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 mu-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.