Initiation factor 2 isolated from rat brain contains kinase activities responsible for its phosphorylation

Purification of a novel eIF-2 alpha protein kinase from calf brain

A new eukaryotic initiation factor 2 kinase has been purified for the first time from calf brain cytosol. The purification of a nonabundant novel protein kinase activity, designated as PKI, that phosphorylates the alpha subunit of eukaryotic initiation factor 2 is described. The protein kinase activity was assayed using purified initiation factor 2 as a substrate and was purified by ammonium sulphate precipitation, conventional chromatography in heparin-Sepharose and phosphocellulose and by high performance size exclusion and anion exchange chromatographies. The protein kinase activity elutes in the region of 140,000 in the size exclusion chromatography and is associated with two different polypeptides a and b, with relative molecular masses of 38,000 and 20,000 and an approximate ratio of 2.5-3.0:1. The protein kinase does not phosphorylate casein or histones and it is independent of cyclic nucleotides. It can be classified as a serine kinase since the phosphorylation of the alpha subunit of eIF-2 is produced in serine residues. Under these conditions none of the kinase subunits are phosphorylated.

An improved purification procedure and properties of casein kinase II from brain

A simple and short purification procedure applicable to casein kinase II has been developed, for fully characterizing the enzyme from calf cerebral cortex cytosol. The procedure consists of four chromatographic steps: DEAE-cellulose, phosphocellulose, phosvitin-Sepharose and ATP-agarose which yields 87% pure casein kinase II. The purified enzyme shows three major bands with apparent molecular masses of 42, 38, and 27 kDa by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and is self-autophosphorylated on its 27 kDa polypeptide. The enzyme shows all the characteristics described for casein kinase II from other sources: it is independent of cyclic nucleotides, calcium/phospholipids, and double-stranded poly(I).poly(C); it can utilize both ATP and GTP as phosphoryl donors and can phosphorylate both casein and phosvitin but not histone. The kinetic studies establish that the Km for ATP is 12.5 microM and 25.1 microM when using phosvitin and casein respectively as phosphoryl acceptors. The Km for phosvitin is 0.91 mg/ml and for casein 1.43 mg/ml, while the Vmax is 315 nmol/min/per mg protein and 479 nmol/min/per mg protein for phosvitin and casein respectively. The activity of the kinase is highly stimulated by KCl or NaCl, and almost completely inhibited by heparin concentrations of 1 microgram/ml (92%). This inhibition is reduced to only 33% in the presence of optimal KCl concentrations (150 mM). Spermine stimulates enzyme activity, whilst hemin produces a slight inhibition.

Specific phosphorylation of the beta subunit of eIF-2 factor from brain by three different protein kinases

The eukaryotic initiation factor 2 (eIF-2) from calf brain has been purified to homogeneity and free of endogenous kinase activity. Phosphorylation of eIF-2 factor has been examined with four different protein kinases. Casein kinase II, calcium/phospholipid-dependent protein kinase and cyclic AMP-dependent protein kinase from brain, phosphorylate the beta subunit of eIF-2, whilst hemin-controlled inhibitor phosphorylate the alpha subunit of the factor. According to the peptide maps obtained, the phosphorylation sites of the factor by the three beta kinases are specific and distinct. These data suggest a different regulation for the beta subunit through this modification.

Differential subcellular distribution of guanine nucleotide exchange factor in suckling and adult rat brain

Guanine nucleotide exchange factor (GEF) activity in ribosomal high salt wash and cytosolic fractions from suckling (4-10-day-old) and adult (60-day-old) rats was assayed by two different methods, by measuring: (i) its ability to promote binding of [3H]Met-tRNAi to eukaryotic initiation factor-2 (eIF-2) preparations that are partially or wholly in the form of eIF-2-GDP complexes (at Mg2+ concentrations near the optimum for protein synthesis), and (ii) under similar conditions, its ability to catalyze the displacement of [3H]GDP, previously bound to eIF-2, by unlabelled GDP. A purified eIF-2 (GEF-free) from brain was used as the source of eIF-2 activity. GEF activity in ribosomal fractions is higher in the brain of suckling than adults rats, and a direct correlation therefore exists between ribosomal GEF activity and the previously observed age-related decrease in eIF-2 activity in ribosomal high salt wash protein fractions. On the other hand GEF activity in the postmicrosomal supernatant is lower in the brain of suckling than adult rats. These findings further support the hypothesis that the progressive decrease in protein synthesis during brain development is controlled through regulation of the initiation step, by modulation of eIF-2/GEF activities.

Protein kinase activities associated with ribosomes of developing rat brain. Identification of eukaryotic initiation factor 2 kinases

Protein kinases associated with ribosomes in the brains of suckling (4-10 days) and adult (2 months) rats were extracted from ribosomal fraction with 0.5 M KCl. The different protein kinase activities were characterized by their ability to phosphorylate three exogenous substrates: casein, histone IIs and histone IIIs in the presence of different modulators. Ribosomal salt wash fractions contain a high casein kinase activity which was partially inhibited by heparin and stimulated by calmodulin in the presence of Ca2+, indicating the presence of casein kinase I and II and calcium/calmodulin-dependent kinases. Cyclic AMP and cyclic GMP-dependent kinases and protein kinase C (calcium/phospholipids-dependent kinase) were also present. No differences were found in the casein kinase activities of suckling and adult animals, but histone kinase activities were higher in adult than in suckling animals. To identify initiation factor 2 kinases, purified factor from adult brains was used as a protein marker. In addition to the phosphorylation of both factor subunits alpha and beta by casein kinase I or II, an increased phosphorylation was detected of alpha subunit in the presence of cyclic AMP, and beta subunit, in the presence of Ca2+/calmodulin or Ca2+/phospholipids. Present results reinforce our hypothesis that, as occurs in other eukaryotic cells, the decreased rate of protein synthesis during brain development may be regulated by phosphorylation of initiation factor 2.