Minimal CK2 activity required for yeast growth

Protein kinase CK2 is essential for the growth of Saccharomyces cerevisiae. Yeast cells that lack the functional genes coding for both the catalytic subunits of protein kinase CK2 can grow only if they are complemented by exogenous cDNAs coding for this subunit. A series of deletion mutants of CK2α from Xenopus laevis was constructed. These mutants that have carboxyl end deletions yield a CK2α product that varies over four orders of magnitude in its capacity to phosphorylate casein in vitro. Complementation of yeast RPG41-1a, a mutant defective in CKA1 and CKA2 genes, with wild-type X. laevis CK2α and with cDNAs coding for truncated CK2α having amino acids 1–328 and 1–327 resulted in cells that grew in gal-minimal media at 30 ^∘C as well as the cells harboring the yeast CKA2 gene. However, the growth was significantly diminished when cells were complemented with X. laevis CK2α containing 1–326 amido acids. This mutant has 0.6% of the catalytic activity of the wild-type enzyme. Yeast cells that expressed CK2α 1–324 and 1–323 which have 10-and 100-fold less activity, respectively, were not able to grow. The growth of cells containing the CK2α 1–326 mutant was very sensitive to temperature, and minimal growth was observed at 37 ^∘C. This mutant was also more sensitive to UV radiation but was not significantly affected by 0.4 M NaCl.Both authors contributed equally to this work     

Drosophila Casein Kinase I Alpha Regulates Homolog Pairing and Genome Organization by Modulating Condensin II Subunit Cap-H2 Levels

The spatial organization of chromosomes within interphase nuclei is important for gene expression and epigenetic inheritance. Although the extent of physical interaction between chromosomes and their degree of compaction varies during development and between different cell-types, it is unclear how regulation of chromosome interactions and compaction relate to spatial organization of genomes. Drosophila is an excellent model system for studying chromosomal interactions including homolog pairing. Recent work has shown that condensin II governs both interphase chromosome compaction and homolog pairing and condensin II activity is controlled by the turnover of its regulatory subunit Cap-H2. Specifically, Cap-H2 is a target of the SCF^Slimb E3 ubiquitin-ligase which down-regulates Cap-H2 in order to maintain homologous chromosome pairing, chromosome length and proper nuclear organization. Here, we identify Casein Kinase I alpha (CK1α) as an additional negative-regulator of Cap-H2. CK1α-depletion stabilizes Cap-H2 protein and results in an accumulation of Cap-H2 on chromosomes. Similar to Slimb mutation, CK1α depletion in cultured cells, larval salivary gland, and nurse cells results in several condensin II-dependent phenotypes including dispersal of centromeres, interphase chromosome compaction, and chromosome unpairing. Moreover, CK1α loss-of-function mutations dominantly suppress condensin II mutant phenotypes in vivo. Thus, CK1α facilitates Cap-H2 destruction and modulates nuclear organization by attenuating chromatin localized Cap-H2 protein.     

Autophosphorylation at the regulatory β subunit reflects the supramolecular organization of protein kinase CK2

Among the features of protein kinase CK2, autophosphorylation at its β-subunit(s) upon incubation with ATP/Mg⁺⁺ was early detected as a rapid and stoichiometric event occurring through an intramolecular mechanism as judged from kinetic analyses. The autophosphorylation site was mapped to Ser2 and, to a lesser extent, Ser3 both fulfilling the CK2 consensus sequence (MSSSEEV). The crystal structure of the heterotetrameric holoenzyme, however, is not compatible with an intramolecular autophosphorylation of the N-terminal stretch of either of the two β subunits. Here we show that efficient “intramolecular” autophosphorylation of the β subunit is crucially dependent on the formation of oligomers composed by several holoenzyme heterotetrameric protomers. Increasing ionic strength of the incubation medium promoting dissociation of the supramolecular oligomers abrogates β subunit autophosphorylation, although CK2 catalytic activity, as judged from the phosphorylation of exogenous substrates, is still quite evident. These findings, in conjunction with graphic modelization, support the view that CK2 autophosphorylation at its β subunits takes place through an “intraoligomeric” mechanism where the β subunits of a protomer are phosphorylated by the catalytic subunits of another adjacent protomer. It appears therefore that in vivo β autophosphorylation is symptomatic of supramolecular CK2 oligomers.     

