The casein kinase Ialpha isoform is both physically positioned and functionally competent to regulate multiple events of mRNA metabolism

The DUF1669 domain of FAM83H is required for its localization to nuclear speckles

Autosomal-dominant hypocalcified amelogenesis imperfecta (ADHCAI) is caused by mutations in the FAM83H gene. Mutated FAM83H genes encode truncated FAM83H proteins with amino acid lengths between amino acids 1-286 and 1-693, in contrast to wild-type FAM83H (1-1179). Deletion of the C-terminus of FAM83H results in its subcellular translocation from the cytoplasmic compartment to the nuclear speckles, where splicing factors accumulate. However, the amino acid region of FAM83H required for nuclear speckle localization has not yet been determined, and whether all FAM83H-truncated proteins associated with ADHCAI localize to nuclear speckles remains unknown. Here, we examined the subcellular localization of FAM83H mutant proteins with truncations or deletions at various amino acid positions. Deletions within residues 1-300, which corresponds to the DUF1669 domain (17-281), attenuated or abolished the nuclear speckle localization of FAM83H. Meanwhile, some ADHCAI-related FAM83H-truncated proteins did not localize to nuclear speckles, despite the presence of the DUF1669 domain. These results suggest that the DUF1669 domain is required, but not sufficient, for nuclear speckle localization of FAM83H, demonstrating that nuclear speckle localization is not a common feature among FAM83H-truncated proteins related to ADHCAI.

Casein Kinase 1α as a Novel Factor Affects Thyrotropin Synthesis via PKC/ERK/CREB Signaling

Casein kinase 1α (CK1α) is present in multiple cellular organelles and plays various roles in regulating neuroendocrine metabolism. Herein, we investigated the underlying function and mechanisms of CK1α-regulated thyrotropin (thyroid-stimulating hormone (TSH)) synthesis in a murine model. Immunohistochemistry and immunofluorescence staining were performed to detect CK1α expression in murine pituitary tissue and its localization to specific cell types. Tshb mRNA expression in anterior pituitary was detected using real-time and radioimmunoassay techniques after CK1α activity was promoted and inhibited in vivo and in vitro. Relationships among TRH/L-T4, CK1α, and TSH were analyzed with TRH and L-T4 treatment, as well as thyroidectomy, in vivo. In mice, CK1α was expressed at higher levels in the pituitary gland tissue than in the thyroid, adrenal gland, or liver. However, inhibiting endogenous CK1α activity in the anterior pituitary and primary pituitary cells significantly increased TSH expression and attenuated the inhibitory effect of L-T4 on TSH. In contrast, CK1α activation weakened TSH stimulation by thyrotropin-releasing hormone (TRH) by suppressing protein kinase C (PKC)/extracellular signal-regulated kinase (ERK)/cAMP response element binding (CREB) signaling. CK1α, as a negative regulator, mediates TRH and L-T4 upstream signaling by targeting PKC, thus affecting TSH expression and downregulating ERK1/2 phosphorylation and CREB transcriptional activity.

Knockdown of Csnk1a1 results in preimplantation developmental arrest in mice

Casein kinase 1, alpha 1 (CSNK1A1), is a member of the highly conserved serine/threonine protein kinase family. This study was established to analyze the expression and localization of CSNK1A1 and its function in early embryonic development in mice. Csnk1a1 mRNA and protein are expressed in multiple mouse tissues including the ovary. After ovulation and fertilization, Csnk1a1 mRNA and protein were detected in preimplantation embryos and their expression was highest in two-cell-stage embryos. CSNK1A1 protein was also mainly localized in the cytoplasm of preimplantation embryos. Moreover, knockdown of Csnk1a1 in zygotes led to a significant decrease in the rate of blastocyst formation. Furthermore, treatment of zygotes with the CSNK1A1-specific inhibitor D4476 also resulted in embryonic developmental arrest. These results provide the first evidence for a novel function of CSNK1A1 in early embryonic development in mice.

