An Updated View on an Emerging Target: Selected Papers from the 8th International Conference on Protein Kinase CK2

The 8th International Conference on Protein Kinase CK2 took place in Homburg, Germany, from 6 September to 9 September 2016. Over 80 scientists from Australia, China, Japan, USA, Canada, Denmark, France, Italy, Spain, Poland and Germany participated. After the opening lecture by Lorenzo A. Pinna, Padova, Italy, entitled Exploring the CK2 Paradox: Restless, Dangerous, Dispensable, the scientists reported their latest research on the structural characterization of CK2, hence leading directly to the development of CK2 inhibitors. The driving force behind the development of inhibitors is their use in the treatment of various diseases, which was the next topic of the conference. New findings on protein kinase CK2 were addressed in the following session. The final topic of the conference addressed the role of CK2 in differentiation and development

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

Golgi apparatus casein kinase phosphorylates bioactive Ser-6 of bone morphogenetic protein 15 and growth and differentiation factor 9

AbstractBone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9 (GDF-9) are oocyte-secreted factors that play essential roles in human folliculogenesis and ovulation. Their bioactivity is tightly regulated through phosphorylation, likely to occur within the Golgi apparatus of the secretory pathway. Here we show that Golgi apparatus casein kinase (G-CK) catalyzes the phosphorylation of rhBMP-15 and rhGDF-9. rhBMP-15, in particular, is an excellent substrate for G-CK. In each protein a single residue is phosphorylated by G-CK, corresponding to the serine residue at the sixth position of the mature region of both rhBMP-15 and rhGDF-9, whose phosphorylation is required for biological activity

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

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

Phosphorylation of FAM134C by CK2 controls starvation-induced ER-phagy.

Selective degradation of the endoplasmic reticulum (ER) via autophagy (ER-phagy) is initiated by ER-phagy receptors, which facilitate the incorporation of ER fragments into autophagosomes. FAM134 reticulon family proteins (FAM134A, FAM134B, and FAM134C) are ER-phagy receptors with structural similarities and nonredundant functions. Whether they respond differentially to the stimulation of ER-phagy is unknown. Here, we describe an activation mechanism unique to FAM134C during starvation. In fed conditions, FAM134C is phosphorylated by casein kinase 2 (CK2) at critical residues flanking the LIR domain. Phosphorylation of these residues negatively affects binding affinity to the autophagy proteins LC3. During starvation, mTORC1 inhibition limits FAM134C phosphorylation by CK2, hence promoting receptor activation and ER-phagy. Using a novel tool to study ER-phagy in vivo and FAM134C knockout mice, we demonstrated the physiological relevance of FAM134C phosphorylation during starvation-induced ER-phagy in liver lipid metabolism. These data provide a mechanistic insight into ER-phagy regulation and an example of autophagy selectivity during starvation.We thank G. Diez Roux and P. Ashley-Norman for critical reading of the manuscript. We thank the microscopy, MS, advanced histopathology, and FACS facilities at TIGEM Institute. We thank E. Nusco for helping us with AAV injections. Funding: This work was supported by European Research Council (ERC) (714551), Telethon intramural grants, and Associazione Italiana per la Ricerca sul Cancro (AIRC) (IG 2015 Id 17717) (to C.S.) and Telethon Foundation (TMPGCBX16TT), AFM Telethon (Trampoline Grant), and AIRC (MFAG-2020-24856) (to P.G.). G.D.L. is a recipient of AIRC fellowship “Francesco Alicino” (25407). V.L. acknowledges funding from the ERC (101001784), the Italian MIUR-PRIN 2017 (2017FJZZRC), and the Swiss National Supercomputing Center (CSCS) (project ID u8). The work of A.S. was supported by the German Research Foundation DFG (SFB1177/2 and WO210/20-2) and the Dr. Rolf M. Schwiete Stiftung (13/2017). A.E. is supported by the RETOS projects Programme of Spanish Ministry of Science, Innovation and Universities, Spanish State Research Agency (grants SAF2015-67538-R and PID2019-104012RB-I00), and the ERC (638891). A.B.P.-G. is a recipient of Ph.D. fellowship from MICIU/AEI (BES-2017-081381). A.R. is a recipient of Umberto Veronesi Foundation postdoctoral fellowship. Author contributions: G.D.L. and F.I. performed most of the experiments. F.I. and A.B.P.-G. performed in vivo experiments. M.M. performed mutagenesis experiments. S.A. and V.L. performed LC3-FAM134C binding analysis. C.P.Q.M. performed in vitro phosphorylation assays. L.C. analyzed CK2 substrate phosphorylation. F.S., A.P., C.C., and A.S. analyzed proteomic data. G.N. provided critical suggestions. A.R. performed proteomic experiments. A.E. supervised in vivo experiments. M.R., L.A.P., and O.M. supervised CK2 experiments. C.S. designed the study. P.G. and C.S. conceived and supervised the experiments. C.S., P.G., V.L., and M.R. wrote the paper. G.D.L. and F.I. prepared the figures. All the authors read the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.S

Phosphorylation of the phosphatase modulator subunit (inhibitor-2) by casein kinase-1 Identification of the phosphorylation sites

AbstractThe isolated modulator subunit of the inactive protein phosphatase-1 is phosphorylated in vitro by casein kinase-1 at two different site Ser-86 and Ser-174. The Ser-86 site is a common target for casein kinase-1 and casein kinase-2, but is preferentially phosphorylated by the former enzyme. The Ser-174 site seems to be specific for casein kinase-1, and is phosphorylated at a slower rate. These results give a new insight into the in vitro phosphorylation pattern of the modulator subunit of the phosphatase and provides additional data on the specificity of casein kinase-1

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