A DIFFERENCE IN THE RATE OF RIBOSOMAL ELONGATION BALANCES THE SYNTHESIS OF EUKARYOTIC TRANSLATION INITIATION-FACTOR (EIF)-2 ALPHA AND EIF-2-BETA

Eukaryotic translation initiation factor 2 (eIF-2) is a heterotrimer c omposed of three subunits designated alpha, beta, and gamma. These pro teins exist in equimolar amounts in the cell and have not been detecte d as isolated subunits. Our research examines the basis of their balan ced synthesis. Northern analysis of K562 cell mRNA revealed that eIF-2 beta was five times more abundant than eIF-2alpha. However, immunoprec ipitation of pulse-labeled K562 cells showed an equimolar rate of synt hesis of eIF-2alpha and -beta despite the 5-fold difference in the siz e of their mRNA pools. Addition of equal amounts of synthetic capped m RNA for eIF-2alpha and eIF-2beta to an in vitro translation reaction p roduced five times more eIF-2alpha protein than eIF-2beta. Determinati on of the polysome profile for alpha and beta mRNA in K562 cells indic ated eIF-2alpha was translated more efficiently than eIF-2beta. Substi tution of either the initiation codon context or the leader of the bet a mRNA for that of alpha had only a minor effect on the translational efficiency of beta. Comparison of the rate of ribosomal elongation for the two mRNAs indicated that ribosomes associated with the beta mRNA elongate at a rate 4-fold less than that of eIF-2alpha. Thus, the bala nced translation of alpha and beta mRNA is primarily the result of a 4 -fold difference in the rate of ribosomal elongation.