FKBP association with RyR channels: effect of CLIC2 binding on sub-conductance opening and FKBP binding

Abstract

Ryanodine receptor (RyR) calcium channels are central to striated muscle function and influence signalling in neurones and other cell types. Beneficially low RyR activity and maximum conductance opening may be stabilised when RyRs bind to FK506 binding proteins (FKBPs) and destabilised by FKBP dissociation, with submaximal opening during RyR hyperactivity associated with myopathies and neurological disorders. However this is debated and quantitative evidence is lacking. Here we have measured altered FKBP binding to RyRs and submaximal activity with addition of wild-type (WT) CLIC2, an inhibitory RyR ligand, or its H101Q mutant that hyperactivates RyRs, likely causing cardiac and intellectual abnormalities. The proportion of sub-conductance opening increases with WT and H101Q CLIC2 and is correlated with reduced FKBP/RyR association. The sub-conductance opening reduces RyR currents in the presence of WT CLIC2. In contrast, sub-conductance openings contribute to excess RyR “leak” with H101Q CLIC2. There are significant FKBP and RyR isoform-specific actions of CLIC2, rapamycin and FK506 on FKBP/RyR association. The results show that FKBPs do influence RyR gating and that this would contribute to excess Ca2+ release in one RyR channelopathy.