Actin-depolymerizing effect of dimeric macrolides, bistheonellide A and swinholide A.

We compared the effects of dimeric marine toxins, bistheonellide A, and swinholide A, on actin polymerization. Bistheonellide A and swinholide A possess two identical side chains with similar structures to those of other marine toxins, mycalolide B, and aplyronine A. By monitoring changes in fluorescent intensity of pyrenyl-actin, bistheonellide A was found to inhibit polymerization of G-actin and to depolymerize F-actin in a concentration-dependent manner. The relationship between the concentration of bistheonellide A and its inhibitory activity on actin polymerization suggested that one molecule of bistheonellide A binds two molecules of G-actin. We demonstrated by SDS-PAGE that the complex of G-actin with bistheonellide A, swinholide A, or mycalolide B could 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 depolymerizing effect of bistheonellide A or mycalolide B is irreversible. Bistheonellide A increased, while swinholide A decreased, the rate of nucleotide exchange in G-actin, suggesting that binding of these toxins induces different conformational changes in the actin molecule. These results suggest that bistheonellide A intervenes between two actin molecules, forms a tertiary complex with each of its side chains bound to G-actin, and inhibits polymerization by sequestering G-actin from 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.

Mycalolide B, a novel actin depolymerizing agent.

We investigated the effects of a novel marine toxin, mycalolide B, on actin polymerization and actin-activated myosin Mg(2+)-ATPase activity using purified actin and myosin from rabbit skeletal muscle. The results were compared with cytochalasin D which inhibits actin polymerization by binding to the barbed end of F-actin. By monitoring fluorescent intensity of pyrenyl-actin, mycalolide B did not accelerate actin polymerization but quickly depolymerized F-actin, whereas cytochalasin D accelerated actin nucleation and depolymerized F-actin at slower rate. The kinetics of depolymerization suggest that mycalolide B severs F-actin. The relationship between the concentration of total actin and F-actin at different concentration of mycalolide B suggests that mycalolide B forms 1:1 complex with G-actin. Viscometry and electron microscopic observation further suggest that actin filament was depolymerized by mycalolide B. Unlike cytochalasin D, furthermore, mycalolide B suppressed actin-activated myosin Mg(2+)-ATPase activity. We concluded that mycalolide B severs F-actin and sequesters G-actin and may serve as a novel pharmacological tool for analyzing actin-mediated cell functions.

[Actin depolymerizing action by marine toxin, pectenotoxin-2].

Pectenotoxin-2 (PCTX-2), which is one of Diarrhetic Shellfish Poisoning (DSP), is a family of cyclic polyether macrolide toxin isolated from scallop Patinopecten yessoensis. Although PCTX-2 has a potent cytotoxic activities against several cancer cell lines, the biochemical activity of PCTX-2 has not been determined yet. To clarify the biochemical activity of PCTX-2 is the aime in this study. PCTX-2 inhibited the contractions elicited by 72.7 mM KCl or 1 microM phenyrephrine in a concentration dependent manner in the isolated rat aorta. In A10 cells, actin stressfiber in center but not in periphery of the cell was disrupted by PCTX-2 without any visible shape change. By monitoring fluorescent intensity of pyrenyl-actin, PCTX-2 was found to inhibit the velocity and the degree of actin polymerization in a concentration dependent manner. In addition, PCTX-2 decreased viscosity of F-actin measured with falling ball viscometry. Stoichiometric analysis indicated that PCTX-2 forms 1:4 complex with G-actin. These results suggest that PCTX-2 is a potent natural actin depolymerizing compound with unique mode of action.