Apocalmodulin and Ca2+-Calmodulin Bind to Neighboring Locations on the Ryanodine Receptor

Abstract

Calmodulin (CaM) binds to the ryanodine receptor/ calcium release channel of skeletal muscle (RyR1), both in the absence and presence of Ca 2؉ , and regulates the activity of the channel activity by activating and inhibiting it, respectively. Using cryo-electron microscopy and three-dimensional reconstruction, we found that one apoCaM binds per RyR1 subunit along the sides of the cytoplasmic assembly of the receptor. This location is distinct from but close to the location found for Ca 2؉ -CaM, providing a structural basis for efficient switching of CaM between these two positions with the oscillating intracellular Ca 2؉ concentration that generates muscle relaxation/contraction cycles. The locations of apoCaM and Ca 2؉ -CaM at a critical region for RYR1-dihydropyridine receptor interaction are suggestive of a direct role for CaM in the mechanism of excitation-contraction coupling.

Different Regions in Skeletal and Cardiac Muscle Ryanodine Receptors Are Involved in Transducing the Functional Effects of Calmodulin

“…Cryomicroscopy studies revealed that the CaM-binding site in the skeletal muscle RyR isoform is at least 10 nm away from the effector site (pore) and that Ca 2ϩ binding causes an ϳ33-Å shift of the binding site (22). Therefore, long range conformational changes are likely involved in the modulation of RyR activity by CaM.…”

Ryanodine Receptors: Structure, Expression, Molecular Details, and Function in Calcium Release

“…Cryo-EM difference mapping of the three-dimensional structures of RyR1 with and without added CaM has suggested that the CaCaM binding site is located in subdomain 3. The site seems to be displaced to 33 Å in the presence of Ca 2þ with respect to its position for apoCaM Wagenknecht et al 1997;Samsó and Wagenknecht 2002). This displacement could be caused by a movement of the CaM upon binding calcium and/or a movement of the CaM binding site when RyR1 binds Ca 2þ .…”

“…The clamps (Fig. 3, subdomains 5, 6, 7, 8, 9, 10) undergo major conformational changes during the opening and closing of the channel (Serysheva et al 2008;Samsó et al 2009), are likely to participate in intermolecular interactions with neighboring RyRs, and are the sites of interactions with modulators Wagenknecht et al 1996;Wagenknecht et al 1997;Samsó and Wagenknecht 2002;Samsó et al 2006;Sharma et al 2006;Meng et al 2009). Two of the areas of high divergence in the primary sequence of the RyR isoforms were mapped in the clamps Liu et al 2004).…”

FRET-based mapping of calmodulin bound to the RyR1 Ca 2+ release channel

“…6B). By comparison, these simulations predict much weaker energy transfer in the presence of micromolar Ca 2ϩ (R Ca ; predicted FRET ϭ 0.16) because of the 33-Å shift in the apparent center of mass of Ca 2ϩ CaM bound to RyR1 (7). Note that predicted distances to CaM on the adjacent face of the channel are greater (RЈ apo ϭ 122 Ϯ 5 Å, RЈ Ca ϭ 110 Ϯ 5 Å; Fig.…”

“…may be linked to large-scale structural rearrangements, involving translocation of either CaM itself or of the underlying RyR1 CaM-binding domain (7).…”