Identification of a 57S translation complex containing closed-loop factors and the 60S ribosome subunit

In eukaryotic translation the 60S ribosome subunit has not been proposed to interact with mRNA or closed-loop factors eIF4E, eIF4G, and PAB1. Using analytical ultracentrifugation with fluorescent detection system, we have identified a 57S translation complex that contains the 60S ribosome, mRNA, and the closed-loop factors. Previously published data by others also indicate the presence of a 50S-60S translation complex containing these same components. We have found that the abundance of this complex increased upon translational cessation, implying formation after ribosomal dissociation. Stoichiometric analyses of the abundances of the closed-loop components in the 57S complex indicate this complex is most similar to polysomal and monosomal translation complexes at the end of translation rather than at the beginning or middle of translation. In contrast, a 39S complex containing the 40S ribosome bound to mRNA and closed-loop factors was also identified with stoichiometries most similar to polysomal complexes engaged in translation, suggesting that the 39S complex is the previously studied 48S translation initiation complex. These results indicate that the 60S ribosome can associate with the closed-loop mRNA structure and plays a previously undetected role in the translation process.