The marine sponge Ianthella basta synthesizes at least 25 tetrameric bromotyrosine structures that possess a stringent structural requirement for modifying the gating behavior of ryanodine-sensitive Ca2+ channels (ryanodine receptors) (RyR)). Bastadin 5 (B5) was shown to stabilize open and closed channel states with little influence on the sensitivity of the channel to activation by Ca2+ (Mack, M. M., Molinski, T. F., Buck, E. D., and Pessah, I. N. (1994) J. Biol. Chem. 269, 23236-23249). In the present paper, we utilize single channel analysis and measurements of Ca2+ flux across the sarcoplasmic reticulum to identify bastadin 10 (B10) as the structural congener responsible for dramatically stabilizing the open conformation of the RyR channel, possibly by reducing the free energy associated with closed to open channel transitions (ΔG*c→o). The stability of the channel open state induced by B10 sensitized the channel to activation by Ca2+ to such an extent that it essentially obviated regulation by physiological concentrations of Ca2+ and relieved inhibition by physiological Mg2+. These actions of B10 were produced only on the cytoplasmic face of the channel, were selectively eliminated by pretreatment of channels with FK506 or rapamycin, and were reconstituted by human recombinant FKBP12. The actions of B10 were found to be reversible. A structure-activity model is proposed by which substitutions on the Eastern and Western hemispheres of the bastarane macrocycle may confer specificity toward the RyR1-FKBP12 complex to stabilize either the closed or open channel conformation. These results indicate that RyR1-FKBP12 complexes possesses a novel binding domain for phenoxycatechols and raise the possibility of molecular recognition of an endogenous ligand.
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