A high molecular weight reticulocyte protein factor, named Co-eIF-2, contains Co-eIF-2A, Co-eIF-2B, and Co-eIF-2C activities and stimulates Met-tRNA, binding to eIF-2 both in the presence and absence of Mg2+. Some characteristics of this stimulation in the absence of Mg2+ are: (1) Stimulation is most pronounced at low eIF-2 levels. (2) Stimulation is partially resistant to heat and NEM treatment, and thus appears to be due to the combined action of both heat and NEM-insensitive Co-eIF-2A, and heat and NEM-sensitive Co-eIF-2C activities. (3) [3H]GDP bound in eIF-2·[3H]GDP complex is rapidly displaced by unlabelled GTP during ternary complex formation., Co-eIF-2 stimulates. Met-tRNAf binding to eIF-2 even when added after the [3H]GDP from eIF-2·[3H]GDP has been completely displaced. This indicates that Co-eIF-2-stimulation is not due to GDP displacement from eIF-2·GDP. We propose that eIF-2 molecules become inactive in the presence of Mg2 and at high dilution, and Co-eIF-2 restores the inactive eIF-2 molecules into an active form.
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