Distribution of reversing factor in reticulocyte lysates during active protein synthesis and on inhibition by heme deprivation or double-stranded RNA.

We have recently shown a direct correlation between protein synthetic activity and the function of reversing factor (RF) as a catalyst of GDP-GTP exchange in whole reticulocyte lysates under normal conditions and on inhibition of protein synthesis by heme deficiency, double-stranded RNA, or oxidized glutathione. In this paper we report that RF is detectable as a nonribosomal complex with eukaryotic initiation factor 2 phosphorylated in its alpha subunit [eIF-2(alpha P)] in whole lysates inhibited by heme deprivation or by double-stranded RNA. The complex contains no unphosphorylated eIF-2 alpha, and the GDP present is freely dissociable. All nonribosomal eIF-2(alpha P) is complexed with RF in fully inhibited lysates; we have not detected free eIF-2(alpha P). RF in this [RF X eIF-2(alpha P)] complex is unavailable to catalyze the release of GDP from eIF-2-GDP. Dephosphorylation of eIF-2(alpha P) present in nonribosomal fractions releases active RF, which is able to carry out its normal guanine nucleotide exchange function.