title: Actin polymerization. The mechanism of action of cytochalasin D.
abstract: Fluorescence changes using actin covalently labeled with N-(1-pyrenyl)iodoacetamide have been used to determine the effect of cytochalasin D on actin polymerization. A mechanism for the effect of cytochalasin D on actin polymerization is presented, which explains the experimental observation of a cytochalasin D-induced increase in the initial rate of polymerization and a decrease in the final extent of the reaction. Central to this mechanism is the Mg2+-dependent formation of cytochalasin D-induced dimers. The dimers serve as nuclei to enhance the polymerization rate. Binding of Mg2+ to a low affinity site on the dimer induces a conformational change which can be observed as a rapid fluorescence increase. A subsequent time-dependent fluorescence decrease observed prior to polymerization appears to represent ATP hydrolysis resulting in dissociation of the dimer and release of actin monomers containing ADP. We postulate that a slow rate of exchange of ATP for bound ADP relative to hydrolysis results in the accumulation of monomers containing ADP. As these monomers have a high critical concentration, the final extent of polymerization is reduced dramatically. The Mg2+ dependence of the final extent of polymerization in the presence of cytochalasin D is also explained in the context of this mechanism.
mesh_terms: D000199:Actins / Q000378:metabolism*; D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000494:pharmacology; D015638:Cytochalasin D; D003572:Cytochalasins / Q000378:metabolism / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005453:Fluorescence; D018260:Gelsolin; D006868:Hydrolysis; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008840:Microfilament Proteins / Q000494:pharmacology; D011108:Polymers / Q000378:metabolism*; D011487:Protein Conformation; D013997:Time Factors
chemical_list: D000199:Actins; D002135:Calcium-Binding Proteins; D003572:Cytochalasins; D018260:Gelsolin; D008840:Microfilament Proteins; D011108:Polymers; D015638:Cytochalasin D; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D008274:Magnesium