Characterization of mammalian translation initiation factor 5 (eIF-5): Demonstration that eIF-5 is a phosphoprotein and is present in cells as a single molecular form of apparent M_r 58,000

Eukaryotic translation initiation factor 5 (eIF-5) promotes the hydrolysis of GTP bound to the 40 S initiation complex (40 S·mRNA·Met-tRNA_f·eIF-2· GTP). Using assays that measure (a) the release of ³²P_i from [γ-³²P]GTP bound to the 40 S initiation complex and (6) the eIF-5-dependent formation of an 80 S initiation complex from a preformed 40 S initiation complex containing bound [³⁵S]Met-tRNAf, we devised a novel and rapid procedure for purifying eIF-5 from rabbit reticulocyte lysates in high yield. Highly purified eIF-5 is a monomeric protein with a M_r of about 58,000. However, in partially purified preparations, 58-kDa eIF-5 is associated with other cellular protein(s) and sediments in glycerol gradient centrifugation as a protein of M_r ≈ 160,000. Chicken antibodies to native eIF-5 were isolated from egg yolks of laying hens immunized with rabbit reticulocyte eIF-5. ³⁵S-Labeled eIF-5, isolated from rat anterior pituitary GHs cells using affinity-purified anti-eIF-5 antibodies, also has an apparent M_r of 58,000. eIF-5 immunoprecipitated from extracts of ³²P-labeled GH₃ cells was phosphorylated on serine residues. Phosphopeptide mapping revealed two major sites of phosphorylation, which are distinct from rabbit reticulocyte eIF-5 sites phosphorylated in vitro by casein kinase II. These results demonstrate that eIF-5 is a phosphoprotein that is present in cells as a single molecular form of apparent molecular weight 58,000.