Protein synthesis in Drosophila melanogaster embryos

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Abstract
Eukaryotic initiation factor 2 (eIF‐2) was purified from the high‐salt wash fraction of Drosophila melanogaster embryos. This factor, with a molecular mass of about 90 kDa, consists of two subunits of 47 kDa and 39 kDa on dodecylsulfate/polyacrylamide gel electrophoresis. The 39‐kDa subunit is phosphorylated by the hemin‐controlled inhibitor of rabbit reticulocytes in a terminal fragment which can be cleaved by mild treatment with trypsin. Drosophila eIF‐2 is not a substrate for protein kinases capable of phosphorylating the β subunit of eIF‐2 from rabbit reticulocytes. It is also shown that Drosophila eIF‐2 can form a ternary complex with GTP and Met‐tRNAi, which can be efficiently transferred to 40S ribosomes in the presence of AUG and Mg2+. This factor is able to form a binary complex with GDP. Furthermore, purified eIF‐2 contains about 0.3 mol bound GDP/mol suggesting a high affinity of the factor for this nucleotide. Data supporting the notion that this affinity is increased in the presence of Mg2+, which impairs the GDP/GTP exchange on eIF‐2, are presented. The properties of Drosophila eIF‐2 suggest that this factor may be susceptible to regulation by a mechanism like that operating on rabbit reticulocyte eIF‐2.