Research ArticleStructural Biology

A cryo-EM–based model of phosphorylation- and FKBP12.6-mediated allosterism of the cardiac ryanodine receptor

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Sci. Signal.  23 May 2017:
Vol. 10, Issue 480, eaai8842
DOI: 10.1126/scisignal.aai8842

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More flexible with phosphorylation?

The type 2 ryanodine receptor (RyR2) mediates Ca2+ release from the sarcoplasmic reticulum of cardiomyocytes to initiate cardiac muscle contraction. Mutations in this intracellular Ca2+ channel are associated with cardiac diseases that may lead to heart failure. Dhindwal et al. used cryo-EM to determine the structure of rabbit RyR2 in complex with the regulatory protein FKBP12.6 in the closed state at 11.8 Å resolution. They found two conformations of RyR2, which may correspond to the extent of phosphorylation of a domain that harbors several disease-associated mutations. Because the more flexible conformation may correspond to phosphorylated RyR2, the authors suggest that phosphorylation may reduce the energy required for the Ca2+ channel to transition to an open state. These results provide a structural basis for understanding how phosphorylation may affect the activation of RyR2.

Abstract

Type 2 ryanodine receptors (RyR2s) are calcium channels that play a vital role in triggering cardiac muscle contraction by releasing calcium from the sarcoplasmic reticulum into the cytoplasm. Several cardiomyopathies are associated with the abnormal functioning of RyR2. We determined the three-dimensional structure of rabbit RyR2 in complex with the regulatory protein FKBP12.6 in the closed state at 11.8 Å resolution using cryo-electron microscopy and built an atomic model of RyR2. The heterogeneity in the data set revealed two RyR2 conformations that we proposed to be related to the extent of phosphorylation of the P2 domain. Because the more flexible conformation may correspond to RyR2 with a phosphorylated P2 domain, we suggest that phosphorylation may set RyR2 in a conformation that needs less energy to transition to the open state. Comparison of RyR2 from cardiac muscle and RyR1 from skeletal muscle showed substantial structural differences between the two, especially in the helical domain 2 (HD2) structure forming the Clamp domain, which participates in quaternary interactions with the dihydropyridine receptor and neighboring RyRs in RyR1 but not in RyR2. Rigidity of the HD2 domain of RyR2 was enhanced by binding of FKBP12.6, a ligand that stabilizes RyR2 in the closed state. These results help to decipher the molecular basis of the different mechanisms of activation and oligomerization of the RyR isoforms and could be extended to RyR complexes in other tissues.

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