IDENTIFICATION OF POSTTRANSLATIONALLY MODIFIED 18-KILODALTON PROTEIN FROM RICE AS EUKARYOTIC TRANSLATION INITIATION-FACTOR 5A

Using anther-derived rice (Oryza sativa L.) cell-suspension cultures, we have identified an 18-kD protein that is posttranslationally modifi ed by spermidine and is influenced by endogenous polyamine levels. The posttranslationally modified residue has been identified as the unusu al amino acid hypusine [N'-(4-amino-2-hydroxybutyl)lysine] by reverse- phase high-performance liquid chromatography and gas chromatography-ma ss-spectrometry analyses. Differential labeling of the protein with la beled amines provided evidence that the butylamine moiety of spermidin e is the immediate precursor of the hypusine residue in the protein. T he eukaryotic translation initiation factor 5A (elF-5A) is the only kn own mammalian protein that undergoes a similar posttranslational modif ication with hypusine. The purified 18-kD protein co-electrophoreses w ith human translational initiation factor eIF-5A in both isoelectric f ocusing and sodium dodecyl sulfate-polyacrylamide gels. The purified p rotein from rice stimulated methionylpuromycin synthesis in vitro, ind icating its functional similarity to mammalian eIF-5A. The results pre sented provide evidence that the posttranslationally modified 18-kD pr otein from rice containing hypusine is eIF-5A and suggest the conserva tion of hypusine-containing translation initiation factor eIF-5A in eu karyotes.