A novel cytoplasmic interaction between junctin and ryanodine receptor calcium release channels

Abstract

Junctin, a non-catalytic splice variant of the aspartate-β-hydroxylase gene, is inserted into the membrane of the sarcoplasmic reticulum (SR) Ca2+ store where it modifies Ca2+ signalling in the heart and skeletal muscle through its regulation of ryanodine receptor (RyR) Ca2+ release channels. Junctin is required for normal muscle function as its knockout leads to abnormal Ca2+ signalling, muscle dysfunction and cardiac arrhythmia. However, junctin's binding interactions with RyRs are largely unknown and have been assumed to occur only in the SR lumen. We find robust binding of RyRs to full junctin, its luminal and unexpectedly its cytoplasmic domain, each with distinct effects on RyR1 and RyR2 activity. Full junctin in the luminal solution increases channel activity by ∼3-fold. The C-terminal luminal interaction inhibits RyR channel activity by ∼50%. The N-terminal cytoplasmic binding produces a ∼5-fold increase in RyR activity. The cytoplasmic interaction is required for luminal binding to replicate the influence of full junctin on RyR1 and RyR2 activity. The C-terminal domain of junctin binds to residues including S1–S2 linker of RyR1 and N-terminal junctin binds between RyR1 residues 1078-2156.