We have characterized the structure and function of RNA sequences that direct beta-cytoplasmic actin mRNA to the cell periphery were mapped to two segments of 3'-untranslated region by expression of LacZ/beta-actin chimeric mRNAs in chicken embryo fibroblasts (CEFs). A 54-nt segment, the "RNA zipcode," and a homologous but less active 43-nt segment each localized beta-galactosidase activity to the leading lamellae. This zipcode contains the full activity, and mutations or deletions within it reduce, but do not eliminate, its activity, indicating that several motifs contribute to the activity. Two of these motifs, when multimerized, can regenerate almost full activity. These sequences are highly conserved in evolution, since the human beta-actin zipcode, positioned identically in the 3'UTR localizes equally well in chicken cells. Complementary phosphorothioate oligonucleotides against the zipcode delocalized endogenous beta-actin mRNA, whereas those complementary to the region just outside the zipcode, or sense oligonucleotides, did not. Actin mRNA or protein levels were unaffected by the antisense treatments, but a dramatic change in lamellipodia structure, and actin stress fiber organization was observed using the same antizipcode oligonucleotides which delocalized the mRNA. Hence, discrete 3'UTR sequences direct beta-actin isoform synthesis to the leading lamellae and affect cell morphology, presumably through the actin cytoskeleton.

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