Expression, purification, crystallization, and preliminary x-ray analysis of casein kinase-1 from Schizosaccharomyces pombe

Quantification of Methylation and Phosphorylation Stoichiometry

Tauopathies including Alzheimer's disease (AD) are neurodegenerative disorders accompanied by the conversion of functional forms of the microtubule associated protein Tau into non-functional aggregates. A variety of post-translational modifications (PTMs) on Tau precede or accompany the conversion, placing them in position to modulate Tau function as well as its propensity to aggregate. Although Tau PTMs can be characterized by their sites of modification, their total stoichiometry when summed over all sites also is an important metric of their potential impact on function. Here we provide a protocol for rapidly producing recombinant Tau with enzyme-specific PTMs at high stoichiometry in vitro and demonstrate its utility in the context of hyperphosphorylation. Additionally, protocols for estimating phosphorylation and methylation stoichiometry on Tau proteins isolated from any source are presented. Together these methods support experimentation on Tau PTM function over a wide range of experimental conditions.

Canonical and noncanonical Wnt signaling: Multilayered mediators, signaling mechanisms and major signaling crosstalk

Wnt signaling plays a major role in regulating cell proliferation and differentiation. The Wnt ligands are a family of 19 secreted glycoproteins that mediate their signaling effects via binding to Frizzled receptors and LRP5/6 coreceptors and transducing the signal either through β-catenin in the canonical pathway or through a series of other proteins in the noncanonical pathway. Many of the individual components of both canonical and noncanonical Wnt signaling have additional functions throughout the body, establishing the complex interplay between Wnt signaling and other signaling pathways. This crosstalk between Wnt signaling and other pathways gives Wnt signaling a vital role in many cellular and organ processes. Dysregulation of this system has been implicated in many diseases affecting a wide array of organ systems, including cancer and embryological defects, and can even cause embryonic lethality. The complexity of this system and its interacting proteins have made Wnt signaling a target for many therapeutic treatments. However, both stimulatory and inhibitory treatments come with potential risks that need to be addressed. This review synthesized much of the current knowledge on the Wnt signaling pathway, beginning with the history of Wnt signaling. It thoroughly described the different variants of Wnt signaling, including canonical, noncanonical Wnt/PCP, and the noncanonical Wnt/Ca2+ pathway. Further description involved each of its components and their involvement in other cellular processes. Finally, this review explained the various other pathways and processes that crosstalk with Wnt signaling.

Functions and regulation of the serine/threonine protein kinase CK1 family: moving beyond promiscuity

Regarded as constitutively active enzymes, known to participate in many, diverse biological processes, the intracellular regulation bestowed on the CK1 family of serine/threonine protein kinases is critically important, yet poorly understood. Here, we provide an overview of the known CK1-dependent cellular functions and review the emerging roles of CK1-regulating proteins in these processes. We go on to discuss the advances, limitations and pitfalls that CK1 researchers encounter when attempting to define relationships between CK1 isoforms and their substrates, and the challenges associated with ascertaining the correct physiological CK1 isoform for the substrate of interest. With increasing interest in CK1 isoforms as therapeutic targets, methods of selectively inhibiting CK1 isoform-specific processes is warranted, yet challenging to achieve given their participation in such a vast plethora of signalling pathways. Here, we discuss how one might shut down CK1-specific processes, without impacting other aspects of CK1 biology.

Regulation of Multifunctional Calcium/Calmodulin Stimulated Protein Kinases by Molecular Targeting

Multifunctional calcium/calmodulin-stimulated protein kinases control a broad range of cellular functions in a multitude of cell types. This family of kinases contain several structural similarities and all are regulated by phosphorylation, which either activates, inhibits or modulates their kinase activity. As these protein kinases are widely or ubiquitously expressed, and yet regulate a broad range of different cellular functions, additional levels of regulation exist that control these cell-specific functions. Of particular importance for this specificity of function for multifunctional kinases is the expression of specific binding proteins that mediate molecular targeting. These molecular targeting mechanisms allow pools of kinase in different cells, or parts of a cell, to respond differently to activation and produce different functional outcomes.

Structure, regulation, and (patho-)physiological functions of the stress-induced protein kinase CK1 delta (CSNK1D)

Members of the highly conserved pleiotropic CK1 family of serine/threonine-specific kinases are tightly regulated in the cell and play crucial regulatory roles in multiple cellular processes from protozoa to human. Since their dysregulation as well as mutations within their coding regions contribute to the development of various different pathologies, including cancer and neurodegenerative diseases, they have become interesting new drug targets within the last decade. However, to develop optimized CK1 isoform-specific therapeutics in personalized therapy concepts, a detailed knowledge of the regulation and functions of the different CK1 isoforms, their various splice variants and orthologs is mandatory. In this review we will focus on the stress-induced CK1 isoform delta (CK1δ), thereby addressing its regulation, physiological functions, the consequences of its deregulation for the development and progression of diseases, and its potential as therapeutic drug target.

The kinase domain of CK1δ can be phosphorylated by Chk1

Members of the casein kinase 1 (CK1) family are key regulators in numerous cellular signal transduction pathways and in order to prevent the development of certain diseases, CK1 kinase activity needs to be tightly regulated. Modulation of kinase activity by site-specific phosphorylation within the C-terminal regulatory domain of CK1δ has already been shown for several cellular kinases. By using biochemical methods, we now identified residues T161, T174, T176, and S181 within the kinase domain of CK1δ as target sites for checkpoint kinase 1 (Chk1). At least residues T176 and S181 show full conservation among CK1δ orthologues from different eukaryotic species. Enzyme kinetic analysis furthermore led to the hypothesis that site-specific phosphorylation within the kinase domain finally contributes to fine-tuning of CK1δ kinase activity. These data provide a basis for the extension of our knowledge about the role of site-specific phosphorylation for regulation of CK1δ and associated signal transduction pathways.

Spatiotemporal regulation of the Dma1-mediated mitotic checkpoint coordinates mitosis with cytokinesis

During cell division, the timing of mitosis and cytokinesis must be ordered to ensure that each daughter cell receives a complete, undamaged copy of the genome. In fission yeast, the septation initiation network (SIN) is responsible for this coordination, and a mitotic checkpoint dependent on the E3 ubiquitin ligase Dma1 and the protein kinase CK1 controls SIN signaling to delay cytokinesis when there are errors in mitosis. The participation of kinases and ubiquitin ligases in cell cycle checkpoints that maintain genome integrity is conserved from yeast to human, making fission yeast an excellent model system in which to study checkpoint mechanisms. In this review, we highlight recent advances and remaining questions related to checkpoint regulation, which requires the synchronized modulation of protein ubiquitination, phosphorylation, and subcellular localization.