Highlighting protein kinase CK2 movement in living cells

Protein kinase CK2 has traditionally been described as a stable heterotetrameric complex (α < eqid1 > β₂) but new approaches that effectively capture the dynamic behavior of proteins, are bringing a new picture of this complex into focus. To track the spatio-temporal dynamics of CK2 in living cells, we fused its catalytic α and regulatory β subunits with GFP and analog proteins. Beside the mostly nuclear localization of both subunits, and the identification of specific domains on each subunit that triggers their localization, the most significant finding was that the association of both CK2 subunits in a stable tetrameric holoenzyme eliminates their nuclear import (Mol Cell Biol {23}: 975–987, 2003). Molecular movements of both subunits in the cytoplasm and in the nucleus were analyzed using different new and updated fluorescence imaging methods such as: fluorescence recovery after photo bleaching (FRAP), fluorescence loss in photo bleaching (FLIP), fluorescence correlation spectroscopy (FCS), and photoactivation using a biphoton microscope. These fluorescence-imaging techniques provide unprecedented ways to visualize and quantify the mobility of each individual CK2 subunit with high spatial and temporal resolution. Visualization of CK2 heterotetrameric complex formation could also be recorded using the fluorescence resonance energy transfer (FRET) technique. FRET imaging revealed that the assembling of this molecular complex can take place both in the cytoplasmic and nuclear compartments. The spatio–temporal organization of individual CK2 subunits and their dynamic behavior remain now to be correlated with the functioning of this kinase in the complex environment of the cell.     

Human Regulatory Subunit RIβ of cAMP-Dependent Protein Kinases: Expression, Holoenzyme Formation and Microinjection into Living Cells

The human regulatory subunit RIβ of cAMP-dependent protein kinases was expressed in Escherichia coli as a fusion protein with glutathione S -transferase. Purification was performed by affinity chromatography on glutathione-agarose beads after cleavage with thrombin. The human recombinant Riff protein migrated at 55 kDa on SDS-PAGE and displayed immunoreactivity with an anti-human RIβ antiserum. Furthermore, the purified recombinant RIβ protein was shown to exist as a dimer that was able to form holoenzyme with the catalytic subunit Cα. The rate of RIβ₂Cα₂ holoenzyme formation was faster in the presence than in the absence of MgATP. The kinase activity measured before and after adding cAMP to the holoenzyme showed that the presence of cAMP resulted in holoenzyme dissociation and release of active Cα-subunit, due to cAMP binding to RIβ. Compared to a RIα₂Cα₂ holoenzyme, the RIβ₂Cα₂ holoenzyme exhibited a more than twofold higher sensitivity to cAMP. The subcellular localization of Riff was analyzed in quiescent REF-52 fibroblasts and Wistar rat thyroid (WRT) cells after microinjection of fluorescently labeled proteins into the cytoplasm. A cytoplasmic distribution was observed when free RIβ was injected, whereas free Cα injected into the cytoplasm appeared in the nucleus. When holoenzymes with labeled Riff and unlabeled Cα, or unlabeled RIβ and labeled Cα, were injected, unstimulated cells showed fluorescence in the cytoplasm of both cell types. REF-52 cells stimulated with 8-bromo-cAMP (8-Br-cAMP) and WRT cells treated with thyrotropin (TSH) showed fluorescence mainly in the cytoplasm when RIβ was the labeled subunit of the in vivo dissociated bioenzyme. In contrast, nuclear fluorescence was evident from the release and translocation of labeled Cα from the holoenzyme complex after stimulation with 8-Br-cAMP or TSH.     

