The autophosphorylation reaction in the mechanism of activation of pig brain cyclic AMP-dependent protein kinase

Evidence for a regulatory action of vanadate on protein phosphorylation in brain microvessels

We investigated the action of vanadate on protein phosphorylation in microvessels isolated from rat brain. We found that a stimulation of protein phosphorylation from 32P-ATP occurs, in the presence of different concentrations of vanadate, 10(-3) M being the most effective dose. This action was time-dependent, and it was more evident after 60 s of treatment. The contribution of ATPase inhibition caused by vanadate appears to be negligible. In addition a stimulation of cAMP-dependent protein kinase activity was observed. The pattern of protein phosphorylation showed that exposure to 10(-3) M vanadate resulted in a nonspecific stimulation of protein phosphorylation concomitantly with a selective inhibition of the 55 KDa protein phosphorylation. The nature of this protein is also discussed.

Regulation of casein kinase 2 by phosphorylation/dephosphorylation

The effects of various polycation-stimulated (PCS) phosphatases and of the active catalytic subunit of the ATPMg-dependent (AMDc) protein phosphatase on the activity of casein kinase 2 (CK-2) were investigated by using the synthetic peptide substrate Ser-Glu-Glu-Glu-Glu-Glu, whose phosphorylated derivative is entirely insensitive to these protein phosphatases. Previous dephosphorylation of native CK-2 enhances its specific activity 2-3-fold. Such an effect, accounted for by an increase in Vmax, is more readily promoted by the PCS phosphatases than by the AMDc phosphatase. The phosphate incorporated by autophosphorylation could not be removed by the protein phosphatases, suggesting the involvement of phosphorylation site(s) other than the one(s) affected by intramolecular autophosphorylation. The activation of CK-2 by the phosphatase pretreatment is neutralized during the kinase assay; the mechanism of this phenomenon, which is highly dependent on the kinase concentration, is discussed.

In vitro protein phosphorylation in head preparations from normal and mutant Drosophila melanogaster

We have characterized protein phosphorylation in vitro in subcellular fractions from Drosophila melanogaster heads. Optimal conditions for the incorporation of 32P into proteins, and its dependence on ATP, divalent cations, and cyclic nucleotides have been determined, as well as the effect of inhibitors of ATPase, protein phosphatase, and protein kinase on protein phosphorylation. Among these inhibitors, Zn2+ was found to affect the incorporation of 32P into specific bands and p-hydroxymercuribenzoate was found to be most suited for freezing the activity of both kinases and phosphatases. Cyclic AMP-dependent protein kinase (cAMP-dPK) activity was present in both supernatant (S2) and particulate (P2) fractions, with the majority (60-85%, depending on the homogenization medium) being associated with S2, as determined by phosphorylation of exogenous synapsin I. cAMP-dPK catalyzed the phosphorylation of at least 18 endogenous polypeptides in S2 and at least 10 endogenous polypeptides in P2. These proteins could be classified on the basis of the extent of stimulation of phosphorylation by cyclic nucleotides, dependence on cyclic nucleotide concentration, and rate of phosphorylation. A phosphoprotein of 51 kilodaltons (pp51) was a major component of the S2 and P2 fractions and displayed properties expected from the regulatory subunit of the cAMP-dPK, R-II. A phosphoprotein doublet of approximately 37 kilodaltons (pp37) was stimulated to the largest extent by cAMP in the P2 and S2 fractions. The phosphorylation of several proteins in both fractions was significantly lowered by the mammalian Walsh inhibitor of cAMP-dPK, whereas in some cases the stimulation of phosphorylation of the same proteins by exogeneous cAMP was relatively small. Phosphoproteins from two learning mutants known to be deficient in cAMP metabolism, dnc and rut, were analyzed for their extent of phosphorylation in the presence of a stable cAMP analogue; no significant differences from normal were detected, suggesting that the genetic defect in cAMP metabolism is not accompanied by constituent abnormalities in phosphorylated substrates in the adult fly, and that the physiological defects in these mutants result from aberrations in the interaction of the cAMP cascade with normal substrates. The majority of Ca2+/calmodulin kinase activity (80-90%, depending on the homogenization procedure) was associated with S2, as revealed by phosphorylation of exogenous synapsin I. Two endogenous substrates for this kinase in P2 had molecular masses of approximately 45 and 87 kilodaltons. At least 11 substrates for the Ca2+/calmodulin-dependent kinase were detected in S2.(ABSTRACT TRUNCATED AT 400 WORDS)

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.

Subunit structure and autophosphorylation mechanism of casein kinase-TS (type-2) from rat liver cytosol

Type-2 casein kinase-TS (Ck-TS) purified to homogeneity from rat liver cytosol exhibits a molecular mass of 130000 daltons in non-denaturating media and a subunit composition consistent with an alpha 2 beta 2 heterotetramer. The quaternary structure of Ck-TS is not compromised by limited proteolysis with trypsin which converts the 38-kDa alpha subunit into 36-kDa (alpha') and 34-kDa (alpha") derivatives, inducing a parallel decrease of enzymatic activity. Since the 25-kDa beta subunit is unaffected under comparable conditions, the catalytic activity seemingly resides in the alpha subunits. The beta subunit, on the other hand, undergoes a very rapid phosphorylation upon incubation of Ck-TS with ATP/Mg2+: 0.8-1.5 mol P/mol Ck-TS are incorporated within 30 s. Such a fast autophosphorylation is neither prevented nor slowed down by the addition of a large excess of phosphorylatable substrates and takes place through an intra-molecular rather than inter-molecular process. This conclusion is supported by the following data. (a) The autophosphorylation rate is linearly proportional to the concentration of Ck-TS. (b) Thermally inactivated Ck-TS is not phosphorylated by catalytic amounts of active enzyme. (c) Basic polypeptides like protamine and polylysine stimulate the activity of Ck-TS toward phosphorylatable substrates while preventing the autophosphorylation reaction. Since the effectors that inhibit autophosphorylation also induce a remarkable decrease of the Km values for the protein substrates, the possibility is discussed that autophosphorylation might represent a regulatory device by which Ck-TS could be converted into a partially inactivated form exhibiting reduced affinity toward its endogenous targets.