The kinase domain of CK1 enzymes contains the localization cue essential for compartmentalized signaling at the spindle pole

CK1 protein kinases contribute to multiple biological processes, but how they are tailored to function in compartmentalized signaling events is largely unknown. Hhp1 and Hhp2 (Hhp1/2) are the soluble CK1 family members in Schizosaccharomyces pombe. One of their functions is to inhibit the septation initiation network (SIN) during a mitotic checkpoint arrest. The SIN is assembled by Sid4 at spindle pole bodies (SPBs), and though Hhp1/2 colocalize there, it is not known how they are targeted there or whether their SPB localization is required for SIN inhibition. Here, we establish that Hhp1/2 localize throughout the cell cycle to SPBs, as well as to the nucleus, cell tips, and division site. We find that their catalytic domains but not their enzymatic function are used for SPB targeting and that this targeting strategy is conserved in human CK1δ/ε localization to centrosomes. Further, we pinpoint amino acids in the Hhp1 catalytic domain required for SPB interaction; mutation of these residues disrupts Hhp1 association with the core SPB protein Ppc89, and the inhibition of cytokinesis in the setting of spindle stress. Taken together, these data have enabled us to define a molecular mechanism used by CK1 enzymes to target a specific cellular locale for compartmentalized signaling.

CK1δ activity is modulated by CDK2/E- and CDK5/p35-mediated phosphorylation

CK1 protein kinases form a family of serine/threonine kinases which are highly conserved through different species and ubiquitously expressed. CK1 family members can phosphorylate numerous substrates thereby regulating different biological processes including membrane trafficking, cell cycle regulation, circadian rhythm, apoptosis, and signal transduction. Deregulation of CK1 activity and/or expression contributes to the development of neurological diseases and cancer. Therefore, CK1 became an interesting target for drug development and it is relevant to further understand the mechanisms of its regulation. In the present study, Cyclin-dependent kinase 2/Cyclin E (CDK2/E) and Cyclin-dependent kinase 5/p35 (CDK5/p35) were identified as cellular kinases able to modulate CK1δ activity through site-specific phosphorylation of its C-terminal domain. Furthermore, pre-incubation of CK1δ with CDK2/E or CDK5/p35 reduces CK1δ activity in vitro, indicating a functional impact of the interaction between CK1δ and CDK/cyclin complexes. Interestingly, inhibition of Cyclin-dependent kinases by Dinaciclib increases CK1δ activity in pancreatic cancer cells. In summary, these results suggest that CK1δ activity can be modulated by the interplay between CK1δ and CDK2/E or CDK5/p35. These findings extend our knowledge about CK1δ regulation and may be of use for future development of CK1-related therapeutic strategies in the treatment of neurological diseases or cancer.

Analysis of isoform-specific tau aggregates suggests a common toxic mechanism involving similar pathological conformations and axonal transport inhibition

Misfolded tau proteins are characteristic of tauopathies, but the isoform composition of tau inclusions varies by tauopathy. Using aggregates of the longest tau isoform (containing 4 microtubule-binding repeats and 4-repeat tau), we recently described a direct mechanism of toxicity that involves exposure of the N-terminal phosphatase-activating domain (PAD) in tau, which triggers a signaling pathway that disrupts axonal transport. However, the impact of aggregation on PAD exposure for other tau isoforms was unexplored. Here, results from immunochemical assays indicate that aggregation-induced increases in PAD exposure and oligomerization are common features among all tau isoforms. The extent of PAD exposure and oligomerization was larger for tau aggregates composed of 4-repeat isoforms compared with those made of 3-repeat isoforms. Most important, aggregates of all isoforms exhibited enough PAD exposure to significantly impair axonal transport in the squid axoplasm. We also show that PAD exposure and oligomerization represent common pathological characteristics in multiple tauopathies. Collectively, these results suggest a mechanism of toxicity common to each tau isoform that likely contributes to degeneration in different tauopathies.

Analyzing Tau Aggregation with Electron Microscopy

Conversion of monomeric tau protein into filamentous aggregates is a defining event in the pathogenesis of Alzheimer's disease. To gain insight into disease pathogenesis, the mechanisms that trigger and mediate tau aggregation are under intense investigation. Characterization efforts have relied primarily on recombinant tau protein preparations and high-throughput solution-based detection methods such as thioflavin-dye fluorescence and laser-light-scattering spectroscopies. Transmission electron microscopy (TEM) is a static imaging tool that complements these approaches by detecting individual tau filaments at nanometer resolution. In doing so, it can provide unique insight into the quality, quantity, and composition of synthetic tau filament populations. Here we describe protocols for analysis of tau filament populations by TEM for purposes of dissecting aggregation mechanism.

The CK1 Family: Contribution to Cellular Stress Response and Its Role in Carcinogenesis

Members of the highly conserved and ubiquitously expressed pleiotropic CK1 family play major regulatory roles in many cellular processes including DNA-processing and repair, proliferation, cytoskeleton dynamics, vesicular trafficking, apoptosis, and cell differentiation. As a consequence of cellular stress conditions, interaction of CK1 with the mitotic spindle is manifold increased pointing to regulatory functions at the mitotic checkpoint. Furthermore, CK1 is able to alter the activity of key proteins in signal transduction and signal integration molecules. In line with this notion, CK1 is tightly connected to the regulation and degradation of β-catenin, p53, and MDM2. Considering the importance of CK1 for accurate cell division and regulation of tumor suppressor functions, it is not surprising that mutations and alterations in the expression and/or activity of CK1 isoforms are often detected in various tumor entities including cancer of the kidney, choriocarcinomas, breast carcinomas, oral cancer, adenocarcinomas of the pancreas, and ovarian cancer. Therefore, scientific effort has enormously increased (i) to understand the regulation of CK1 and its involvement in tumorigenesis- and tumor progression-related signal transduction pathways and (ii) to develop CK1-specific inhibitors for the use in personalized therapy concepts. In this review, we summarize the current knowledge regarding CK1 regulation, function, and interaction with cellular proteins playing central roles in cellular stress-responses and carcinogenesis.

TOC1: a valuable tool in assessing disease progression in the rTg4510 mouse model of tauopathy

All tauopathies result in various forms of cognitive decline and neuronal loss. Although in some diseases, tau mutations appear to cause neurodegeneration, the toxic "form" of tau remains elusive. Tau is the major protein found within neurofibrillary tangles (NFTs) and therefore it seemed rational to assume that aggregation of tau monomers into NFTs was causal to the disease process. However, the appearance of oligomers rather than NFTs coincides much better with the voluminous neuronal loss in many of these diseases. In this study, we utilized the bigenic mouse line (rTg4510) which conditionally expresses P301L human tau. A novel tau antibody, termed Tau Oligomer Complex 1 (TOC1) was employed to probe mouse brains and assess disease progression. TOC1 selectively recognizes dimers/oligomers and appears to constitute an early stage marker of tau pathology. Its peak reactivity is coincident with other well-known early stage pathological markers such as MC1 and the early-stage phospho-marker CP13. TOC1's reactivity depends on the conformation of the tau species since it does not react with monomer under native conditions, although it does react with monomers under SDS-denaturation. This indicates a conformational change must occur within the tau aggregate to expose its epitope. Tau oligomers preferentially form under oxidizing conditions and within this mouse model, we observe tau oligomers forming at an increased rate and persisting much longer, most likely due to the aggressive P301L mutation. With the help of other novel antibodies, the use of this antibody will aid in providing a better understanding of tau toxicity within Alzheimer's disease and other tauopathies.

