F-actin sequesters elongation factor 1 alpha from interaction with aminoacyl-tRNA in a pH-dependent reaction

The machinery of eukaryotic protein synthesis is found in association with the actin cytoskeleton. A major component of this translational apparatus, which is involved in the shuttling of aa-tRNA, is the actin-binding protein elongation factor 1 alpha (EF-1 alpha). To investigate the consequences for translation of the interaction of EF-1 alpha with F-actin, we have studied the effect of F-actin on the ability of EF-1 alpha to bind to aa-tRNA. We demonstrate that binding of EF-1 alpha:GTP to aa-tRNA is not pH sensitive with a constant binding affinity of similar to 0.2 mu M over the physiological range of pH. However, the sharp pH dependence of binding of EF-1 alpha to F-actin is sufficient to shift the binding of EF-1 alpha from F-actin to aa-tRNA as pH increases. The ability of EF-1 alpha to bind either F-actin or aa-tRNA in competition binding experiments is also consistent with the observation that EF-1 alpha's binding to F-actin and aa-tRNA is mutually exclusive. Two pH-sensitive actin- binding sequences in EF-1 alpha are identified and are predicted to overlap with the aa-tRNA-binding sites. Our results suggest that pH-regulated recruitment and release of EF-1 alpha from actin filaments in vivo will supply a high local concentration of EF-1 alpha to facilitate polypeptide elongation by the F-actin-associated translational apparatus.