Protein Synthesis in Brine Shrimp Embryos. Dormant and Developing Embryos of Arternia salina Contain Equivalent Amounts of Chain Initiation Factor 2

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Abstract
Dormant and developing embryos of A. salina contain equivalent amounts of eIF-2, the eukaryotic initiation factor which forms a ternary complex with GTP and Met-tRNAf. The factor was purified from 0.5 M NH4Cl ribosomal washes by (NH4)2SO4 fractionation, followed by chromatography on heparin-Sepharose, DEAE-cellulose, hydroxyapatite and phosphocellulose. Purified preparations from dormant and developing embryos have similar specific activities and nucleotide requirements. The mobility of both proteins in dodecylsulfate gel electrophoresis is indistinguishable, and each contains 3 major polypeptide chains of MW 52,000, 45,000 and 42,000. Both proteins are also immunologically identical, and each stimulates amino acid incorporation in a cell-free system of protein synthesis. The binding of [35S]Met-tRNAf to 40-S ribosomal subunits is catalyzed by eIF-2 isolated from dormant or developing embryos and is dependent upon GTP and AUG. Binding of [35S]Met-tRNAf to 40-S ribosomal subunits, and ternary complex formation with eIF-2, GTP and [35S]Met-tRNAf is stimulated 2 to 3-fold by a factor present in the 0.5 M NH4Cl ribosomal wash and which elutes from DEAE-cellulose at 50 mM KCl. This protein does not exhibit GTP-dependent binding of [35S]Met-tRNAf. Binding of GDP and GTP was investigated with purified eIF-2 from developing embryos. The factor forms a binary complex with GDP or GTP, and eIF-2-bound [3H]GDP exchanges very slowly with free nucleotides. eIF-2 does not limit resumption of embryo development following encystment nor does it limit mRNA translation in extracts from dormant embryos.