CK1δ kinase activity is modulated by Chk1-mediated phosphorylation

CK1δ, a member of the casein kinase 1 family, is involved in the regulation of various cellular processes and has been associated with the pathophysiology of neurodegenerative diseases and cancer. Therefore recently, interest in generating highly specific inhibitors for personalized therapy has increased enormously. However, the efficacy of newly developed inhibitors is affected by the phosphorylation state of CK1δ. Cellular kinases phosphorylating CK1δ within its C-terminal domain have been identified but still more information regarding the role of site-specific phosphorylation in modulating the activity of CK1δ is required. Here we show that Chk1 phosphorylates rat CK1δ at serine residues 328, 331, 370, and threonine residue 397 as well as the human CK1δ transcription variants 1 and 2. CK1δ mutant proteins bearing one, two or three mutations at these identified phosphorylation sites exhibited significant differences in their kinetic properties compared to wild-type CK1δ. Additionally, CK1δ co-precipitates with Chk1 from HT1080 cell extracts and activation of cellular Chk1 resulted in a significant decrease in cellular CK1δ kinase activity. Taken together, these data point towards a possible regulatory relationship between Chk1 and CK1δ.

RNA helicase DDX3 is a regulatory subunit of casein kinase 1 in Wnt-β-catenin signaling

Casein kinase 1 (CK1) members play key roles in numerous biological processes. They are considered "rogue" kinases, because their enzymatic activity appears unregulated. Contrary to this notion, we have identified the DEAD-box RNA helicase DDX3 as a regulator of the Wnt-β-catenin network, where it acts as a regulatory subunit of CK1ε: In a Wnt-dependent manner, DDX3 binds CK1ε and directly stimulates its kinase activity, and promotes phosphorylation of the scaffold protein dishevelled. DDX3 is required for Wnt-β-catenin signaling in mammalian cells and during Xenopus and Caenorhabditis elegans development. The results also suggest that the kinase-stimulatory function extends to other DDX and CK1 members, opening fresh perspectives for one of the longest-studied protein kinase families.

Ligand electronic properties modulate tau filament binding site density

Small molecules that bind tau-bearing neurofibrillary lesions are being sought for premortem diagnosis, staging, and treatment of Alzheimer's disease and other tauopathic neurodegenerative diseases. The utility of these agents will depend on both their binding affinity and binding site density (B(max)). Previously we identified polarizability as a descriptor of protein aggregate binding affinity. To examine its contribution to binding site density, we investigated the ability of two closely related benzothiazole derivatives ((E)-2-[[4-(dimethylamino)phenyl]azo]-6-methoxybenzothiazole) and ((E)-2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methoxybenzothiazole) that differed in polarizability to displace probes of high (Thioflavin S) and low (radiolabeled (E,E)-1-iodo-2,5-bis(3-hydroxycarbonyl-4-methoxy)styrylbenzene; IMSB) density sites. Consistent with their site densities, Thioflavin S completely displaced radiolabeled IMSB, but IMSB was incapable of displacing Thioflavin S. Although both benzothiazoles displaced the low B(max) IMSB probe, only the highly polarizable analog displaced near saturating concentrations of the Thioflavin S probe. Quantum calculations showed that high polarizability reflected extensive pi-electron delocalization fostered by the presence of electron donating and accepting groups. These data suggest that electron delocalization promotes ligand binding at a subset of sites on tau aggregates that are present at high density, and that optimizing this aspect of ligand structure can yield tau-directed agents with superior diagnostic and therapeutic performance.

Tau isoform composition influences rate and extent of filament formation

The risk of developing tauopathic neurodegenerative disease depends in part on the levels and composition of six naturally occurring Tau isoforms in human brain. These proteins, which form filamentous aggregates in disease, vary only by the presence or absence of three inserts encoded by alternatively spliced exons 2, 3, and 10 of the Tau gene (MAPT). To determine the contribution of alternatively spliced segments to Tau aggregation propensity, the aggregation kinetics of six unmodified, recombinant human Tau isoforms were examined in vitro using electron microscopy assay methods. Aggregation propensity was then compared at the level of elementary rate constants for nucleation and extension phases. We found that all three alternatively spliced segments modulated Tau aggregation but through differing kinetic mechanisms that could synergize or compete depending on sequence context. Overall, segments encoded by exons 2 and 10 promoted aggregation, whereas the segment encoded by exon 3 depressed it with its efficacy dependent on the presence or absence of a fourth microtubule binding repeat. In general, aggregation propensity correlated with genetic risk reported for multiple tauopathies, implicating aggregation as one candidate mechanism rationalizing the correlation between Tau expression patterns and disease.

The role of molecular regulation and targeting in regulating calcium/calmodulin stimulated protein kinases

Calcium/calmodulin-stimulated protein kinases can be classified as one of two types - restricted or multifunctional. This family of kinases contains several structural similarities: all possess a calmodulin binding motif and an autoinhibitory region. In addition, all of the calcium/calmodulin-stimulated protein kinases examined in this chapter are regulated by phosphorylation, which either activates or inhibits their kinase activity. However, as the multifunctional calcium/calmodulin-stimulated protein kinases are ubiquitously expressed, yet regulate a broad range of cellular functions, additional levels of regulation that control these cell-specific functions must exist. These additional layers of control include gene expression, signaling pathways, and expression of binding proteins and molecular targeting. All of the multifunctional calcium/calmodulin-stimulated protein kinases examined in this chapter appear to be regulated by these additional layers of control, however, this does not appear to be the case for the restricted kinases.

Heat shock protein 70 prevents both tau aggregation and the inhibitory effects of preexisting tau aggregates on fast axonal transport

Aggregation and accumulation of the microtubule-associated protein tau are associated with cognitive decline and neuronal degeneration in Alzheimer's disease and other tauopathies. Thus, preventing the transition of tau from a soluble state to insoluble aggregates and/or reversing the toxicity of existing aggregates would represent a reasonable therapeutic strategy for treating these neurodegenerative diseases. Here we demonstrate that molecular chaperones of the heat shock protein 70 (Hsp70) family are potent inhibitors of tau aggregation in vitro, preventing the formation of both mature fibrils and oligomeric intermediates. Remarkably, addition of Hsp70 to a mixture of oligomeric and fibrillar tau aggregates prevents the toxic effect of these tau species on fast axonal transport, a critical process for neuronal function. When incubated with preformed tau aggregates, Hsp70 preferentially associated with oligomeric over fibrillar tau, suggesting that prefibrillar oligomeric tau aggregates play a prominent role in tau toxicity. Taken together, our data provide a novel molecular basis for the protective effect of Hsp70 in tauopathies.

