Gregory M.L. Patterson
Department of Chemistry and Cancer Research Center of Hawaii, University of Hawaii, Honolulu, HI 96822, USA.
The scytophycins are antifungal, cytotoxic macrolides produced by cyanobacteria (blue green algae) of the genera Tolypothrix and Scytonema. Tolytoxin, the most potent of the scytophycin compounds, has been shown to inhibit cell proliferation, induce morphological changes, and disrupt stress fiber organization in cultured mammalian. These effects are manifested rapidly (less than 15 minutes) and at concentrations significantly lower than other F-actin disrupting agents such as cytochalasins B or D, latrunculin A, or swinholide A.Tolytoxin also inhibits G-actin polymerization and induces F-actin depolymerization in vitro (Cell Motil. Cytoskeleton 24:39). We have investigated the interaction of tolytoxin with purified rabbit skeletal muscle actin by the use of a [3H]-labelled derivative of tolytoxin. This derivative, which has biological potency equivalent to tolytoxin, binds to actin at a site distinct from the cytochalasin B binding site. High affinity, saturable binding of this ligand to actin mixtures could be observed after extended incubation periods. Analysis of the binding data indicated approximately 0.5 binding sites per actin monomer, suggesting that tolytoxin binds to actin dimers. Gel filtration chromatography of the ligand-actin complex indicated that radioactivity was associated with actin dimers, without significant binding to monomeric actin. Bubb et al. have recently reported that the marine toxin swinholide A stabilizes actin dimers and severs actin filaments (J. Biol. Chem. 270:3463). While tolytoxin shares some structural features with swinholide A, tolytoxin lacks the two-fold axis of symmetry that Bubb et al. suggest is responsible for interaction with actin dimers. Tolytoxin appears to promote microfilament depolymerization through a mechanism that involves sequestration of actin as non-functional dimers, mimicking the function of endogenous regulatory proteins such as brevin or gelsolin. These properties suggest that tolytoxin and or other scytophycin macrolides may be useful in studies of actin sequestering proteins.