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 skele tal isoform of the ryanodine-sensitive sarcoplasmic reticulum calcium channel by a novel mechanism involving the FKBP12/ RyR-1 complex. Bast adins 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 [H-3]ryanodine. In physiological salt, bastadin 5 (5 m u M) increases the [H-3]ryanodine binding capacity of SR membranes 5-f old, by stabilizing the high affinity conformation of RyR-1 for ryanod ine without shifting the affinity of the activator site for Ca2+ or al tering the response to caffeine or adenine nucleotides. Bastadin 5 dec reases the inhibitory potency of Mg2+ 8-fold and high (> 100 mu M) Ca2 + 5-fold. Bastadin 5 inhibits Ca2+ uptake into SR vesicles and enhance s Ca2+-induced Ca2+ release 8-fold. Bastadin 5 increases single-channe l open dwell time, tau(1) and tau(2) 65- and 92-fold, respectively, wi thout changing unitary conductance for Cs+ (450 picosiemans) or open p robability. Most significant is the finding that the unique actions of bastadin 5 on [H-3]ryanodine binding and Ca2+ transport are antagoniz ed by the immunosuppressant FK506. FK506 alone weakly enhances the bin ding of [H-3]ryanodine, compared to bastadin 5. However, FK506 diminis hes bastadin 5-induced changes in [H-3]ryanodine binding and Ca2+ tran sport without altering the efficacy of adenine nucleotides. Unlike FK5 06, bastadin 5 does not directly promote the dissociation of FKBP12 fr om the RyR-1 membrane complex; however, it markedly enhances the relea se of FKBP12 induced by FK506, These results suggest that the bastadin 5 effector site is a novel modulatory domain on FKBP12. Bastadins rep resent a new class of compounds to gain insight into the functional in teractions between FKBP12 and RyR-1.