Characterization of prefibrillar Tau oligomers in vitro and in Alzheimer disease

Neurofibrillary tangles, composed of insoluble aggregates of the microtubule-associated protein Tau, are a pathological hallmark of Alzheimer disease (AD) and other tauopathies. However, recent evidence indicates that neuronal dysfunction precedes the formation of these insoluble fibrillar deposits, suggesting that earlier prefibrillar Tau aggregates may be neurotoxic. To determine the composition of these aggregates, we have employed a photochemical cross-linking technique to examine intermolecular interactions of full-length Tau in vitro. Using this method, we demonstrate that dimerization is an early event in the Tau aggregation process and that these dimers self-associate to form larger oligomeric aggregates. Moreover, using these stabilized Tau aggregates as immunogens, we generated a monoclonal antibody that selectively recognizes Tau dimers and higher order oligomeric aggregates but shows little reactivity to Tau filaments in vitro. Immunostaining indicates that these dimers/oligomers are markedly elevated in AD, appearing in early pathological inclusions such as neuropil threads and pretangle neurons as well as colocalizing with other early markers of Tau pathogenesis. Taken as a whole, the work presented herein demonstrates the existence of alternative Tau aggregates that precede formation of fibrillar Tau pathologies and raises the possibility that these hierarchical oligomeric forms of Tau may contribute to neurodegeneration.

Highly selective and rapid enrichment of phosphorylated peptides using gallium oxide-coated magnetic microspheres for MALDI-TOF-MS and nano-LC-ESI-MS/MS/MS analysis

In this work, we present, to our knowledge, the first demonstration of the utility of iron oxide magnetic microspheres coated with gallium oxide for the highly selective enrichment of phosphopeptide prior to mass spectrometric analysis. These microspheres that we prepared not only have a shell of gallium oxide, giving them a high-trapping capacity for the phosphopeptides, but also their magnetic property enables easy isolation by positioning an external magnetic field. Tryptic digest products of phosphoproteins including beta-casein, ovalbumin, casein, as well as five protein mixtures were used as the samples to exemplify the feasibility of this approach. In very short time (only 0.5 min), phosphopeptides sufficient for characterization by MALDI-TOF-MS were selectively enriched by the Ga(2)O(3)-coated Fe(3)O(4) microspheres. The performance of the Ga(2)O(3)-coated Fe(3)O(4) microspheres were further compared with Fe(3+)-immobilized magnetic silica microspheres, commercial Fe(3+)-IMAC resin, and TiO2 beads for enrichment of peptides originating from tryptic digestion of beta-casein and BSA with a molar ratio of 1:50, and the results proved a stronger selective ability of Ga(2)O(3)-coated Fe(3)O(4) microspheres over the other materials. Finally, the Ga(2)O(3)-coated Fe(3)O(4) microspheres were successfully utilized for enrichment of phosphopeptides from digestion products of rat liver extract. All results show that Ga(2)O(3)-coated Fe(3)O(4) microsphere is an effective material for selective isolation and concentration of phosphopeptides.

Phosphorylation of CK1delta: identification of Ser370 as the major phosphorylation site targeted by PKA in vitro and in vivo

The involvement of CK1 (casein kinase 1) delta in the regulation of multiple cellular processes implies a tight regulation of its activity on many different levels. At the protein level, reversible phosphorylation plays an important role in modulating the activity of CK1delta. In the present study, we show that PKA (cAMP-dependent protein kinase), Akt (protein kinase B), CLK2 (CDC-like kinase 2) and PKC (protein kinase C) alpha all phosphorylate CK1delta. PKA was identified as the major cellular CK1deltaCK (CK1delta C-terminal-targeted protein kinase) for the phosphorylation of CK1delta in vitro and in vivo. This was implied by the following evidence: PKA was detectable in the CK1deltaCK peak fraction of fractionated MiaPaCa-2 cell extracts, PKA shared nearly identical kinetic properties with those of CK1deltaCK, and both PKA and CK1deltaCK phosphorylated CK1delta at Ser370 in vitro. Furthermore, phosphorylation of CK1delta by PKA decreased substrate phosphorylation of CK1delta in vitro. Mutation of Ser370 to alanine increased the phosphorylation affinity of CK1delta for beta-casein and the GST (gluthatione S-transferase)-p53 1-64 fusion protein in vitro and enhanced the formation of an ectopic dorsal axis during Xenopus laevis development. Anchoring of PKA and CK1delta to centrosomes was mediated by AKAP (A-kinase-anchoring protein) 450. Interestingly, pre-incubation of MiaPaCa-2 cells with the synthetic peptide St-Ht31, which prevents binding between AKAP450 and the regulatory subunit RII of PKA, resulted in a 6-fold increase in the activity of CK1delta. In summary, we conclude that PKA phosphorylates CK1delta, predominantly at Ser370 in vitro and in vivo, and that site-specific phosphorylation of CK1delta by PKA plays an important role in modulating CK1delta-dependent processes.

Casein kinase 1 is a novel negative regulator of E-cadherin-based cell-cell contacts

Cadherins are the most crucial membrane proteins for the formation of tight and compact cell-cell contacts. Cadherin-based cell-cell adhesions are dynamically established and/or disrupted during various physiological and pathological processes. However, the molecular mechanisms that regulate cell-cell contacts are not fully understood. In this paper, we report a novel functional role of casein kinase 1 (CK1) in the regulation of cell-cell contacts. Firstly, we observed that IC261, a specific inhibitor of CK1, stabilizes cadherin-based cell-cell contacts, whereas the overexpression of CK1 disrupts them. CK1 colocalizes with E-cadherin and phosphorylates the cytoplasmic domain of E-cadherin in vitro and in a cell culture system. We show that the major CK1 phosphorylation site of E-cadherin is serine 846, a highly conserved residue between classical cadherins. Constitutively phosphorylated E-cadherin (S846D) is unable to localize at cell-cell contacts and has decreased adhesive activity. Furthermore, phosphorylated E-cadherin (S846D) has weaker interactions with beta-catenin and is internalized more efficiently than wild-type E-cadherin. These data indicate that CK1 is a novel negative regulator of cadherin-based cell-cell contacts.

