Protein synthesis in Drosophila melanogaster embryos.. Two mechanisms for guanine nucleotide exchange on eukaryotic initiation factor 2

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Abstract
The mechanism for guanine nucleotide exchange with eukaryotic initiation factor-2 (eIF-2) from Drosophila melanogaster embryos was studied using the reaction eIF-2 .cntdot. [3H]GDP + GDP (GTP) .dblarw. eIF-2 .cntdot. GDP (GTP) + [3H]GDP. When highly purified eIF-2 is used the rate of nucleotide exchange is greatly reduced by Mg2+ and this reduction is overcome by the guanine-nucleotide-exchange factor (GEF) of rabbit reticulocytes. This GEF-dependent exchange is inhibited when Drosophila eIF-2 is either phosphorylated by the hemin-controlled inhibitor (HCI) of rabbit reticulocytes or treated with phosphatidylserine or a rabbit eIF-2 .cntdot. phosphatidylserine complex. The Mg2+ impairment of guanine nucleotide exchange is less severe when highly purified eIF-2 is incubated at a higher temperature (37.degree. C) and is not observed at any temperature if partially purified eIF-2 is used instead of the highkly purified factor. In the latter two cases the exchange is not inhibited by either phosphorylation with HCI or phospholipid tratment of Drosophila eIF-2, possibly suggesting that the observed exchange is not mediated by a GEF-like factor. Our data support two possible mechanisms for GDP/GTP exchange with Drosophila embryos eIF-2: a GEF-dependent exchange, similar to that described in rabbit reticulocytes, which may be regulated by phosphorylation of eIF-2, and a factor-independent exchange which appearsto be insensitive to this type of control.