Oocyte Casein kinase 1α deletion causes defects in primordial follicle formation and oocyte loss by impairing oocyte meiosis and enhancing autophagy in developing mouse ovary

Casein kinase 1α is a member of CK1 family, which is ubiquitously expressed and plays multiple functions, including its potential roles in regulating cell division. But the functions of CK1α in mammalian oogenesis and folliculogenesis remain elusive. In this study, we assayed the cell type of CK1α expression in the developing mouse ovary and confirmed that CK1α was highly expressed in ovaries after birth. The oocyte-specific CK1α knockout (cKO) mouse model was then established by crossing Ddx4-Cre mice with Csnk1a1-floxp mice, and the effects of CK1α deletion on oogenesis and folliculogenesis were identified. The results showed that oocyte CK1α deletion impaired the progression of oocyte meiosis and primordial follicle formation during meiotic prophase I, which subsequently caused oocyte loss and mouse infertility. Further, the in vivo CK1α deletion and in vitro inhibition of CK1 activity resulted in the defects of DNA double-strand break (DSB) repair, whereas apoptosis and autophagy were enhanced in the developing ovary. These may contribute to oocyte loss and infertility in cKO mice. It is thus concluded that CK1α is essential for mouse oogenesis and folliculogenesis by involving in regulating the processes of oocyte meiosis and DNA DSB repair during meiotic prophase I of mouse oocytes. However, the related signaling pathway and molecular mechanisms need to be elucidated further.

The conserved C-terminal residues of FAM83H are required for the recruitment of casein kinase 1 to the keratin cytoskeleton

The casein kinase 1 (CK1) family of serine/threonine protein kinases is involved in diverse cellular events at discrete subcellular compartments. FAM83H acts as a scaffold protein that recruits CK1 to the keratin cytoskeleton or to the nuclear speckles, which are storage sites for splicing factors. We determined the amino acid region of FAM83H required for recruiting CK1 to the keratin cytoskeleton. The subcellular localization of mutant FAM83H proteins with deletions of amino acid residues at different positions was evaluated via immunofluorescence. FAM83H mutants with deleted C-terminal residues 1134–1139, which are conserved among vertebrates, lost the ability to localize and recruit CK1 to the keratin cytoskeleton, suggesting that these residues are required for recruiting CK1 to the keratin cytoskeleton. The deletion of these residues (1134–1139) translocated FAM83H and CK1 to the nuclear speckles. Amino acid residues 1 to 603 of FAM83H were determined to contain the region responsible for the recruitment of CK1 to the nuclear speckles. Our results indicated that FAM83H recruits CK1 preferentially to the keratin cytoskeleton and alternatively to the nuclear speckles.

Omics Data Mining for multiPTMs in Oral Cancer: Cellular Proteome and Secretome of Chronic Tobacco-Treated Oral Keratinocytes

Oral cancer is common worldwide but lacks robust diagnostics and therapeutics. Lifestyle factors, such as tobacco chewing and smoking, are significantly associated with oral cancers. Mapping the changes in the global proteome, secretome and post-translational modifications (PTMs) during tobacco exposure of oral keratinocytes hold great potential for understanding the mechanisms of oral carcinogenesis, not to mention for innovation toward clinical interventions in the future. On the other hand, although advances in mass spectrometry (MS)-based techniques have enabled the deep mining of complex proteomes, a large portion of the mass spectrometric data remains unassigned. These unassigned spectral data can be researched for multiple post-translational modifications (multiPTMs). Using data mining of publicly available proteomics data, we report, in this study, a multiPTM analysis of high-resolution MS-derived datasets on cellular proteome and secretome of chronic tobacco-treated oral keratinocytes. We identified 800 PTM sites in 496 proteins. Among them, 43 PTM sites in 37 proteins were found to be differentially expressed, accounting for their protein-level expression. Enrichment analysis of the proteins with altered phosphosite expression and the known kinases of these phosphosites discovered the overrepresentation of certain biological processes such as splicing and hemidesmosome assembly. These findings contribute to a deeper understanding of omics level changes in chronic tobacco-treated oral keratinocytes, and by extension, pathophysiology of oral cancers.

