Actin-latrunculin A structure and function. Differential modulation of actin-binding protein function by latrunculin A.

Latrunculin A is used extensively as an agent to sequester monomeric actin in living cells. We hypothesize that additional activities of latrunculin A may be important for its biological activity. Our data are consistent with the formation of a 1:1 stoichiometric complex with an equilibrium dissociation constant of 0.2 to 0.4 micrometer and provide no evidence that the actin-latrunculin A complex participates in the elongation of actin filaments. Profilin and latrunculin A bind independently to actin, whereas binding of thymosin beta(4) to actin is inhibited by latrunculin A. Potential implications of this differential effect on actin-binding proteins are discussed. From a structural perspective, if latrunculin A binds to actin at a site that sterically influences binding by thymosin beta(4), then the observation that latrunculin A inhibits nucleotide exchange on actin implies an allosteric effect on the nucleotide binding cleft. Alternatively, if, as previously postulated, latrunculin A binds in the nucleotide cleft of actin, then its ability to inhibit binding by thymosin beta(4) is a surprising result that suggests that significant allosteric changes affect the thymosin beta(4) binding site. We show that latrunculin A and actin form a crystalline structure with orthorhombic space group P2(1)2(1)2(1) and diffraction to 3.10 A. A high resolution structure with optimized crystallization conditions should provide insight regarding these remarkable allosteric properties.