Isolation and characterisation of the guanine nucleotide exchange factor from rat liver.

A factor possessing guanine nucleotide exchange factor (GEF) activity has been isolated from microsomal high salt wash fractions derived from rat liver. The subsequent purification procedure employed ion-exchange chromatography on phosphocellulose (which resolved it from protein synthesis initiation factor-2 (eIF-2] and on carboxymethyl-Sephadex. The factor stimulated the formation of initiation complexes by eIF-2 and this stimulation was inhibited by phosphorylation of eIF-2 on its alpha-subunit. In particular the factor promoted the exchange of GDP bound to eIF-2 for GTP, and its functional properties therefore closely resemble those of GEF from other sources, including rabbit reticulocytes. However, its native molecular mass (450-480 kDa as estimated by gel filtration or density gradient centrifugation) was greater than those reported for GEF from other types of cells. Analysis of the rat liver GEF preparation on SDS-polyacrylamide gels revealed components of molecular weights similar to those reported for reticulocyte GEF.