Casein kinase 1α: biological mechanisms and theranostic potential

Casein kinase 1α (CK1α) is a multifunctional protein belonging to the CK1 protein family that is conserved in eukaryotes from yeast to humans. It regulates signaling pathways related to membrane trafficking, cell cycle progression, chromosome segregation, apoptosis, autophagy, cell metabolism, and differentiation in development, circadian rhythm, and the immune response as well as neurodegeneration and cancer. Given its involvement in diverse cellular, physiological, and pathological processes, CK1α is a promising therapeutic target. In this review, we summarize what is known of the biological functions of CK1α, and provide an overview of existing challenges and potential opportunities for advancing theranostics.

Induction of Huh‑7 cell apoptosis by HCV core proteins via CK1α‑p53‑Bid signaling pathway

Hepatitis C virus (HCV)-infected liver cells sensitize host cells to tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-induced cell apoptosis; however, the precise mechanisms are unknown. In the present study, flow cytometry demonstrated that the Annexin V-positive Huh-7 cell number was higher in groups transfected with core proteins when compared with the pcDNA3.1 group. The mRNA and protein expression levels of B-cell lymphoma 2 (Bcl-2) were negatively associated, while Bcl-2-associated X protein (Bax) were positively correlated, with cell apoptotic rate, which, were verified by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and western blotting. There were no significant differences in the expressions of casein kinase 1 (CK1)-ε, CK1γ or CK1δ; however, the mRNA and protein levels of CK1α were markedly higher in groups transfected with the T (those derived from the HCV-J6 strain), NT (those derived from non-tumor tissues) and C191 (those derived from tumor tissues) HCV core proteins than in mock group. When compared with the Mock and Negative Control (control known-down) groups, the mRNA and protein levels of CK1α were lower in the CK1α known-down group, and there were no marked Huh-7 cell morphological changes among the 3 groups. There was more sensitivity to cell apoptosis in CK1α-silenced, however, not in non-CK1α-silenced, Huh-7 cells. BH3 interacting-domain death agonist (Bid) protein levels in CK1α-silenced Huh-7 cells were higher when compared with non-CK1α-silenced Huh-7 cells, and the level of p53 that translocated to the nucleus increased. Chromatin immunoprecipitation-PCR demonstrated that p53 bound to human Bid gene promoter. The level of the Bid promoter in CK1α-silenced Huh-7 cells was significantly higher than in the non-CK1α-silenced Huh-7 cells. Electron microscopy indicated that p53 knockdown decreased HCV core protein and TRAIL-induced cell apoptosis. Bid/caspase-8 protein levels in CK1α-silenced Huh-7 cells that were transfected with p53 siRNA were lower than in the control group. The present study demonstrated that HCV core proteins sensitize host cells to TRAIL-induced cell apoptosis by activating the CK1α-p53-Bid dependent pathway.

Casein kinase 1 is recruited to nuclear speckles by FAM83H and SON

In some fibroblasts, casein kinase 1α (CK1α) is localized to nuclear speckles, which are sub-nuclear compartments supplying splicing factors, whereas it is recruited on keratin filaments in colorectal cancer cells such as DLD1 cells. In order to obtain a deeper understanding of why CK1α is localized to these different subcellular sites, we herein elucidated the mechanisms underlying its localization to nuclear speckles. CK1α and FAM83H were localized to nuclear speckles in RKO and WiDr colorectal cancer cells, which do not express simple epithelial keratins, and in DLD1 cells transfected with siRNAs for type I keratins. The localization of FAM83H to nuclear speckles was also detected in colorectal cancer cells with a poorly organized keratin cytoskeleton in colorectal cancer tissues. Using an interactome analysis of FAM83H, we identified SON, a protein present in nuclear speckles, as a scaffold protein to which FAM83H recruits CK1α. This result was supported by the knockdown of FAM83H or SON delocalizing CK1α from nuclear speckles. We also found that CK1δ and ε are localized to nuclear speckles in a FAM83H-dependent manner. These results suggest that CK1 is recruited to nuclear speckles by FAM83H and SON in the absence of an intact keratin cytoskeleton.

