The identification of an eukaryotic initiation factor 4D precursor in spermidine-depleted Chinese hamster ovary cells

When Chinese hamster ovary cells are incubated with terminal methylenes-3H]spermidine, radioactivity is incorporated into a single cellular protein, eukaryotic initiation factor 4D (eIF-4D), through posttranslational synthesis of the amino acid hypusine (N epsilon-(4-amino-2-hydroxybuyly)lysine). The effect of spermidine depletion on this protein modification reaction was studied by high resolution two-dimensional gel electrophoresis. Factor eIF-4D containing both 3H]lysine and 3H]hypusine was detected as one of the major labeled cellular proteins on the fluorographic map of the proteins from Chinese hamster ovary cells that had been incubated with 3H]lysine. When these cells were depleted of spermidine by the use of DL-alpha-difluoromethylornithine before addition of 3H]lysine, no radiolabeling of this mature eIF-4D (hypusine form, Mr approximately 18,000; pI approximately 5.3) occurred. Instead, a new radiolabeled protein (Mr 18,000; pI 5.1) that contained 3H]lysine but no 3H]hypusine or 3H]deoxyhypusine was seen. This protein was identified as an eIF-4D precursor by comparison of the two-dimensional map of its tryptic peptides with that of the tryptic peptides from 3H]lysine-labeled eIF-4D. Further comparisons also suggest that additional post-translational modification processes are involved in the biogenesis of eIF-4D.