AUTOREGULATION OF ACTIN SYNTHESIS RESPONDS TO MONOMERIC ACTIN LEVELS

Regulation of the assembly and expression of actin is of major importa nce in diverse cellular functions such as motility and adhesion and in defining cellular and tissue architecture. These biological processes are controlled by changing the balance between polymerized (F) and so luble (G) actin. Previous stud ies have indicated the existence of an autoregulatory pathway that links the state of assembly and expression of actin, resulting in the reduction of actin synthesis after actin f ilaments are depolymerized. We have employed the marine toxins swinhol ide A and latrunculin A, both disrupting the organization of the actin -cytoskeleton, to determine whether this autoregulatory response is ac tivated by a decrease in the level of polymerized actin or by an incre ase in monomeric actin concentrations in the cell. We showed that in c ells treated with swinholide A the level of filamentous actin is decre ased, and using a reversible cross-linking reagent, we found that acti n dimers are formed. Latrunculin A also disassembled actin filaments, bur produced monomeric actin, followed by a reduction in actin and vin culin expression, while swinholide A treatment elevated the synthesis of these proteins. In cells treated with both latrunculin A and swinho lide A, dimeric actin was formed, and actin and vinculin synthesis wer e higher than in control cells. These results suggest that the substra te that confers an autoregulated reduction in actin expression is mono meric actin, and when its level is decreased by dimeric actin formatio n, actin synthesis is increased. J. Cell. Biochem. 65:469-378. (C) 199 7 Wiley-Liss, Inc.