Within the field of eukaryotic protein synthesis, one factor remained putative for decades: eukaryotic translation initiation factor (eIF) 5A. Because eIF5A is an essential protein required for cell proliferation, and one easily targeted by inhibitors, identifying its role in the cell remains important and urgent. Recent reports support early findings that eIF5A stimulates protein synthesis and newly assign the factor a role in elongation rather than initiation. Here we show that eIF5A directly stimulates protein synthesis on native mRNAs, that rapid depletion of eIF5A in vivo immediately leads to a 2-fold inhibition of protein synthesis, and that both the immediate and lasting effects of eIF5A depletion are a reduction in polysome size concomitant with eIF5A depletion. Addition of purified eIF5A to a depleted lysate results in a roughly 2-fold stimulation of protein synthesis in vitro, a result consistent with both older methionyl-puromycin synthesis data and more recently published findings. We find that although eIF5A is not required for protein synthesis, it stimulates the process by about 2- to 3-fold. Our data, along with other published results, reinforce the conclusion that eIF5A stimulates protein synthesis with one important difference: Polysome profiles observed immediately after eIF5A depletion are diagnostic for a role in initiation. This discrepancy is discussed.
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