Bundling of actin filaments by elongation factor 1 α inhibits polymerization at filament ends

Elongation factor 1 α (EF1α) is an abundant protein that binds aminoacyl-tRNA and ribosomes in a GTP-dependent manner. EF1α also interacts with the cytoskeleton by binding and bundling actin filaments and microtubules. In this report, the effect of purified EF1α on actin polymerization and depolymerization is examined. At molar ratios present in the cytosol, EF1α significantly blocks both polymerization and depolymerization of actin filaments and increases the final extent of actin polymer, while at high molar ratios to actin, EF1α nucleates actin polymerization. Although EF1α binds actin monomer, this monomer-binding activity does not explain the effects of EF1α on actin polymerization at physiological molar ratios. The mechanism for the inhibition of polymerization is related to the actin-bundling activity of EF1α. Both ends of the actin filament are inhibited for polymerization and both bundling and the inhibition of actin polymerization are affected by pH within the same physiological range; at high pH both bundling and the inhibition of actin polymerization are reduced. Additionally, it is seen that the binding of aminoacyl-tRNA to EF1α releases EF1α's inhibiting effect on actin polymerization. These data demonstrate that EF1α can alter the assembly of F-actin, a filamentous scaffold on which non-membrane-associated protein translation may be occurring in vivo.