Eukaryotic initiation factor 2 (eIF-2) purified from rabbit reticulocyte lysates consists of equimolar amounts of two polypeptide chains of Mr 48,000 and 38,000. Determination of the molecular weight of the native factor gave a value which is consistent with a Mr of 86,000, indicating that the factor is composed of one Mr 48,000 and one Mr 38,000 polypeptide. The purified factor exhibited all the binding activities characteristic of eIF-2. The factor formed ternary complexes with Met-tRNA? et and GTP; it bound GDP to form a binary complex; and it also possessed the property of binding a wide variety of RNA species, including reoviral mRNA, phage T3 mRNA, rRNAs, and tRNA. Furthermore, the ternary complex formed by purified eIF-2 interacted with the 40S ribosomal subunit in the presence of AUG codon to form a 40S initiation complex. These results indicate that all binding activities attributed to eIF-2 are contained in the 48,000-and 38,000-dalton polypeptides.We have shown that eukaryotic initiation factor 2 (eIF-2) purified from ribosomal salt wash proteins from calf liver is composed of one 48,000-and one 38,000-dalton polypeptide corresponding to a molecular mass of 86,000 for the native factor (1). These results were in contrast to numerous reports that eIF-2 from other eukaryotic sources, and in particular from rabbit reticulocytes, consisted of three nonidentical polypeptide chains with molecular masses in the ranges 55,000-52,000, 50,000-48,000, and 38,000-35,000 daltons (2-11). In order to resolve the anomaly in the subunit composition (and molecular mass) of eIF-2 prepared from these two mammalian sources, we purified eIF-2 from rabbit reticulocyte lysates by the procedure used for preparation of the factor from calf liver.In this paper we show that eIF-2 from rabbit reticulocytes that was purified by our rigorous purification procedure also produced only two polypeptide bands, of 48,000 and 38,000 daltons, on 15% polyacrylamide gels in the presence of sodium dodecyl sulfate. Similar to the eIF-2 from calf liver, our highly purified eIF-2 from rabbit reticulocytes exhibited all the activities known for eIF-2.
MATERIALS AND METHODS
Materials