Alteration of the major phosphorylation site of eukaryotic protein synthesis initiation factor 4E prevents its association with the 48 S initiation complex.

Abstract

Site-directed mutagenesis was used to replace the serine residue at the primary phosphorylation site of human eukaryotic initiation factor (eIF) 4E with an alanine residue. The mutated cDNA was transcribed in vitro, and the transcript was used to direct protein synthesis in a reticulocyte lysate system. The variant protein (eIF-4EAla) was retained on a 7-methylguanosine 5'-triphosphate (m7GTP)-Sepharose affinity column and was specifically eluted by m7GTP. Examination of eIF-4EAla by isoelectric focusing revealed two species which had the same pI values as the phosphorylated and nonphosphorylated forms of unaltered eIF-4E (here designated eIF-4ESer). However, conversion of unphosphorylated eIF-4EAla to the putative phosphorylated eIF-4EAla in the reticulocyte lysate system was slower than the corresponding conversion of eIF-4ESer. The possibility that the more acidic form of eIF-4EAla was due to NH2-terminal acetylation was ruled out by an experiment in which the acetyl-CoA pool of the reticulocyte lysate system was depleted with oxaloacetate and citrate synthase. The more acidic form of eIF-4EAla was, however, eliminated by treatment with calf intestine alkaline phosphatase, suggesting that it results from a second-site phosphorylation. When translation reaction mixtures were resolved on sucrose density gradients, the 35S-labeled eIF-4ESer was found on the 48 S initiation complex in the presence of guanylyl imidodiphosphate, as reported earlier (Hiremath, L.S., Hiremath, S.T., Rychlik, W., Joshi, S., Domier, L.L., and Rhoads, R.E. (1989) J. Biol. Chem. 264, 1132-1138). eIF-4EAla, by contrast, was not found on the 48 S complex, suggesting that phosphorylation of eIF-4E is necessary for it to carry out its role of transferring mRNA to the 48 S complex. Supporting this interpretation was the finding that eIF-4ESer isolated from 48 S initiation complexes consisted predominantly of the phosphorylated form.