Posttranslational formation of hypusine in a single major protein occurs generally in growing cells and is associated with activation of lymphocyte growth.

Growing lymphocytes perform a novel chemical modification of a single protein (Hy+: approximately 18 kd, pI approximately 5.1), resulting in the formation of the unusual amino acid, hypusine (N epsilon-[4-amino-2-hydroxybutyl]lysine). This posttranslational event occurs only following activation of lymphocyte growth. Hypusine formation increases at a rate parallel to protein synthesis during the first 24 hr of growth stimulation, beginning before 6 hr of growth. At all times, hypusine is restricted primarily to the single protein, Hy+. In resting cells, the unmodified substrate protein, Hy0, is continuously synthesized and maintained in a steady-state pool of significant size. In several other cell lines, hypusine formation was also observed in a single protein of approximately 18 kd, pI approximately 5.1, indistinguishable electrophoretically from the lymphocyte protein. Thus Hy+ is a ubiquitous protein showing significant conservation among divergent species. Maintenance by resting lymphocytes of a pool of unmodified protein and early activation during growth of the hypusine-forming enzyme system suggest that this posttranslational modification may be of importance to lymphocyte activation.