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.