The skeletal muscle sarcoplasmic reticulum (SR) Ca2+ release channel or ryanodine receptor (RyR1) binds four molecules of FKBP12, and the interaction of FKBP12 with RyR1 regulates both unitary and coupled gating of the channel. We have characterized the physiologic effects of previously identified mutations in RyR1 that disrupt FKBP12 binding (V2461G and V2461I) on excitation-contraction (EC) coupling and intracellular Ca2+ homeostasis following their expression in skeletal myotubes derived from RyR1-knockout (dyspedic) mice. Wild-type RyR1-, V246I-, and V2461G-expressing myotubes exhibited similar resting Ca2+ levels and maximal responses to caffeine (10 mM) and cyclopiazonic acid (30 μM). However, maximal voltage-gated Ca2+ release in V2461G-expressing myotubes was reduced by ∼50% compared with that attributable to wild-type RyR1 (ΔF/Fmax = 1.6 ± 0.2 and 3.1 ± 0.4, respectively). Dyspedic myotubes expressing the V2461I mutant protein, that binds FKBP12.6 but not FKBP12, exhibited a comparable reduction in voltage-gated SR Ca2+ release (ΔF/Fmax = 1.0 ± 0.1). However, voltage-gated Ca2+ release in V2461I-expressing myotubes was restored to a normal level (ΔF/Fmax = 2.9 ± 0.6) following co-expression of FKBP12.6. None of the mutations that disrupted FKBP binding to RyR1 significantly affected RyR1-mediated enhancement of L-type Ca2+ channel activity (retrograde coupling). These data demonstrate that FKBP12 binding to RyR1 enhances the gain of skeletal muscle EC coupling.
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