Beta-actin mRNA is localized in the leading lamellae of chicken embryo fibroblasts (CEFs) (Lawrence, J., and R. Singer. 1986. Cell. 45:407-415), close to where actin polymerization in the lamellipodia drives cellular motility. During serum starvation beta-actin mRNA becomes diffuse and non-localized. Addition of FCS induces a rapid (within 2-5 min) redistribution of beta-actin mRNA into the leading lamellae. A similar redistribution was seen with PDGF, a fibroblast chemotactic factor. PDGF-induced beta-actin mRNA redistribution was inhibited by the tyrosine kinase inhibitor herbimycin, indicating that this process requires intact tyrosine kinase activity, similar to actin filament polymerization and chemotaxis. Lysophosphatidic acid, which has been shown to rapidly induce actin stress fiber formation (Ridley, A., and A. Hall. 1992. Cell. 790:389-399), also increases peripheral beta-actin mRNA localization within minutes. This suggests that actin polymerization and mRNA localization may be regulated by similar signaling pathways. Additionally, activators or inhibitors of kinase A or C can also delocalize steady-state beta-actin mRNA in cells grown in serum, and can inhibit the serum induction of peripherally localized beta-actin mRNA in serum-starved CEFs. These data show that physiologically relevant extracellular factors operating through a signal transduction pathway can regulate spatial sites of actin protein synthesis, which may in turn affect cellular polarity and motility.

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