Complex of Calmodulin with a Ryanodine Receptor Target Reveals a Novel, Flexible Binding Mode

Abstract

Calmodulin regulates ryanodine receptor-mediated Ca(2+) release through a conserved binding site. The crystal structure of Ca(2+)-calmodulin bound to this conserved site reveals that calmodulin recognizes two hydrophobic anchor residues at a novel "1-17" spacing that brings the calmodulin lobes close together but prevents them from contacting one another. NMR residual dipolar couplings demonstrate that the detailed structure of each lobe is preserved in solution but also show that the lobes experience domain motions within the complex. FRET measurements confirm the close approach of the lobes in binding the 1-17 target and show that calmodulin binds with one lobe to a peptide lacking the second anchor. We suggest that calmodulin regulates the Ca(2+) channel by switching between the contiguous binding mode seen in our crystal structure and a state where one lobe of calmodulin contacts the conserved binding site while the other interacts with a noncontiguous site on the channel.

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

“…The atomic structure of Ca 2ϩ CaM in complex with the RyR1 3614 -3643 fragment was recently solved by Mackenzie and coworkers (12). Their findings detail the antiparallel binding of Ca 2ϩ CaM to the RyR1 target helix first suggested by Hamilton and coworkers (10) and reveal a unique wide spacing of hydrophobic anchors at Trp-3620 and Phe-3636.…”

“…In binding the RyR1 target, the 2 lobes of Ca 2ϩ CaM are therefore positioned apart and do not display the close apposition observed in CaM's complexes with kinase targets (12). Notably, Ca 2ϩ -dependent structural rearrangements that underlie CaM regulation are not revealed by the atomic structure of MacKenzie and coworkers (12), and likely involve additional interactions at noncontiguous sites within the full-length RyR1 (12)(13)(14)(15). Thus, the relationship between biochemical evidence suggesting a shared binding site for apo-CaM and Ca 2ϩ CaM and the large-scale translocations of mass suggested by cryo-EM remains unclear.…”

“…Their findings detail the antiparallel binding of Ca 2ϩ CaM to the RyR1 target helix first suggested by Hamilton and coworkers (10) and reveal a unique wide spacing of hydrophobic anchors at Trp-3620 and Phe-3636. In binding the RyR1 target, the 2 lobes of Ca 2ϩ CaM are therefore positioned apart and do not display the close apposition observed in CaM's complexes with kinase targets (12). Notably, Ca 2ϩ -dependent structural rearrangements that underlie CaM regulation are not revealed by the atomic structure of MacKenzie and coworkers (12), and likely involve additional interactions at noncontiguous sites within the full-length RyR1 (12)(13)(14)(15).…”

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

“…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 structure of CaM bound to a RyR1 peptide (3614-3643) has been visualized by NMR residual dipolar coupling (Maximciuc et al 2006). Amino acids 3615 -3628 contact the carboxy lobe of CaM, whereas amino acids 3628 -3637 bind the amino lobe of CaM.…”

Calmodulin has the Potential to Function as a Ca2+-Dependent Adaptor Protein

“…Structures of some Ca 2+ -CaM-IQ domain complexes have revealed wrap-around binding modes, albeit with differences in lobe and peptide orientation compared to other complexes. [18][19][20] For a discussion about the mechanisms of CaM-dependent ion channel regulation (see ref. 16).…”