Dysbindin structural homologue CK1BP is an isoform-selective binding partner of human casein kinase-1

Casein kinase-1 is a family of ubiquitous eukaryotic protein kinases that frequently function in tandem with the ubiquitin modification system to modulate protein turnover and trafficking. In Alzheimer's disease, these enzymes colocalize with ubiquitinated lesions, including neurofibrillary tangles and granulovacuolar degeneration bodies, suggesting they also play a role in disease pathogenesis. To identify binding partners that potentially regulate or recruit these enzymes toward disease lesions, a Sos-recruitment yeast two-hybrid screen was performed with human Ckidelta (the casein kinase-1 isoform most closely linked to granulovacuolar degeneration bodies) and a human brain cDNA library. All interacting clones contained a single open reading frame termed casein kinase-1 binding protein (CK1BP). On the basis of sequence alignments, CK1BP was a structural homologue of the acidic domain of dysbindin, a component of the dystrophin-associated protein complex and the biogenesis of lysosome-related organelles complex-1. CK1BP interacted with full-length Ckidelta, the isolated Ckidelta catalytic domain, Ckigamma2, -gamma3, and -epsilon in the yeast two-hybrid system, and bound Ckidelta and -epsilon in pulldown assays but did not interact with Ckialpha. Interaction with the Ckidelta catalytic domain led to concentration-dependent inhibition of protein kinase activity in the presence of protein substrates tau and alpha-synuclein. Although intact dysbindin did not bind any CK1 isoform, deletion of its coiled-coil domain yielded a protein fragment that behaved much like CK1BP in two-hybrid screens. These data suggest that the acidic domain of dysbindin and its paralogs in humans may function to recruit casein kinase-1 isoforms to protein complexes involved in multiple biological functions.

Analysis of protein phosphorylation by hypothesis-driven multiple-stage mass spectrometry

We describe a strategy, which we term hypothesis-driven multiple-stage mass spectrometry (HMS-MS), for the sensitive detection and identification of phosphopeptides derived from enzymatic digests of phosphoproteins. In this strategy, we postulate that any or all of the potential sites of phosphorylation in a given protein may be phosphorylated. Using this assumption, we calculate the m/z values of all the corresponding singly charged phosphopeptide ions that could, in theory, be produced by the enzyme employed for proteolysis. We test ions at these m/z values for the presence of phosphoserine or phosphothreonine residues using tandem mass spectrometry (MS(2)) in a vacuum MALDI ion trap mass spectrometer, where the neutral loss of the elements of H(3)PO(4) (98 Da) provides a sensitive assay for the presence of phosphopeptides. Subsequent MS(3) analysis of the (M + H - 98)(+) peaks allows us to confirm or reject the hypotheses that the putative phosphopeptides are present in the sample. HMS-MS was successfully applied to the detection and identification of phosphopeptides from substrates of the Saccharomyces cerevisiae cyclin-dependent kinase (Cdk) Cdc28, phosphorylated in vitro (Ipl1) and in vivo (Orc6), basing hypothesis formation on the minimal Cdk consensus phosphorylation motif Ser/Thr-Pro. The method was also used to find in vitro phosphopeptides from a domain of the Drosophila melanogaster protein PERIOD, hypothesizing possible phosphorylations of all Ser/Thr residues without assuming a consensus motif. Our results demonstrate that HMS-MS is a sensitive, highly specific tool for systematically surveying proteins for Ser/Thr phosphorylation, and represents a significant step toward our goal of comprehensive phosphorylation mapping.

The casein kinase 1 family: participation in multiple cellular processes in eukaryotes

Phosphorylation of serine, threonine and tyrosine residues by cellular protein kinases plays an important role in the regulation of various cellular processes. The serine/threonine specific casein kinase 1 and 2 protein kinase families--(CK1 and CK2)--were among the first protein kinases that had been described. In recent years our knowledge of the regulation and function of mammalian CK1 kinase family members has rapidly increased. Extracellular stimuli, the subcellular localization of CK1 isoforms, their interaction with various cellular structures and proteins, as well as autophosphorylation and proteolytic cleavage of their C-terminal regulatory domains influence CK1 kinase activity. Mammalian CK1 isoforms phosphorylate many different substrates among them key regulatory proteins involved in the control of cell differentiation, proliferation, chromosome segregation and circadian rhythms. Deregulation and/or the incidence of mutations in the coding sequence of CK1 isoforms have been linked to neurodegenerative diseases and cancer. This review will summarize our current knowledge about the function and regulation of mammalian CK1 isoforms.

Mutational analysis of stress-responsive peanut dual specificity protein kinase. Identification of tyrosine residues involved in regulation of protein kinase activity

We recently reported that Arachis hypogaea serine/threonine/tyrosine (STY) protein kinase is developmentally regulated and is induced by abiotic stresses (Rudrabhatla, P., and Rajasekharan, R. (2002) Plant Physiol. 130, 380-390). Other than MAPKs, the site of tyrosine phosphorylation has not been documented for any plant kinases. To study the role of tyrosines in the phosphorylation of STY protein kinase, four conserved tyrosine residues were sequentially substituted with phenylalanine and expressed as histidine fusion proteins. Mass spectrometry experiments showed that STY protein kinase autophosphorylated within the predicted kinase ATP-binding motif, activation loop, and an additional site in the C terminus. The protein kinase activity was abolished by substitution of Tyr(297) with Phe in the activation loop between subdomains VII and VIII. In addition, replacing Tyr(148) in the ATP-binding motif and Tyr(317) in the C-terminal domain with Phe not only obliterated the ability of the STY protein kinase protein to be phosphorylated, but also inhibited histone phosphorylation, suggesting that STY protein kinase is phosphorylated at multiple sites. Replacing Tyr(213) in the Thr-Glu-Tyr sequence motif with Phe resulted in a 4-fold increase in autophosphorylation and 2.8-fold increase in substrate phosphorylation activities. Mutants Y148F, Y297F, and Y317F displayed dramatically lower phosphorylation efficiency (k(cat)/K(m)) with ATP and histone, whereas mutant Y213F showed increased phosphorylation. Our results suggest that autophosphorylation of Tyr(148), Tyr(213), Tyr(297), and Tyr(317) is important for the regulation of STY protein kinase activity. Our study reveals the first example of Thr-Glu-Tyr domain-mediated autoinhibition of kinases.

Plasma membrane localization of the Yck2p yeast casein kinase 1 isoform requires the C-terminal extension and secretory pathway function

The S. cerevisiae Yck2 protein is a plasma membrane-associated member of the casein kinase 1 protein kinase family that, with its homolog Yck1p, is required for bud morphogenesis, cytokinesis, endocytosis and other cellular processes. Membrane localization of Yckp is critical for its function, since soluble mutants do not provide sufficient biological activity to sustain normal growth. Yck2p has neither a predicted signal sequence nor obvious transmembrane domain to achieve its plasma membrane localization, but has a C-terminal -Cys-Cys sequence that is likely to be palmitoylated. We demonstrate here that Yck2p is targeted through association with vesicular intermediates of the classical secretory pathway. Yck2p lacking C-terminal Cys residues fails to associate with any membrane, whereas substitution of these residues with a farnesyl transferase signal sequence allows sec-dependent plasma membrane targeting and biological function, suggesting that modification is required for interaction with early secretory membranes but that targeting does not require a particular modification. Deletion analysis within the 185 residue C-terminus indicates that the final 28 residues are critical for membrane association, and additional sequences just upstream are required for proper plasma membrane targeting.

