Interactions of Elongation Factor 1α with F-Actin and β-Actin mRNA: Implications for Anchoring mRNA in Cell Protrusions

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Abstract
The targeting of mRNA and local protein synthesis is important for the generation and maintenance of cell polarity. As part of the translational machinery as well as an actin/microtubule-binding protein, elongation factor 1α (EF1α) is a candidate linker between the protein translation apparatus and the cytoskeleton. We demonstrate in this work that EF1α colocalizes with β-actin mRNA and F-actin in protrusions of chicken embryo fibroblasts and binds directly to F-actin and β-actin mRNA simultaneously in vitro in actin cosedimentation and enzyme-linked immunosorbent assays. To investigate the role of EF1α in mRNA targeting, we mapped the two actin-binding sites on EF1α at high resolution and defined one site at the N-terminal 49 residues of domain I and the other at the C-terminal 54 residues of domain III. In vitro actin-binding assays and localization in vivo of recombinant full-length EF1α and its various truncates demonstrated that the C terminus of domain III was the dominant actin-binding site both in vitro and in vivo. We propose that the EF1α–F-actin complex is the scaffold that is important for β-actin mRNA anchoring. Disruption of this complex would lead to delocalization of the mRNA. This hypothesis was tested by using two dominant negative polypeptides: the actin-binding domain III of EF1α and the EF1α-binding site of yeast Bni1p, a protein that inhibits EF1α binding to F-actin and also is required for yeast mRNA localization. We demonstrate that either domain III of EF1α or the EF1α-binding site of Bni1p inhibits EF1α binding to β-actin mRNA in vitro and causes delocalization of β-actin mRNA in chicken embryo fibroblasts. Taken together, these results implicate EF1α in the anchoring of β-actin mRNA to the protrusion in crawling cells.