An Artemia salina factor which stimulates the activity of highly purified initiation factor eIF‐2 from A. salina and reticulocytes

In a variety of eukaryotic cells, there is a protein synthesis initiation factor which makes a ternary complex with GTP (or non-hydrolyzable GTP analogs) and eukaryotic initiator Met-TRNAi [l-lo] . This factor is referred to as eIF-2 according to the nomenclature in [ 111. A protein factor from reticulocytes which stimulates the GTP-dependent binding of MettRNAi to highly purified reticulocyte eIF-2 was isolated [ 121. This stimulatory factor is referred to as CO-eIF-2 here. The discovery of CO&F-2 in reticulocytes is significant since about 90% of the input eIF-2 was bound to Met-tRN4 in a GTP-dependent manner in the presence of COeIF-2 [ 121 . In contrast, it has been reported that only a relatively small fraction of highly purified or homogeneous reticulocyte eIF-2 makes a ternary complex with GTP and Met-tRN4 in the absence of added reticulocyte CO-eIF-2 [ 12,131. eIF-2 is a key initiation factor which is involved in one of the early steps of chain initiation, namely the formation of 40 S subunit . Met-tRNAi preinitiation complexes [ 1 l] . Any factor (like COeIF-2) which modulates the activity of eIF-2 is also likely to modulate the overall rate of polypeptide chain initiation. It is, therefore, of considerable interest to establish the presence of COeIF-2 in a system other than reticulocytes and to study its mechanism of action. Evidence is provided here which shows that a factor, functionally analogous to reticulocyte CO&F-2, is also present in developing Artemia salina embryos. Furthermore, we have observed that A. salina and reticulocyte COeIF-2 preparations are interchangeable, i.e., COeIF-2 isolated from either source can stimulate the activity of highly purified eIF-2 from