Biochemical and enzymatic characterization of a partially purified casein kinase-1 like activity from Trypanosoma cruzi

Two protein kinase activities that use casein as a substrate, Q-I and Q-II, were identified in the epimastigote stage of Trypanosoma cruzi upon chromatography on Q-Sepharose. Q-I was purified further through concanavalin A-sepharose (Q-I*) to remove any trace of the contaminating protease cruzipain. The optimal activity for Q-I* was obtained at pH 8.0, 25 degreesC, 5 mM MgCl(2) and 75 mM NaCl. The size and pI of Q-I* were determined to be 33-36 kDa and 9.6, respectively. When two selective peptide substrates for casein kinases (CKs) (P1: RRKDLHDDEEDEAMSITA for CK1 and P2: RRRADDSDDDDD for CK2) were used, Q-I* was shown to specifically phosphorylate P1. Kinetic studies showed that Q-I* has a K(m) of 5.3 +/- 0.34 mg/ml for casein, 157.6 +/- 5.3 microM for P1 and 35.9 +/- 3.9 microM for ATP. The enzyme was inhibited by N-(2-amino-ethyl)-5-chloroisoquinoline-8-sulfonamide (CKI-7) or 1-(5-chloroisoquinoline-8-sulfonyl) (CKI-8), two inactivators of mammalian CKs. CKI-7 behaved as a competitive inhibitor with respect to ATP, with a K(I) of 75-100 microM. Treatment with high concentrations of polylysine or heparin also resulted in a significant inhibition of Q-I*. Two well-known activators of mammalian CKs, spermine and spermidine, were also tested. Spermine and spermidine activated Q-I* in a dose-dependent manner. Based on the following characteristics: (1) the ionic strength required for elution from anion-exchange resins; (2) its molecular size and monomeric structure; (3) pI; (4) high level of specificity for P1; (5) inactivation by CKI-7 and CKI-8; and (6) insensitivity to GTP and low concentrations of heparin, we conclude that Q-I* belongs to the CK1 family of protein kinases.

Analysis of phosphorylated proteins and peptides by mass spectrometry

Phosphorylation on serine, threonine and tyrosine residues is an extremely important modulator of protein function. Therefore, there is a great need for methods capable of accurately elucidating sites of phosphorylation. Although full characterization of phosphoproteins remains a formidable analytical challenge, mass spectrometry has emerged as an increasingly viable tool for this task. This review summarizes the methodologies currently available for the analysis of phosphoproteins by mass spectrometry, including enrichment of compounds of interest using immobilized metal affinity chromatography and chemical tagging techniques, detection of phosphopeptides using mass mapping and precursor ion scans, localization of phosphorylation sites by peptide sequencing, and quantitation of phosphorylation by the introduction of mass tags. Despite the variety of powerful analytical methods that are now available, complete characterization of the phosphorylation state of a protein isolated in small quantities from a biological sample remains far from routine.

Mass spectrometry as a tool for protein crystallography

Atomic resolution structure determinations of proteins by X-ray crystallography are formidable multidisciplinary undertakings, requiring protein construct design, expression and purification, crystallization trials, phase determination, and model building. Modern mass spectrometric methods can greatly facilitate these obligate tasks. Thus, mass spectrometry can be used to verify that the desired protein construct has been correctly expressed, to define compact domains in the target protein, to assess the components contained within the protein crystals, and to screen for successful incorporation of seleno-methionine and other heavy metal reagents used for phasing. In addition, mass spectrometry can be used to address issues of modeling, topology, and side-chain proximity. Here, we demonstrate how rational use of mass spectrometry assists and expedites high resolution X-ray structure determination through each stage of the process of protein crystallography.

A large family of eukaryotic-like protein Ser/Thr kinases of Myxococcus xanthus, a developmental bacterium

Myxococcus xanthus is a gram-negative bacterium that forms multicellular fruiting bodies upon starvation. Here, we demonstrate that it contains at least 13 eukaryotic-like protein Ser/Thr kinases (Pkn1 to Pkn13) individually having unique features. All contain the kinase domain of approximately 280 residues near the N-terminal end, which share highly conserved features in eukaryotic Ser/Thr kinases. The kinase domain is followed by a putative regulatory domain consisting of 185 to 692 residues. These regulatory domains share no significant sequence similarities. The C-terminal regions of 11 kinases contain at least 1 transmembrane domain, suggesting that they function as transmembrane sensor kinases. From the recent genomic analysis, protein Ser/Thr kinases were found in various pathogenic bacteria and coexist with protein His kinases. Phylogenetic analysis of these Ser/Thr kinases reveals that all bacterial Ser/Thr kinases were evolved from a common ancestral kinase together with eukaryotic Tyr and Ser/Thr kinases. Coexistence of both Ser/Thr and His kinases in some organisms may be significant in terms of functional differences between the two kinases. We argue that both kinases are essential for some bacteria to adapt optimally to severe environmental changes.

Crystal structure of a conformation-selective casein kinase-1 inhibitor

Members of the casein kinase-1 family of protein kinases play an essential role in cell regulation and disease pathogenesis. Unlike most protein kinases, they appear to function as constitutively active enzymes. As a result, selective pharmacological inhibitors can play an important role in dissection of casein kinase-1-dependent processes. To address this need, new small molecule inhibitors of casein kinase-1 acting through ATP-competitive and ATP-noncompetitive mechanisms were isolated on the basis of in vitro screening. Here we report the crystal structure of 3-[(2,4,6-trimethoxyphenyl) methylidenyl]-indolin-2-one (IC261), an ATP-competitive inhibitor with differential activity among casein kinase-1 isoforms, in complex with the catalytic domain of fission yeast casein kinase-1 refined to a crystallographic R-factor of 22.4% at 2.8 A resolution. The structure reveals that IC261 stabilizes casein kinase-1 in a conformation midway between nucleotide substrate liganded and nonliganded conformations. We propose that adoption of this conformation by casein kinase-1 family members stabilizes a delocalized network of side chain interactions and results in a decreased dissociation rate of inhibitor.

In vitro polymerization of tau protein monitored by laser light scattering: method and application to the study of FTDP-17 mutants

Tau polymerization into the filaments that compose neurofibrillary tangles is seminal to the development of many neurodegenerative diseases. It is therefore important to understand the mechanisms involved in this process. However, a consensus method for monitoring tau polymerization in vitro has been lacking. Here we demonstrate that illuminating tau polymerization reactions with laser light and measuring the increased scattering at 90 degrees to the incident beam with a digital camera results in data that closely approximate the mass of tau polymer formation in vitro. The validity of the technique was demonstrated over a range of tau concentrations and through multiple angle scattering measurements. In addition, laser light scattering data closely correlated with quantitative electron microscopy measurements of the mass of tau filaments. Laser light scattering was then used to measure the efficiency with which the mutant tau proteins found in frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) form filamentous structures. Several of these mutant proteins display enhanced polymerization in the presence of arachidonic acid, suggesting a direct role for these mutations in tau the filament formation that characterizes FTDP-17.

