G-actin conformational change and polymerization induced by paraquat

Paraquat (1,1'-dimethyl-4,4'-bipyridilium dichloride) is a broad-spectrum h erbicide that is highly toxic to animals (including man), the major lesion being in the lung. In mammalian cells, paraquat causes deep alterations in the organization of the cytoskeleton, marked decreases in cytoskeletal prot ein synthesis, and alterations in cytoskeletal protein composition; therefo re, the involvement of the cytoskeleton in cell injury by paraquat was sugg ested. We previously demonstrated that monomeric actin binds paraquat; more over, prolonged actin exposure to paraquat, in depolymerizing medium, induc es the formation of actin aggregates, which are built up by F-actin. In thi s work we have shown that the addition of paraquat to monomeric actin resul ts in a strong quenching of Trp-79 and Trp-86 fluorescence. Trypsin digesti on experiments demonstrated that the sequence 61-69 on actin subdomain 2 un dergoes paraquat-dependent conformational changes. These paraquat-induced s tructural changes render actin unable to completely inhibit DNase I. By usi ng intermolecular cross-linking to characterize oligomeric species formed d uring paraquat-induced actin assembly, we found that the herbicide causes t he formation of actin oligomers characterized by subunit-subunit contacts l ike those occurring in oligomers induced by polymerizing salts (i.e., betwe en subdomain 1 on one actin subunit and subdomain 4 on the adjacent subunit ). Furthermore, the oligomerization of G-actin induced by paraquat is paral leled by ATP hydrolysis.