Primary and secondary interactions between CK2α and CK2β lead to ring-like structures in the crystals of the CK2 holoenzyme

Protein kinase CK2 predominantly exists as a heterotetrameric holoenyzme consisting of two catalytic subunits (CK2α) and two non-catalytic subunits (CK2β). Early investigations which we review here had revealed the presence of two types of contacts between CK2α and CK2β: a primary interaction responsible for the stability of the CK2 holoenzyme and stimulatory for the catalytic activity, and a secondary interaction which is inhibitory and in which the acidic loop of CK2β associates with the basic stretch and the (p+1)-loop of CK2α. At the end of the last decade both types of interactions were assumed to occur within the same tetrameric complex. The CK2 holoenyzme structure, however, suggested that the secondary interactions must happen between different CK2 tetramers. Such a behaviour should lead to higher-ordered aggregates consistent with several previous reports about a distinct aggregation propensity of CK2. We demonstrate here that in the CK2 holoenzyme crystals contacts between different CK2 tetramers exists which provide structural details of the secondary CK2α/CK2β interactions. These mainly ionic interactions lead to trimeric rings of CK2 holoenzymes in the crystal. In these rings each CK2 tetramer possesses one CK2α subunit open for substrate binding and another one whose active site is blocked by a secondary contact with CK2β from a neighbouring tetramer. This observation fits to previous findings that salt-sensitive ring-like aggregates of CK2 holoenzymes can exist which possess significant catalytic activity. Furthermore it suggests that earlier ideas about a regulatory role of the enzyme’s aggregation propensity may be worth to be revitalised.     

Yeast surviving factor Svf1 as a new interacting partner, regulator and in vitro substrate of protein kinase CK2

Since Svf1 is phosphoprotein, we investigated whether it was a substrate for protein kinase CK2. According to the amino acid sequence Svf1 harbours 20 putative CK2 phosphorylation sites. Here, we have reported cloning, overexpression, purification and characterization of yeast Svf1 as a substrate for three forms of yeast CK2. Svf1 serves as a substrate for both the recombinant CK2α (K _m 0.35 μM) and CK2α′ (K _m 0.18 μM) as well as CK2 holoenzyme (K _m 1.1 μM). Different K _m values argue that CK2β(β′) subunit has an inhibitory effect on the activity of both CK2α and CK2α′ towards surviving factor Svf1. Reconstitution of α′₂ββ′ isoform of CK2 holoenzyme shows that β/β′ subunits have regulatory effect depending on the kind of CK2 catalytic subunit. This effect was not observed in the case of α₂ββ′ isoform, which may be due to interaction between Svf1 and regulatory CK2β subunit (shown by co-immunoprecipitation experiments). Interactions between CK2 subunits and Svf1 protein may have influence on ATP as well as ATP-competitive inhibitors (TBBt and TBBz) binding. CK2 phosphorylates up to six serine residues in highly acidic peptide K¹⁹⁹EVIPESDEEESSADEDDNEDEDEESGDSEEESGSEEESDSEEVEITYED²⁴⁸ of the Svf1 protein in vitro. Presented data may help to elucidate the role of protein kinase CK2 and Svf1 in the regulation of cell survival pathways.     

Drosophila Sirt2/mammalian SIRT3 deacetylates ATP synthase β and regulates complex V activity

Adenosine triphosphate (ATP) synthase β, the catalytic subunit of mitochondrial complex V, synthesizes ATP. We show that ATP synthase β is deacetylated by a human nicotinamide adenine dinucleotide (NAD⁺)–dependent protein deacetylase, sirtuin 3, and its Drosophila melanogaster homologue, dSirt2. dsirt2 mutant flies displayed increased acetylation of specific Lys residues in ATP synthase β and decreased complex V activity. Overexpression of dSirt2 increased complex V activity. Substitution of Lys 259 and Lys 480 with Arg in human ATP synthase β, mimicking deacetylation, increased complex V activity, whereas substitution with Gln, mimicking acetylation, decreased activity. Mass spectrometry and proteomic experiments from wild-type and dsirt2 mitochondria identified the Drosophila mitochondrial acetylome and revealed dSirt2 as an important regulator of mitochondrial energy metabolism. Additionally, we unravel a ceramide–NAD⁺–sirtuin axis wherein increased ceramide, a sphingolipid known to induce stress responses, resulted in depletion of NAD⁺ and consequent decrease in sirtuin activity. These results provide insight into sirtuin-mediated regulation of complex V and reveal a novel link between ceramide and Drosophila acetylome.     

