Monomeric (G)
actin was shown to be involved in inhibiting its own synthesis by an autoregulatory mechanism that includes enhanced degradation of the actin mRNA [Bershadsky et al., 1995;Lyubimova et al., 1997]. We show that the 3Π-untranslated region (3Π-UTR) of β€-actin mRNA, but not its 5Π-untranslated region, is important for this regulation. The level of full-length β€-actin mRNA in cells was reduced when actin filaments were depolymerized by treatment with latrunculin A and elevated when actin polymerization was induced by jasplakinolide. By contrast, the level of actin mRNA lacking the 3Π-UTR remained unchanged when these drugs modulated the dynamics of actin assembly in the cell. Moreover, the transfection of cells with a construct encoding the autoregulation-deficient form of β€-actin mRNA led to very high levels of actin expression compared with transfection with the control actin construct and was accompanied by characteristic changes in cell morphology and the structure of the actin cytoskeleton. These results suggest that the autoregulatory mechanism working via the 3Π-UTR of actin mRNA is involved in controlling the maintenance of a defined pool of actin monomers that could be necessary for the proper organization of the microfilament system and the cytoskeleton-mediated signaling.