Identification of phosphorylation sites in the recombinant catalytic subunit of cAMP-dependent protein kinase

Role of the glycine triad in the ATP-binding site of cAMP-dependent protein kinase

A glycine-rich loop in the ATP-binding site is one of the most highly conserved sequence motifs in protein kinases. Each conserved glycine (Gly-50, Gly-52, and Gly-55) in the catalytic (C) subunit of cAMP-dependent protein kinase (cAPK) was replaced with Ser and/or Ala. Active mutant proteins were expressed in Escherichia coli, purified to apparent homogeneity, separated into phosphoisoforms, and characterized. Replacing Gly-55 had minimal effects on steady-state kinetic parameters, whereas replacement of either Gly-50 or Gly-52 had major effects on both Km and kcat values consistent with the prediction of the importance of the tip of the glycine-rich loop for catalysis. Substitution of Gly-50 caused a 5-8-fold reduction in Km (ATP), an 8-12-fold increase in Km (peptide), and a 3-5-fold drop in kcat. The Km (ATP) and Km (peptide) values of C(G52S) were increased 8- and 18-fold, respectively, and the kcat was decreased 6-fold. In contrast to catalytic efficiency, the ATPase rates of C(G50S) and C(G52S) were increased by more than an order of magnitude. The thermostability of each mutant was slightly increased. Unphosphorylated C(G52S) was characterized as well as several isoforms phosphorylated at a single site, Ser-338. All of these phosphorylation-defective mutants displayed a substantial decrease in both enzymatic activity and thermal stability that correlated with the missing phosphate at Thr-197. These results are correlated with the crystal structure, models of the respective mutant proteins, and conservation of the Glys within the protein kinase family.

Recombinant strategies for rapid purification of catalytic subunits of cAMP-dependent protein kinase

Knowledge of the crystal structure of the catalytic subunit (C) of cAMP-dependent protein kinase provided for the first time a molecular basis for probing function by site-directed mutagenesis. The purification of mutant C-subunits, however, presented new and unanticipated challenges due to instability, insolubility, and underphosphorylation of the altered proteins. To overcome these barriers, a rapid and efficient method for purifying recombinantly expressed C-subunits was developed. Purification to near homogeneity is achieved in less than 5 h. The procedure is based on colysis of bacteria that overexpress the C-subunit with bacteria that overexpress a poly-His-tagged mutant of the type II regulatory subunit H6RII (R213K). This mutant R-subunit with an altered cAMP binding site A forms holoenzyme rapidly in bacterial extracts, and the Ka (cAMP) for the resulting holoenzyme, 27-37 microM, is nearly 50-fold increased compared to holoenzyme formed with wild-type RII. Thus, after batchwise immobilizing the holoenzyme on Ni(2+)-resin, the free C-subunit can be directly eluted batchwise with high concentrations of cAMP. The method is described for the purification of wild-type C, with yields of approximately 5 mg/liter. In addition, a mutant subunit, C[G52S], which is defective in ATP binding and could not be isolated using previously described methods, was purified with equal efficiency.

The catalytic domain of acanthamoeba myosin I heavy chain kinase. II. Expression of active catalytic domain and sequence homology to p21-activated kinase (PAK)

Acanthamoeba myosin I heavy chain (MIHC) kinase is a monomeric 97-kDa protein that is activated by binding to acidic phospholipids or by autophosphorylation. Activation by phospholipids is inhibited by Ca2+-calmodulin. In the accompanying paper (Brzeska, H., Martin, B., and Korn, E. D. (1996) J. Biol. Chem. 271, 27049-27055), we identified the catalytic domain as the COOH-terminal 35 kDa produced by trypsin digestion of phosphorylated MIHC kinase. In this paper, we report the cloning and sequencing of the corresponding cDNA and expression of fully active catalytic domain. The expressed catalytic domain has substrate specificity similar to that of native kinase and resistance to trypsin similar to that of fully phosphorylated MIHC kinase. MIHC kinase catalytic domain has only 25% sequence identity to the catalytic domain of protein kinase A and similarly low sequence identity to the catalytic domains of protein kinase C- and calmodulin-dependent kinases, but 50% sequence identity and 70% similarity to the p21-activated kinase (PAK) and STE20 family of kinases. This suggests that MIHC kinase is (at least) evolutionarily related to the PAK family, whose activities are regulated by small GTP-binding proteins. The homology includes the presence of a potential MIHC kinase autophosphorylation site as well as conserved Tyr and Ser/Thr residues in the region corresponding to the P+1 loop of protein kinase A. A synthetic peptide corresponding to this region of MIHC kinase is phosphorylated by both the expressed catalytic domain and native MIHC kinase.