Mutations on the N-Terminal Edge of the DELSEED Loop in either the α or β Subunit of the Mitochondrial F1-ATPase Enhance ATP Hydrolysis in the Absence of the Central γ Rotor

F₁-ATPase is a rotary molecular machine with a subunit stoichiometry of α₃β₃γ₁δ₁ε₁. It has a robust ATP-hydrolyzing activity due to effective cooperativity between the three catalytic sites. It is believed that the central γ rotor dictates the sequential conformational changes to the catalytic sites in the α₃β₃ core to achieve cooperativity. However, recent studies of the thermophilic Bacillus PS3 F₁-ATPase have suggested that the α₃β₃ core can intrinsically undergo unidirectional cooperative catalysis (T. Uchihashi et al., Science 333:755-758, 2011). The mechanism of this γ-independent ATP-hydrolyzing mode is unclear. Here, a unique genetic screen allowed us to identify specific mutations in the α and β subunits that stimulate ATP hydrolysis by the mitochondrial F₁-ATPase in the absence of γ. We found that the F446I mutation in the α subunit and G419D mutation in the β subunit suppress cell death by the loss of mitochondrial DNA (ρ^o) in a Kluyveromyces lactis mutant lacking γ. In organello ATPase assays showed that the mutant but not the wild-type γ-less F₁ complexes retained 21.7 to 44.6% of the native F₁-ATPase activity. The γ-less F₁ subcomplex was assembled but was structurally and functionally labile in vitro. Phe446 in the α subunit and Gly419 in the β subunit are located on the N-terminal edge of the DELSEED loops in both subunits. Mutations in these two sites likely enhance the transmission of catalytically required conformational changes to an adjacent α or β subunit, thereby allowing robust ATP hydrolysis and cell survival under ρ^o conditions. This work may help our understanding of the structural elements required for ATP hydrolysis by the α₃β₃ subcomplex.     

Properties and purification of the catalytic subunit of cyclic AMP-dependent protein kinase of adipose tissue

The catalytic subunit of cAMP-dependent protein kinase from rat adipose tissue was purified to apparent homogeneity by making use of the differential binding of the holoenzyme and the free catalytic subunit to CM-Sephadex and by gel chromatography. Stability and yield was improved by inclusion of nonionic detergent in all steps after dissociation of the holoenzyme. Isoelectric focusing separated enzyme species with pI values of 7.8 and 8.6–8.8. The amino acid composition was similar to the enzyme purified from other tissues. Enzyme activity was markedly unstable in dilute solutions (<5 μg/ml). Additions of nonionic detergent, glycerol, bovine serum albumin and, especially, histones stabilized the enzyme. With protamine, the catalytic subunit had an apparent K_m of 60 μM and V_max of 20 μmol·min⁻¹·mg⁻¹, corresponding values with mixed histones were 12 μM and 1.2 μmol·min⁻¹·mg⁻¹. With both protein substrates the apparent K_m for ATP was 11 μM. Concentrations of Mg²⁺ above 10 mM were inhibitory. Histone phosphorylation was inhibited by NaCl (50% at 0.5 M NaCl) while protamine phosphorylation was stimulated (4-fold at 1 M NaCl). Inorganic phosphate inhibited both substrates (histones: 50% at 0.3 M, and protamine: 50% at 0.5 M). pH optimum was around pH 9 with both substrates. The catalytic subunit contained 2.0 (range of three determinations, 1.7–2.3) mol phosphate/mol protein. It was autophosphorylated and incorporated ³²P_i from [γ-³²P]ATP in a time-dependent process, reaching saturation when approx. 0.1 mol phosphate/mol catalytic subunit was incorporated.     

