Translation of specific mRNAs is tightly regulated in primary T cells to ensure correct timing and precise delivery of protein syntheses in response to environmental cues. Translation factor eIF5a is known to be important for protein synthesis and is expected to regulate translation of certain difficult mRNA motifs such as consecutive prolines. Functional eIF5a is dynamically regulated upon T cell activation largely through addition of hypusine, a unique post-translational modification, to the mature protein and remarkably the eIF5a family are the only proteins to carry this modification. Here we show that knockout of eIF5a and its hypusination pathway affected both global and gene-specific protein synthesis, especially regulators of cytokine production and cell cycle progression. Furthermore, mRNA targets regulated by eIF5a in activated mouse primary CD8 T cells were systematically identified. AHA-labelled nascent proteins in GC7 (Dhps inhibitor)-treated and eIF5a knockout OT-1 cells (generated using CRSPR-Cas9) were measured using LC-MS. Authors: Thomas CJ Tan, Van Kelly, Xiaoyan Zhou, Tony Ly and Rose Zamoyska