Protein Kinase CK2 Mutants Defective in Substrate Recognition PURIFICATION AND KINETIC ANALYSIS

Five mutants of protein kinase CK2 α subunit in which altogether 14 basic residues were singly to quadruply replaced by alanines (K74A,K75A,K76A,K77A; K79A, R80A,K83A; R191A,R195A,K198A; R228A; and R278A, K279A,R280A) have been purified to near homogeneity either as such or after addition of the recombinant β subunit. By this latter procedure five mutated tetrameric holoenzymes were obtained as judged from their subunit composition, sedimentation coefficient on sucrose gradient ultracentrifugation, and increased activity toward a specific peptide substrate as compared with the isolated α subunits. The kinetic constants and the phosphorylation efficiencies (Vmax/Km) of all the mutants with the parent peptide RRRADDSDDDDD and a series of derivatives, in which individual aspartic acids were replaced by alanines, have been determined. Three mutants, namely K74A,K75A,K76A,K77A; K79A,R80A, K83A; and R191A,R195A,K198A display dramatically lower phosphorylation efficiency and 8-50-fold higher Km values with the parent peptide, symptomatic of reduced attitude to bind the peptide substrate as compared with CK2 wild type. Such differences either disappear or are attenuated if the mutants R191A,R195A, K198A; K79A,R80A,K83A; and K74A,K75A,K76A,K77A are assayed with the peptides RRRADDSADDDD, RRRADDSDDADD, and RRRADDSDDDAA, respectively. In contrast, the phosphorylation efficiencies of the other substituted peptides decrease more markedly with these mutants than with CK2 wild type. These data show that one or more of the basic residues clustered in the 191-198, 79-83, and 74-77 sequences are implicated in the recognition of the acidic determinants at positions +1, +3, and +4/+5, respectively, and that if these residues are mutated, the relevance of the other acidic residues surrounding serine is increased. In contrast the other two mutants, namely R228A and R278A,K279A, R280A, display with all the peptides Vmax values higher than CK2 wild type, counterbalanced however by somewhat higher Kmvalues. It can be concluded from these data that all the five mutations performed are compatible with the reconstitution of tetrameric holoenzyme, but all of them influence the enzymatic efficiency of CK2 to different extents. Although the basic residues mutated in the 74-77, 79-83, and 191-198 sequences are clearly implicated in substrate recognition by interacting with acidic determinants at variable positions downstream from serine, the other basic residues seem to play a more elusive and/or indirect role in catalysis

Protein Kinase CK2α′ Is Induced by Serum as a Delayed Early Gene and Cooperates with Ha-ras in Fibroblast Transformation

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (alpha and/or alpha') and two noncatalytic (beta) subunits forming a heterotetrameric holoenzyme involved in cell growth and differentiation. Here we report the identification, cloning, and oncogenic activity of the murine CK2alpha' subunit. Serum treatment of quiescent mouse fibroblasts induces CK2alpha' mRNA expression, which peaks at 4 h. The kinetics of CK2alpha' expression correlate with increased kinase activity toward a specific CK2 holoenzyme peptide substrate. The ectopic expression of CK2alpha' (or CK2alpha) cooperates with Ha-ras in foci formation of rat primary embryo fibroblasts. Moreover, we observed that BALB/c 3T3 fibroblasts transformed with Ha-ras and CK2alpha' show a faster growth rate than cells transformed with Ha-ras alone. In these cells the higher growth rate correlates with an increase in calmodulin phosphorylation, a protein substrate specifically affected by isolated CK2 catalytic subunits but not by CK2 holoenzyme, suggesting that unbalanced expression of a CK2 catalytic subunit synergizes with Ha-ras in cell transformation

The ATP,Mg-dependent protein phosphatase: Regulation by casein kinase-1

AbstractThe free modulator subunit of the ATP,Mg-dependent phosphatase is phosphorylated up to 1 mol per mol by casein kinase-1, up to 1.85 mol per mol after dephosphorylation by the PCSH1 phosphatase, but 10-fold less when purified in the presence of NaF, suggesting an in vivo phosphorylation of the casein kinase-1 sites. Peptide mapping of 32P-modulator labeled by casein kinase-1 or -2 shows a different phosphorylation pattern. Phosphorylation of the inactive phosphatase by casein kinase-1 prevents the subsequent kinase FA-mediated activation, while it does not impair the activated phosphatase

Discrimination between the activity of protein kinase CK2 holoenzyme and its catalytic subunits

AbstractThe acronym CK2 denotes a highly pleiotropic Ser/Thr protein kinase whose over-expression correlates with neoplastic growth. A vexed question about the enigmatic regulation of CK2 concerns the actual existence in living cells of the catalytic (α and/or α′) and regulatory β-subunits of CK2 not assembled into the regular heterotetrameric holoenzyme. Here we take advantage of novel reagents, namely a peptide substrate and an inhibitor which discriminate between the holoenzyme and the catalytic subunits, to show that CK2 activity in CHO cells is entirely accounted for by the holoenzyme. Transfection with individual subunits moreover does not give rise to holoenzyme formation unless the catalytic and regulatory subunits are co-transfected together, arguing against the existence of free subunits in CHO cells