Differential assembly of human tau isoforms in the presence of arachidonic acid

Six tau isoforms arise from the alternative splicing of a single gene in humans. Insoluble, filamentous deposits of tau protein occur in a number of neurodegenerative diseases, and in some of these diseases, the deposition of polymers enriched in certain tau isoforms has been documented. Because of these findings, we have undertaken studies on the efficacy of fatty acid-induced polymerization of the individual tau isoforms found in the adult human CNS. The polymerization of each tau isoform in the presence of two concentrations of arachidonic acid indicated that isoforms lacking N-terminal exons e2 and e3 formed small, globular oligomers that did not go on to elongate into straight (SF) or paired helical (PHF) filaments under our buffer conditions. The polymerization of all isoforms containing e2 or e2 and e3 occurred readily at a high arachidonic acid concentration. Conversely, at a lower arachidonic acid concentration, only tau isoforms containing four microtubule binding repeats assembled well. Under all buffer conditions employed, filaments formed from three of the isoforms containing e2 and e3 resembled SFs in morphology but began to form PHF-like structures following extended incubation at 37 degrees C. These results indicate that polymerization of the intact tau molecule may be facilitated by e2 and e3. Moreover, tau isoforms containing three versus four microtubule binding repeats display different assembly properties depending on the solvent conditions employed.

Identification of inhibitory autophosphorylation sites in casein kinase I epsilon

Casein kinase I epsilon (CKIepsilon) is a widely expressed protein kinase implicated in the regulation of diverse cellular processes including DNA replication and repair, nuclear trafficking, and circadian rhythm. CKIepsilon and the closely related CKIdelta are regulated in part through autophosphorylation of their carboxyl-terminal extensions, resulting in down-regulation of enzyme activity. Treatment of CKIepsilon with any of several serine/threonine phosphatases causes a marked increase in kinase activity that is self-limited. To identify the sites of inhibitory autophosphorylation, a series of carboxyl-terminal deletion mutants was constructed by site-directed mutagenesis. Truncations that eliminated specific phosphopeptides present in the wild-type kinase were used to guide construction of specific serine/threonine to alanine mutants. Amino acids Ser-323, Thr-325, Thr-334, Thr-337, Ser-368, Ser-405, Thr-407, and Ser-408 in the carboxyl-terminal tail of CKIepsilon were identified as probable in vivo autophosphorylation sites. A recombinant CKIepsilon protein with serine and threonine to alanine mutations eliminating these autophosphorylation sites was 8-fold more active than wild-type CKIepsilon using IkappaBalpha as a substrate. The identified autophosphorylation sites do not conform to CKI substrate motifs identified in peptide substrates.

Aminoglycoside antibiotic phosphotransferases are also serine protein kinases

BACKGROUND

Bacterial resistance to aminoglycoside antibiotics occurs primarily through the expression of modifying enzymes that covalently alter the drugs by O-phosphorylation, O-adenylation or N-acetylation. Aminoglycoside phosphotransferases (APHs) catalyze the ATP-dependent phosphorylation of these antibiotics. Two particular enzymes in this class, APH(3')-IIIa and AAC(6')-APH(2"), are produced in gram-positive cocci and have been shown to phosphorylate aminoglycosides on their 3' and 2" hydroxyl groups, respectively. The three-dimensional structure of APH (3')-IIIa is strikingly similar to those of eukaryotic protein kinases (EPKs), and the observation, reported previously, that APH(3')-IIIa and AAC(6')-APH(2") are effectively inhibited by EPK inhibitors suggested the possibility that these aminoglycoside kinases might phosphorylate EPK substrates.

RESULTS

Our data demonstrate unequivocally that APHs can phosphorylate several EPK substrates and that this phosphorylation occurs exclusively on serine residues. Phosphorylation of Ser/Thr protein kinase substrates by APHs was considerably slower than phosphorylation of aminoglycosides under identical assay conditions, which is consistent with the primary biological roles of the enzymes.

CONCLUSIONS

These results demonstrate a functional relationship between aminoglycoside and protein kinases, expanding on our previous observations of similarities in protein structure, enzyme mechanism and sensitivity to inhibitors, and suggest an evolutionary link between APHs and EPKs.

The role of methionine 156 in cross-subunit nucleotide interactions in the iron protein of nitrogenase

A variant Fe protein has been created at the completely conserved residue methionine 156 by changing it to cysteine. The Azotobacter vinelandii strain expressing M156C is unable to grow under nitrogen-fixing conditions, and the purified protein cannot support substrate reduction in vitro. This mutation has an effect on the Fe protein's ability to undergo the MgATP-induced conformational change as evidenced by the fact that M156C is chelated in the presence of MgATP with a lower observed rate than wild-type. While the electron paramagnetic resonance spectra of this protein are similar to those of the wild-type Fe protein, the circular dichroism spectrum is markedly different in the presence of MgATP, showing that the conformation adopted by M156C following nucleotide binding is different from the wild-type conformation. Although competition activity and chelation assays show that this Fe protein can still form a complex with the MoFe protein, this altered conformation only supports MgATP hydrolysis at 1% the rate of wild-type Fe protein. A model based on x-ray crystallographic information is presented to explain the importance of Met-156 in stabilization of the correct conformation of the Fe protein via critical interactions of the residue with Asp-43 and nucleotide in the other subunit.

Autoinhibition of casein kinase I epsilon (CKI epsilon) is relieved by protein phosphatases and limited proteolysis

Casein kinase I epsilon (CKI epsilon) is a member of the CKI gene family, members of which are involved in the control of SV40 DNA replication, DNA repair, and cell metabolism. The mechanisms that regulate CKI epsilon activity and substrate specificity are not well understood. We report that CKI epsilon, which contains a highly phosphorylated 123-amino acid carboxyl-terminal extension not present in CKI alpha, is substantially less active than CKI alpha in phosphorylating a number of substrates including SV40 large T antigen and is unable to inhibit the initiation of SV40 DNA replication. Two mechanisms for the activation of CKI epsilon have been identified. First, limited tryptic digestion of CKI epsilon produces a protease-resistant amino-terminal 39-kDa core kinase with several-fold enhanced activity. Second, phosphatase treatment of CKI epsilon activates CKI epsilon 5-20-fold toward T antigen. Similar treatment of a truncated form of CKI epsilon produced only a 2-fold activation. Notably, this activation was transient; reautophosphorylation led to a rapid down-regulation of the kinase within 5 min. Phosphatase treatment also activated CKI epsilon toward the novel substrates I kappa B alpha and Ets-1. These mechanisms may serve to regulate CKI epsilon and related forms of CKI in the cell, perhaps in response to DNA damage.

The effect of heparin on the activity of Trichosporon cutaneum casein kinase I

Casein kinase I from Trichosporon cutaneum ribosome-free extracts was purified. Its molecular mass was calculated for 33 kDa. It was shown that casein, phosvitin and Trichosporon cutaneum ribosomal protein of 15 kDa were preferable substrates for the enzyme. It was found that heparin can stimulate or inhibit CKI activity depending on the substrate used. Stimulation of casein and inhibition of phosvitin phosphorylation was observed. In addition it was shown that ribosomal proteins of 19 kDa and 38 kDa were phosphorylated by CKI only in the presence of heparin.

