Purification, phosphorylation and control of the guanine‐nucleotide‐exchange factor from rabbit reticulocyte lysates

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Abstract
A simple, improved procedure for the isolation of guanine-nucleotide-exchange factor (GEF) and for eukaryotic initiation factor 2 (eIF-2) from rabbit reticulocyte lysates has been developed using ion-exchange chromatography on S-Sepharose, Q-Sepharose, Mono Q and Mono S. The majority of the eIF-2 is separated from GEF at an early stage in the procedure and the remaining small amount of eIF-2 · GEF complex is separated from the bulk of the GEF by FPLC on Mono S. The procedure yields approximately 2 mg each of eIF-2 and GEF, of 90% and greater than 80% purity, respectively, from the blood of ten rabbits. All fractions of purified GEF contain four subunits of molecular masses 84, 66, 54 and 39 kDa, with various amounts of a fifth, 30-kDa subunit. The modulation of GEF activity was investigated using the highly purified factor in a guanine-nucleotide-exchange assay. The activity of GEF was stimulated by physiological concentrations of the polyamines, spermine and spermidine, but was unaffected by another polycationic compound, polylysine. Activity was also found to be inhibited by 1 mM NADP+ or NAD+, and this inhibition was overcome by the presence of 1 mM NADPH. Stoichiometric amounts of GEF were unable to release GDP from eIF-2 · GDP complexes in the absence of free guanine nucleotides, suggesting that GEF operates by a ternary-complex mechanism. Casein kinase 1 or casein kinase 2 can each phosphorylate the largest subunit (84 kDa) of GEF. These enzymes both phosphorylate serine residues in GEF but they phosphorylate distinct sites, as demonstrated by phosphopeptide mapping following proteolytic or cyanogen bromide digestion. Neither of these kinases phosphorylated any of the other subunits of GEF to any significant extent and several other kinases were inactive against GEF. No effect of phosphorylation on activity could be demonstrated.

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