ACTIN-DEPOLYMERIZING EFFECT OF DIMERIC MACROLIDES, BISTHEONELLIDE A AND SWINHOLIDE A

We compared the effects of dimeric marine toxins, bistheonellide A, an d swinholide A, on actin polymerization, Bistheonellide A and swinholi de A possess two identical side chains with similar structures to thos e of other marine toxins, mycalolide B, and aplyronine A, By monitorin g changes in fluorescent intensity of pyrenyl-actin, bistheonellide A was found to inhibit polymerization of G-actin and to depolymerize F-a ctin in a concentration-dependent manner, The relationship between the concentration of bistheonellide A and its inhibitory activity on acti n polymerization suggested that one molecule of bistheonellide A binds two molecules of G-actin. We demonstrated by SDS-PAGE that the comple x of G-actin with bistheonellide A, swinholide A, or mycalolide B coul d not interact with myosin, No evidence was found that bistheonellide A severs F-actin at the concentrations examined (molar ratio to actin; 0.025-2.5), while swinholide A showed severing activity, although it was weaker than that of mycalolide B, We also demonstrated that the de polymerizing effect of bistheonellide A or mycalolide B is irreversibl e, Bistheonellide A increased, while swinholide A decreased, the rate of nucleotide exchange in G-actin, suggesting that binding of these to xins induces different conformational changes in the actin molecule, T hese results suggest that bistheonellide A intervenes between two acti n molecules, forms a tertiary complex with each of its side chains bou nd to G-actin, and inhibits polymerization by sequestering G-actin fro m incorporation into F-actin, A difference in structure at the end of the side chain between dimeric macrolides and mycalolide B may account for the weak severing activity of the former.