Identification, cloning, and mutational analysis of the casein kinase 1 cDNA of the malaria parasite, Plasmodium falciparum. Stage-specific expression of the gene

The cDNA for casein kinase 1 (CK1) of Plasmodium falciparum was cloned, sequenced, and expressed in bacteria. The single major open reading frame of the 1.2-kilobase pair cDNA coded for a 324-amino acid polypeptide of approximately 37 kDa, the predicted sequence of which showed strong identity with known CK1 isoforms. The purified recombinant enzyme exhibited properties characteristic of CK1, such as inhibition by CK1-7, the ability to phosphorylate a highly specific peptide substrate, and a strong preference for ATP over GTP. A casein kinase activity, partially purified from soluble extracts of P. falciparum by affinity chromatography through CK1-7 columns displayed identical properties. The activity showed a stage-specific expression in the parasite, in the order trophozoite > ring >> schizont. Northern analysis indicated the existence of two major CK1 mRNAs, 2.4 and 3.2 kilobase pairs long, the levels of which were in the order ring > schizont > trophozoite. Mutagenesis of recombinant CK1 defined important amino acid residues and their potential role in the conformation of the enzyme. The malarial CK1 appeared to be the one of the smallest and perhaps the most primitive CK1 enzymes known, containing little sequence information beyond the minimal catalytic domain.

The structural basis of monoclonal antibody Alz50's selectivity for Alzheimer's disease pathology

The epitope on tau protein recognized by the monoclonal antibody Alz50 was defined through internal deletion mutagenesis and quantified by affinity measurements. The epitope is discontinuous and requires both a previously identified N-terminal segment and the microtubule binding region for efficient binding of Alz50. The interaction between these regions is consistent with an intramolecular reaction mechanism, suggesting that Alz50 binding depends on the conformation of individual tau monomers. The results suggest that tau adopts a distinct conformation when polymerized into filaments and that this conformation is recognized selectively by Alz50.

Phosphorylation of yeast plasma membrane H+-ATPase by casein kinase I

The plasma membrane H+-ATPase of Saccharomyces cerevisiae is subject to phosphorylation by a casein kinase I activity in vitro. We show this casein kinase I activity to result from the combined function of YCK1 and YCK2, two highly similar and plasma membrane-associated casein kinase I homologues. First, H+-ATPase phosphorylation is severely impaired in the plasma membrane of YCK-deficient yeast strains. Furthermore, the wild-type level of the phosphoprotein is restored by the addition of purified mammalian casein kinase I to the mutant membranes. We used the H+-ATPase as well as a synthetic peptide substrate that contains a phosphorylation site for casein kinase I to compare kinase activity in membranes prepared from yeast cells grown in the presence or absence of glucose. The addition of glucose results in increased H+-ATPase activity which is associated with a decline in the phosphorylation level of the enzyme. Mutations in both YCK1 and YCK2 affect this regulation, suggesting that H+-ATPase activity is modulated by glucose via a combination of a "down-regulating" casein kinase I activity and another, yet uncharacterized, "up-regulating" kinase activity. Biochemical mapping of phosphorylated H+-ATPase identifies a major phosphopeptide that contains a consensus phosphorylation site (Ser-507) for casein kinase I. Site-directed mutagenesis of this consensus sequence indicates that Glu-504 is important for glucose-induced decrease in the apparent Km for ATP.

Role of COOH-terminal phosphorylation in the regulation of casein kinase I delta

Casein kinase I delta is a member of the casein kinase I (CKI) family, a group of second messenger independent protein kinases. We present evidence that the COOH-terminal domain of CKI delta has regulatory properties. CKI delta expressed in Escherichia coli was activated by heparin, as found previously, and by treatment with the catalytic subunit of type-1 protein phosphatase (CS1). Concomitant with activation by CS1, there was a reduction in the apparent molecular weight of CKI delta from 55,000 to 49,000 as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Truncation of CKI delta by removal of the COOH-terminal 110 amino acids eliminated the ability of CS1 to activate or to increase electrophoretic mobility. Casein kinase I alpha, a 37-kDa isoform that lacks an extended COOH-terminal domain, was not activated by CS1 or the presence of heparin. However, a chimeric enzyme consisting of CKI alpha fused to the COOH-terminal domain of CKI delta was activated by both heparin and CS1. Analysis of the effects of CS1 on a series of CKI delta COOH-terminal truncation mutants identified an inhibitory region between His317 and Pro342, which contained six potential phosphorylation sites. From analysis of the specific activites of these truncation mutants, removal of the same region resulted in enzyme with a specific activity nearly 10-fold greater than wild-type. Thus, CKI delta activity can be regulated by phosphorylation of its COOH terminus, which may serve to create an autoinhibitory domain. This mechanism of regulation could have important consequences in vivo.

Isolation and characterization of human casein kinase I epsilon (CKI), a novel member of the CKI gene family

The casein kinase I (CKI) gene family is a rapidly enlarging group whose members have been implicated in the control of cytoplasmic and nuclear processes, including DNA replication and repair. We report here the cloning and characterization of a novel isoform of CKI from a human placental cDNA library. The cDNA for this isoform, hCKI epsilon, predicts a basic polypeptide of 416 amino acids and a molecular mass of 47.3 kDa. It encodes a core kinase domain of 285 amino acids and a carboxyl-terminal tail of 123 amino acids. The kinase domain is 53-98% identical to the kinase domains of other CKI family members and is most closely related to the delta isoform. Localization of the hCKI epsilon gene to chromosome 22q12-13 and the hCKI delta gene to chromosome 17q25 confirms that these are distinct genes in the CKI family. Northern blot analysis shows that hCKI epsilon is expressed in multiple human cell lines. Recombinant hCKI epsilon is an active enzyme that phosphorylates known CKI substrates including a CKI-specific peptide substrate and is inhibited by CKI-7, a CKI-specific inhibitor. A budding yeast isoform of CKI, HRR25, has been implicated in DNA repair responses. Expression of hCKI epsilon but not hCKI alpha rescued the slow-growth phenotype of a Saccharomyces cerevisiae strain with a deletion of HRR25. Human CKI epsilon is a novel CKI isoform with properties that overlap those of previously described CKI isoforms.

Cloning and chromosomal localization of the gene coding for human protein kinase CK1

A cDNA clone coding for human protein kinase CK1 (casein kinase 1) has been isolated and sequenced demonstrating that it corresponds to a homolog of the CK1 alpha form found in bovine brain. The derived amino acid sequence of the human CK1 alpha is identical to the bovine counterpart except that it contains 12 extra amino acids at the carboxyl end. Using this cDNA sequence and PCR amplification, YAC genomic clones that contain this human CK1 alpha sequence have been isolated. These YACs have been used for fluorescent in situ hybridization in order to localize the human CK1 alpha gene to chromosome 13q13.