Yeast holoenzyme of protein kinase CK2 requires both β and β′ regulatory subunits for its activity

Protein kinase CK2 is a highly conserved Ser/Thr protein kinase that is ubiquitous among eucaryotic organisms and appears to play an important role in many cellular functions. This enzyme in yeast has a tetrameric structure composed of two catalytic (α and/or α′) subunits and two regulatory β and β′ subunits. Previously, we have reported isolation from yeast cells four active forms of CK2, composed of αα′ββ′, α₂ββ′, α′₂ββ′ and a free α′-catalytic subunit. Now, we report that in Saccharomyces cerevisiae CK2 holoenzyme regulatory β subunit cannot substitute other β′ subunit and only both of them can form fully active enzymatic unit. We have examined the subunit composition of tetrameric complexes of yeast CK2 by transformation of yeast strains containing single deletion of the β or β′ regulatory subunits with vectors carrying lacking CKB1 or CKB2 genes. CK2 holoenzyme activity was restored only in cases when both of them were present in the cell. Additional, co-immunoprecypitation experiments show that polyadenylation factor Fip1 interacts with catalytic α subunits of CK2 and interaction with beta subunits in the holoenzyme decreases CK2 activity towards this protein substrate. These data may help to elucidate the role of yeast protein kinase CK2β/β′ subunits in the regulation of holoenzyme assembly and phosphotransferase activity.     

In vivo modification of Na,K-ATPase activity in Drosophila

We have constructed and characterized transgenic Drosophila lines with modified Na⁺,K⁺-ATPase activity. Using a temperature dependent promoter from the hsp70 gene to drive expression of wild-type α subunit cDNA, we can conditionally rescue bang-sensitive paralysis and ouabain sensitivity of a Drosophila Na⁺,K⁺-ATPase α subunit hypomorphic mutant, 2206. In contrast, a mutant α subunit (α^D369N) leads to increased bang-sensitive paralysis and ouabain sensitivity. We can also generate temperature dependent phenotypes in wild-type Drosophila using the same hsp70 controlled α transgenes. Ouabain sensitivity was as expected, however, both bang sensitive paralysis or locomotor phenotypes became more severe regardless of the type of α subunit transgene. Using the Gal4-UAS system we have limited expression of α transgenes to cell types that normally express a particular Drosophila Na⁺,K⁺-ATPase β (Nervana) subunit isoform (Nrv1 or 2). The Nrv1-Gal4 driver results in lethality while the Nrv2-Gal4 driver shows reduced viability, locomotor function and uncontrolled wing beating. These transgenic lines will be useful for disrupting function in a broad range of cell types.     

The alpha catalytic subunit of protein kinase CK2 is required for mouse embryonic development

Protein kinase CK2 (formerly casein kinase II) is a highly conserved and ubiquitous serine/threonine kinase that is composed of two catalytic subunits (CK2α and/or CK2α′) and two CK2β regulatory subunits. CK2 has many substrates in cells, and key roles in yeast cell physiology have been uncovered by introducing subunit mutations. Gene-targeting experiments have demonstrated that in mice, the CK2β gene is required for early embryonic development, while the CK2α′ subunit appears to be essential only for normal spermatogenesis. We have used homologous recombination to disrupt the CK2α gene in the mouse germ line. Embryos lacking CK2α have a marked reduction in CK2 activity in spite of the presence of the CK2α′ subunit. CK2α^−/− embryos die in mid-gestation, with abnormalities including open neural tubes and reductions in the branchial arches. Defects in the formation of the heart lead to hydrops fetalis and are likely the cause of embryonic lethality. Thus, CK2α appears to play an essential and uncompensated role in mammalian development.     

Evidence for aggregation of protein kinase CK2 in the cell: a novel strategy for studying CK2 holoenzyme interaction by BRET2

Protein kinase CK2 is a ubiquitous pro-survival kinase whose substrate targets are involved in various cellular processes. Crystal structure analysis confirmed constitutive activity of the kinase, yet CK2 activity regulation in the cell is still obscure. In-vitro studies suggest autoinhibitory aggregation of the hetero-tetrameric CK2 holoenzyme as a basis for CK2 regulation. In this study, we applied bioluminescent resonance energy transfer (BRET) technology to investigate CK2 holoenzyme aggregation in living cells. We designed a BRET² pair consisting of the fusion proteins CK2α-Rluc8 and CK2α-GFP². This BRET² sensor reported specific interaction of CK2 holoenzyme complexes. Furthermore, the BRET² sensor was applied to study modulators of CK2 aggregation. We found that CK2 aggregation is not static and can be influenced by the CK2-binding protein alpha subunit of the heterotrimeric G-protein that stimulates adenylyl cyclase (G_αs) and the polycationic compound polylysine. G_αs, but not the CK2 substrate β-arrestin2, decreased the BRET² signal by up to 50 %. Likewise polylysine, but not the CK2 inhibitor DRB, decreased the signal in a dose-dependent manner up to 50 %. For the first time, we present direct experimental evidence for CK2 holoenzyme aggregates in the cell. Our data suggest that CK2 activity may be controlled by holoenzyme aggregation, to our knowledge a novel mechanism for protein kinase regulation. Moreover, the BRET² sensor used in our study is a novel tool for studying CK2 regulation by aggregation and pharmacological screening for novel allosteric CK2 effectors.     