The catalytic domain of Acanthamoeba myosin I heavy chain kinase. I. Identification and characterization following tryptic cleavage of the native enzyme

The actin-activated Mg2+-ATPase activities of the myosin I isoenzymes from Acanthamoeba castellanii are greatly increased by phosphorylation catalyzed by myosin I heavy chain kinase (MIHC kinase), a monomeric 97-kDa protein whose activity is greatly enhanced by acidic phospholipids and by autophosphorylation of multiple sites. In this paper, we show that the 35-kDa COOH-terminal fragment obtained by trypsin cleavage of maximally activated, autophosphorylated kinase retains the full activity and two to three of the autophosphorylation sites of the native enzyme. Other autophosphorylation sites occur in the middle third of the native enzyme. A trypsin cleavage site within the 35-kDa region is protected in phosphorylated kinase but is readily cleaved in unphosphorylated kinase producing catalytically inactive 25- and 11-kDa fragments from the NH2- and COOH-terminal ends, respectively, of the 35-kDa peptide. This implies that the conformation around the "25/11" cleavage site changes upon phosphorylation of the native enzyme. The position of this site corresponds to the activation loop of protein kinase A (see the accompanying paper: Brzeska, H., Szczepanowska, J., Hoey, J., and Korn, E. D. (1996) J. Biol. Chem. 271, 27056-27062). Exogenously added MIHC kinase phosphorylates the 11-kDa fragment, but not the 25-kDa fragment, indicating that the phosphorylation sites of the 35-kDa catalytic fragment are located within the COOH-terminal 11 kDa. The accompanying paper describes the cloning, sequencing, and expression of a fully active 35-kDa catalytic domain.

Crystal structures of catalytic subunit of cAMP-dependent protein kinase in complex with isoquinolinesulfonyl protein kinase inhibitors H7, H8, and H89. Structural implications for selectivity

The discovery of several hundred different protein kinases involved in highly diverse cellular signaling pathways is in stark contrast to the much smaller number of known modulators of cell signaling. Of these, the H series protein kinase inhibitors (1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7), N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide (H8) N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H89)) are frequently used to block signaling pathways in studies of cellular regulation. To elucidate inhibition mechanisms at atomic resolution and to enable structure-based drug design of potential therapeutic modulators of signaling pathways, we determined the crystal structures of corresponding complexes with the cAPK catalytic subunit. Complexes with H7 and H8 (2.2 A) and with H89 (2.3 A) define the binding mode of the isoquinoline-sulfonamide derivatives in the ATP-binding site while demonstrating effects of ligand-induced structural change. Specific interactions between the enzyme and the inhibitors include the isoquinoline ring nitrogen ligating to backbone amide of Val-123 and an inhibitor side chain amide bonding to the backbone carbonyl of Glu-170. The conservation of the ATP-binding site of protein kinases allows evaluation of factors governing general selectivity of these inhibitors among kinases. These results should assist efforts in the design of protein kinase inhibitors with specific properties.

Phosphorylation of threonine 638 critically controls the dephosphorylation and inactivation of protein kinase Calpha

BACKGROUND

It has been widely reported that multisite phosphorylation plays an essential role in the regulation of protein kinases. However, our understanding of how these events modify protein function in vitro and in vivo is poorly understood. Protein kinase C (PKC) affords an interesting example of how phosphorylation control is coupled to effector control. PKC is acutely regulated by the second messenger diacylglycerol; however, it is also known to undergo multisite phosphorylation. Previously, we and others have shown that one site in the 'activation loop' of PKCalpha (a threonine residue at position 497; T497) and PKCbeta (T500) is essential for the catalytic competence of these proteins. More recently, a carboxy-terminal site (T638 in PKCalpha) has been implicated. In this report, we investigate the role of this site and its interaction with the catalytic core site.

RESULTS

We have analyzed mutant PKCalpha proteins, in which amino-acid substitutions were made at the T638 site, and shown that phosphorylation at this site affects the conformation of the protein, as judged by thermal stability, and sensitivity to oxidation, trypsin and phosphatase treatment. This supersensitivity to dephosphorylation in vitro was also seen in an agonist-dependent context in vivo. We have also shown that phosphorylation of this site is not essential for catalytic activity of the purified protein. The molecular basis of the control operating through the T638 site was provided by the evidence of a functional interaction with the previously described catalytic core site, T497. This inter-relationship was further established by the demonstration that the E497 mutant protein had a thermal instability and phosphatase supersensitivity similar to that of the A638 and E638 mutants.

CONCLUSIONS

The T638 phosphorylation site is not required for the catalytic function of PKCalpha per se, but serves to control the duration of activation by regulating the rate of dephosphorylation and inactivation of the protein. This is achieved through the cooperative interaction between the T638 and T497 sites; if either of these residues is not phosphorylated, the protein is supersensitive to phosphatase action. This model of PKCalpha function is likely to be of general significance to the protein kinase superfamily, where similarly juxtaposed sites exist. We conclude that dephosphorylation of PKCalpha, and, by inference, other protein kinases, is regulated by multisite phosphorylation.

