Novel modulators of skeletal muscle FKBP12/calcium channel complex from Ianthella basta. Role of FKBP12 in channel gating.

Macrocyclic natural products derived from bromotyrosine isolated from the sponge Ianthella basta are shown to selectively modulate the skeletal isoform of the ryanodine-sensitive sarcoplasmic reticulum calcium channel by a novel mechanism involving the FKBP12/RyR-1 complex. Bastadins 5, 7, and the newly identified isomer of bastadin 5, bastadin 19, show marked differences in potency and efficacy toward activation of the binding of [3H]ryanodine. In physiological salt, bastadin 5 (5 microM) increases the [3H]ryanodine binding capacity of SR membranes 5-fold, by stabilizing the high affinity conformation of RyR-1 for ryanodine without shifting the affinity of the activator site for Ca2+ or altering the response to caffeine or adenine nucleotides. Bastadin 5 decreases the inhibitory potency of Mg2+ 8-fold and high (> 100 microM) Ca2+ 5-fold. Bastadin 5 inhibits Ca2+ uptake into SR vesicles and enhances Ca(2+)-induced Ca2+ release 8-fold. Bastadin 5 increases single-channel open dwell time, tau 1 and tau 2, 65- and 92-fold, respectively, without changing unitary conductance for Cs+ (450 picosiemans) or open probability. Most significant is the finding that the unique actions of bastadin 5 on [3H]ryanodine binding and Ca2+ transport are antagonized by the immunosuppressant FK506. FK506 alone weakly enhances the binding of [3H]ryanodine, compared to bastadin 5. However, FK506 diminishes bastadin 5-induced changes in [3H]ryanodine binding and Ca2+ transport without altering the efficacy of adenine nucleotides. Unlike FK506, bastadin 5 does not directly promote the dissociation of FKBP12 from the RyR-1 membrane complex; however, it markedly enhances the release of FKBP12 induced by FK506. These results suggest that the bastadin 5 effector site is a novel modulatory domain on FKBP12. Bastadins represent a new class of compounds to gain insight into the functional interactions between FKBP12 and RyR-1.