Substrate recognition by casein kinase-II: The role of histidine-160

AbstractCasein kinase-II (CK-II) belongs to the protein kinases recognizing serine/threonine in proximity to acidic residues in protein substrates. Crystallography and mutagenesis studies on the cAMP-dependent protein kinase (PKA) disclosed that glutamic acid-170 (E170), is important for interaction of substrates with the enzyme. At a position corresponding to E170 in PKA most Ser/Thr kinases have an aspartic or glutamic acid, while CK-II has a histidine residue (H160). In order to examine the relevance of this substitution for CK-II substrate specificity, a mutant of the catalytic α subunit (H160D), in which H160 was changed to aspartic acid, was made. Our results show that H160 is not primarily involved in canonical substrate recognition, but does interact with an acidic residue located at position −2 with respect to the target Ser/Thr

Structural Features Underlying the Multisite Phosphorylation of the A Domain of the NF-AT4 Transcription Factor by Protein Kinase CK1 †

ABSTRACT: The phosphorylation and dephosphorylation of the NF-AT family of transcription factors play a key role in the activation of T lymphocytes and in the control of the immune response. The mechanistic aspects of NF-AT4 phosphorylation by protein kinase CK1 have been studied in this work with the aid of a series of 27 peptides, reproducing with suitable modifications the regions of NF-AT4 that have been reported to be phosphorylated by this protein kinase. The largest parent peptide, representing the three regions A, Z, and L spanning amino acids 173-218, is readily phosphorylated by CK1 at seryl residues belonging to the A2 segment, none of which fulfill the canonical consensus sequence for CK1. An acidic cluster of amino acids in the linker region between domains A and Z is essential for high-efficiency phosphorylation of the A2 domain, as shown by the increase in K m caused by a deletion of the linker region or a substitution of the acidic residues with glycines. Individual substitutions with alanine of each of the five serines in the A2 domain (S-177, S-180, S-181, S-184, and S-186) reduce the phosphorylation rate, the most detrimental effect being caused by Ser177 substitution which results in a 10-fold drop in V max . On the contrary, the replacement of Ser177 with phosphoserine triggers a hierarchical effect with a dramatic improvement in phosphorylation efficiency, which no longer depends on the linker region for optimal efficiency. These data are consistent with a two-phase phosphorylation mechanism of NF-AT4 by CK1, initiated by the linker region which provides a functional docking site for CK1 and allows the unorthodox phosphorylation of Ser177; once achieved, this phosphoserine residue primes the phosphorylation of other downstream seryl residues, according to a hierarchical mechanism typically exploited by CK1. The large number of protein kinases in eukaryotes, with over 800 genes found in the human genome (1), raises multiple questions as to the function and specificity of these important enzymes. In recent years, several laboratories, including ours, have approached the study of the substrate specificity of protein kinases. These studies have concentrated on the analysis of the amino acid sequences surrounding the immediate vicinity of the sites that are phosphorylated in vivo and in vitro by specific kinases and on the preparation of synthetic peptides that contain these sequences and serve as substrates for these particular enzymes (2-5). These studies have been very useful in determining the consensus sequence recognized preferentially by the active center of these kinases and in predicting the domains of new proteins that are probably phosphorylated by these enzymes. In addition, this approach has allowed us to design several peptides that are highly specific for kinases and that can be employed in assaying for the activity of these kinases in crude extracts of cells and tissues (e.g., refs 5-7). The studies with short peptides, however, demonstrated that these model molecules are sometimes less efficient than the true physiological substrates. In addition, several sequences that contain the defined consensus for phosphorylation by these kinases are not phosphorylated in the native proteins. Conversely, atypical sites that are not acted upon in model peptides serve as good substrates within the context of whole proteins (5). These results clearly indicate that the phosphorylation of proteins by protein kinases involves recognition and interactions that go beyond the immediate vicinity of the acceptor serines or threonines in the substrates. The recent discovery that several protein kinases recognize "docking sites" which are distant from the phosphorylatable residues in their protein substrates constitutes an important step toward the understanding of some of the complexities that provide specificity in kinase-protein substrate interactions (8)

Dual role of calsequestrin as substrate and inhibitor of casein kinase-1 and casein kinase-2

Calsequestrin from different muscle tissues and species has been phosphorylated by casein kinase-1 and casein kinase-2, in the conditions previously reported by Cala and Jones (J. Biol. Chem. 266, 391-398, 1991). Results indicates that rabbit cardiac and skeletal calsequestrin and frog skeletal calsequestrin are phosphorylated by both casein kinase-1 and casein kinase-2, at variance with chicken skeletal calsequestrin which is a poor substrate for both enzymes. We also observed that chicken calsequestrin is able to inhibit phosphorylation of cardiac calsequestrin, as well as other specific substrates, when added together to the assay medium