Dephosphorylation of catalytic subunit of cAMP-dependent protein kinase at Thr-197 by a cellular protein phosphatase and by purified protein phosphatase-2A

Thr-197 phosphate is essential for optimal activity of the catalytic (C) subunit of cAMP-dependent protein kinase enzyme, and, in the C subunit crystal structure, it is buried in a cationic pocket formed by the side chains of His-87, Arg-165, Lys-189, and Thr-195. Because of its apparent role in stabilizing the active conformation of C subunit and its resistance to several phosphatases, the phosphate on Thr-197 has been assumed to be metabolically stable. We now show that this phosphate can be removed from C subunit by a protein phosphatase activity extracted from S49 mouse lymphoma cells or by purified protein phosphatase-2A (PP-2A) with concomitant loss of enzymatic activity. By anion-exchange chromatography, inhibitor sensitivity, and relative activity against glycogen phosphorylase a and C subunit as substrates, the cellular phosphatase resembled a multimeric form of PP-2A. PP-1 was ineffective against native C subunit, but it was able to dephosphorylate Thr-197 in urea-treated C subunit. Accessibility of Thr-197 phosphate to the cellular phosphatase was enhanced by storage of C subunit in a phosphate-free buffer or by inclusion of modest concentrations of urea in the reactions and was reduced by salt concentrations in the physiological range and/or by amino-terminal myristoylation. It is concluded that a multimeric form of PP-2A or a closely related enzyme from cell extracts is capable of removing the Thr-197 phosphate from native C subunit in vitro and could account for significant turnover of this phosphate in intact cells.

Autophosphorylation of creatine kinase: characterization and identification of a specifically phosphorylated peptide

We report that several different chicken and rabbit creatine kinase (CK)1 isoenzymes showed an incorporation of 32P when incubated with [gamma-32P]ATP in an autophosphorylation assay. This modification was was shown to be of covalent nature and resulted from an intramolecular phosphorylation reaction that was not dependent on the CK enzymatic activity. By limited proteolysis and sequence analysis of the resulting peptides, the autophosphorylation sites of chicken brain-type CK could be localized within the primary sequence of the enzyme to a 4.5 kDa peptide, spanning a region that is very likely an essential part of the active site of creatine kinase. Homologous peptides were found to be autophosphorylated in chicken muscle-type CK and a mitochondrial CK isoform. Phosphopeptide as well as mutant enzyme analysis provided evidence that threonine-282(2), threonine-289 and serine-285 are involved in the autophosphorylation of CK. Thr-282 and Ser-285 are located close to the reactive cysteine-283. Thr-289 is located within a conserved glycine-rich region highly homologous to the glycine-rich loop of protein kinases, which is known to be important for ATP binding. Thus, it seems likely that the described region constitutes an essential part of the active site of CK.

Phosphorylation and activation of Ca(2+)-calmodulin-dependent protein kinase IV by Ca(2+)-calmodulin-dependent protein kinase Ia kinase. Phosphorylation of threonine 196 is essential for activation

Purified pig brain Ca(2+)-calmodulin (CaM)-dependent protein kinase Ia kinase (Lee, J. C., and Edelman, A. M. (1994) J. Biol. Chem. 269, 2158-2164) enhances, by up to 24-fold, the activity of recombinant CaM kinase IV in a reaction also requiring Ca(2+)-CaM and MgATP. The addition of brain extract, although capable of activating CaM kinase IV by itself, provides no further activation beyond that induced by purified CaM kinase Ia kinase, consistent with the lack of a requirement of additional components for activation. Activation is accompanied by the development of significant (38%) Ca(2+)-CaM-independent CaM kinase IV activity. In parallel fashion to its activation, CaM kinase IV is phosphorylated in a CaM kinase Ia kinase-, Ca(2+)-CaM-, and MgATP-dependent manner. Phosphorylation occurs on multiple serine and threonine residues with a Ser-P:Thr-P ratio of approximately 3:1. The identical requirements for phosphorylation and activation and a linear relationship between extent of phosphorylation of CaM kinase IV and its activation state indicate that CaM kinase IV activation is induced by its phosphorylation. Replacement of Thr-196 of CaM kinase IV with a nonphosphorylatable alanine by site-directed mutagenesis abolishes both the phosphorylation and activation of CaM kinase IV, demonstrating that Thr-196 phosphorylation is essential for activation.

Three protein kinase structures define a common motif

Structural comparisons between cAMP-dependent protein kinase, cyclin-dependent kinase 2 and mitogen-activated protein kinase reveal which features are common to the protein kinase family and which are enzyme-specific.