The p46 subunit of eukaryotic initiation factor (eIF)-4F exchanges with eIF-4A.

The p46 subunit of eukaryotic initiation factor (eIF)-4F purified from rabbit reticulocyte lysate has previously been found to be composed of eIF-4AI and eIF-4AII in a 4:1 ratio, respectively, whereas the free form of rabbit eIF-4A is composed solely of eIF-4AI. Using sucrose gradient centrifugation and an m7GTP-Sepharose 4B assay, it was shown that eIF-4A exchanges with the p46 subunit of eIF-4F. Incubation of [14C]eIF-4A and eIF-4F resulted in the incorporation of [14C] eIF-4A into the eIF-4F complex. Conversely, the [14C] p46 subunit of [14C]eIF-4F was shown to dissociate from the [14C]eIF-4F complex in the presence of eIF-4A, presumably due to the incorporation of unlabeled eIF-4A. Similar experiments were conducted in which 14C-labeled initiation factors were incubated with rabbit reticulocyte lysate. When [14C]eIF-4A was incubated with lysate, [14C]eIF-4A became incorporated into the eIF-4F complex present in the lysate. Additionally, when [14C]eIF-4F was incubated with lysate, the [14C]p46 subunit of [14C]eIF-4F dissociated from the [14C]eIF-4F complex, most likely due to the exchange of unlabeled eIF-4A (present in the lysate) with the [14C]p46 subunit. The exchange of mouse eIF-4AI and eIF-4AII expressed in Escherichia coli was also investigated in the presence of eIF-4F and rabbit reticulocyte lysate. Both the sucrose gradient experiments and m7GTP-Sepharose 4B assays demonstrated that the [14C]p46 subunit of [14C]eIF-4F was displaced in the presence of eIF-4AI or eIF-4AII and that mouse [14C]eIF-4AI or [14C]eIF-4AII became incorporated into the eIF-4F complex in the same manner as rabbit reticulocyte eIF-4A.