amounts of Ca2+ in the sarcoplasmic reticulum and releases them during muscle contraction. For the first time, the crystal structures of Ca2+ complexes for both human (hCASQ1) and rabbit (rCASQ1) skeletal calsequestrin were determined, clearly defining their Ca2+ sequestration capabilities through resolution of high- and low-affinity Ca2+-binding sites. rCASQ1 crystallized in low CaCl2 buffer reveals three high-affinity Ca2+ sites with trigonal bipyramidal, octahedral, and pentagonal bipyramidal coordination geometries, along with three low-affinity Ca2+ sites. hCASQ1 crystallized in high CaCl2 shows 15 Ca2+ ions, including the six Ca2+ ions in rCASQ1. Most of the low-affinity sites, some of which are μ-carboxylate-bridged, are established by the rotation of dimer interfaces, indicating cooperative Ca2+ binding that is consistent with our atomic absorption spectroscopic data. On the basis of these findings, we propose a mechanism for the observed in vitro and in vivo dynamic high-capacity and low-affinity Ca2+-binding activity of calsequestrin. Background: Calsequestrin is a calcium storage/buffer protein within the sarcoplasmic reticulum and binds large amounts of Ca2+ in a unique manner. Results: The specific coordination, geometry, and cooperative effects of Ca2+ binding were determined. Conclusion: The oligomeric state of calsequestrin is directly related to high-capacity Ca2+ binding. Significance: This is the first report of specific Ca2+ coordination sites in calsequestrin and provides a pathological link to related disorders.