Single Turnover Autophosphorylation Cycle of the PKA RIIβ Holoenzyme

To provide tight spatiotemporal signaling control, the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) holoenzyme typically nucleates a macromolecular complex or a “PKA signalosome.” Using the RIIβ holoenzyme as a prototype, we show how autophosphorylation/dephosphorylation of the RIIβ subunit, as well as cAMP and metal ions, contribute to the dynamics of PKA signaling. While we showed previously that the RIIβ holoenzyme could undergo a single turnover autophosphorylation with adenosine triphosphate and magnesium (MgATP) and trap both products in the crystal lattice, we asked here whether calcium could trap an ATP:RIIβ holoenzyme since the RIIβ holoenzyme is located close to ion channels. The 2.8Å structure of an RIIβ^p ₂:C₂:(Ca₂ADP)₂ holoenzyme, supported by biochemical and biophysical data, reveals a trapped single phosphorylation event similar to MgATP. Thus, calcium can mediate a single turnover event with either ATP or adenosine-5''-(β,γ-imido)triphosphate (AMP-PNP), even though it cannot support steady-state catalysis efficiently. The holoenzyme serves as a “product trap” because of the slow off-rate of the pRIIβ subunit, which is controlled by cAMP, not by phosphorylation of the inhibitor site. By quantitatively defining the RIIβ signaling cycle, we show that release of pRIIβ in the presence of cAMP is reduced by calcium, whereas autophosphorylation at the phosphorylation site (P-site) inhibits holoenzyme reassociation with the catalytic subunit. Adding a single phosphoryl group to the preformed RIIβ holoenzyme thus creates a signaling cycle in which phosphatases become an essential partner. This previously unappreciated molecular mechanism is an integral part of PKA signaling for type II holoenzymes.     

Immunoreactivity of subunits of the (Na + K)-ATPase Cross-reactivity of the α,α+ and β forms in different organs and species

The immunologic cross-reactivity of the α and α+ forms of the large subunit and the β subunit of the (Na⁺ + K⁺)-ATPase from brain and kidney preparations was examined using rabbit antiserum prepared against the purified holo lamb kidney enzyme. As previously reported by Sweadner ((1979) J. Biol. Chem. 254, 6060–6067) phosphorylation of the large subunit of the (Na⁺ + K⁺)-ATPase in the presence of Na⁺, Mg²⁺, and [γ-³²P]ATP revealed that dog and, very likely, rat brain contain two forms of the large subunit (designated α and α+) while dog, rat, and lamb kidney contain only one form (α). The cross-reactivity of the α and α+ forms in these preparations was investigated by resolving the subunits by SDS-polyacrylamide gel electrophoresis. The separated polypeptides were transferred to unmodified nitrocellulose paper, and reacted with rabbit anti-lamb kidney serum, followed by detection of the antigen-antibody complex with ¹²⁵I-labeled protein A and autoradiography. By this method, the α and α+ forms of rat and dog brain, as well as the α form found in kidney, were shown to cross-react. In addition, membranes from human cerebral cortex were shown to contain two immunoreactive bands corresponding to the α and α+ forms of dog brain. In contrast, the brain of the insect Manduca sexta contains only one immunoreactive polypeptide with a molecular weight intermediate to the α and α+ forms of dog brain. The β subunit from lamb, dog and rat kidney and from dog and rat brain cross-reacts with anti-lamb kidney (Na⁺ + K⁺)-ATPase serum. The mobility of the β subunit from dog and rat brain on SDS-polyacrylamide electrophoresis gels is greater than the mobility of the β subunit from lamb, rat or dog kidney.