Phosphorylation regulates the Star-PAP-PIPKIα interaction and directs specificity toward mRNA targets

Star-PAP is a nuclear non-canonical poly(A) polymerase (PAP) that shows specificity toward mRNA targets. Star-PAP activity is stimulated by lipid messenger phosphatidyl inositol 4,5 bisphoshate (PI4,5P2) and is regulated by the associated Type I phosphatidylinositol-4-phosphate 5-kinase that synthesizes PI4,5P2 as well as protein kinases. These associated kinases act as coactivators of Star-PAP that regulates its activity and specificity toward mRNAs, yet the mechanism of control of these interactions are not defined. We identified a phosphorylated residue (serine 6, S6) on Star-PAP in the zinc finger region, the domain required for PIPKIα interaction. We show that S6 is phosphorylated by CKIα within the nucleus which is required for Star-PAP nuclear retention and interaction with PIPKIα. Unlike the CKIα mediated phosphorylation at the catalytic domain, Star-PAP S6 phosphorylation is insensitive to oxidative stress suggesting a signal mediated regulation of CKIα activity. S6 phosphorylation together with coactivator PIPKIα controlled select subset of Star-PAP target messages by regulating Star-PAP-mRNA association. Our results establish a novel role for phosphorylation in determining Star-PAP target mRNA specificity and regulation of 3'-end processing.

PIP kinases define PI4,5P₂signaling specificity by association with effectors

Phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) is an essential lipid messenger with roles in all eukaryotes and most aspects of human physiology. By controlling the targeting and activity of its effectors, PI4,5P₂modulates processes, such as cell migration, vesicular trafficking, cellular morphogenesis, signaling and gene expression. In cells, PI4,5P₂has a much higher concentration than other phosphoinositide species and its total content is largely unchanged in response to extracellular stimuli. The discovery of a vast array of PI4,5P₂ binding proteins is consistent with data showing that the majority of cellular PI4,5P₂is sequestered. This supports a mechanism where PI4,5P₂functions as a localized and highly specific messenger. Further support of this mechanism comes from the de novo synthesis of PI4,5P₂which is often linked with PIP kinase interaction with PI4,5P₂effectors and is a mechanism to define specificity of PI4,5P₂signaling. The association of PI4,5P₂-generating enzymes with PI4,5P₂effectors regulate effector function both temporally and spatially in cells. In this review, the PI4,5P₂effectors whose functions are tightly regulated by associations with PI4,5P₂-generating enzymes will be discussed. This article is part of a Special Issue entitled Phosphoinositides.

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 SCFSlimb 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.

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.

Casein kinase 1 alpha regulates chromosome congression and separation during mouse oocyte meiotic maturation and early embryo development

Casein kinase I alpha (CK1α) is a member of serine/threonine protein kinase, generally present in all eukaryotes. In mammals, CK1α regulates the transition from interphase to metaphase in mitosis. However, little is known about its role in meiosis. Here we examined Ck1α mRNA and protein expression, as well as its subcellular localization in mouse oocytes from germinal vesicle to the late 1-cell stage. Our results showed that the expression level of CK1α was increased in metaphase. Immunostaining results showed that CK1α colocalized with condensed chromosomes during oocyte meiotic maturation and early embryo development. We used the loss-of-function approach by employing CK1α specific morpholino injection to block the function of CK1α. This functional blocking leads to failure of polar body 1 (PB1) extrusion, chromosome misalignment and MII plate incrassation. We further found that D4476, a specific and efficient CK1 inhibitor, decreased the rate of PB1 extrusion. Moreover, D4476 resulted in giant polar body extrusion, oocyte pro-MI arrest, chromosome congression failure and impairment of embryo developmental potential. In addition, we employed pyrvinium pamoate (PP), an allosteric activator of CK1α, to enhance CK1α activity in oocytes. Supplementation of PP induced oocyte meiotic maturation failure, severe congression abnormalities and misalignment of chromosomes. Taken together, our study for the first time demonstrates that CK1α is required for chromosome alignment and segregation during oocyte meiotic maturation and early embryo development.

The novel poly(A) polymerase Star-PAP is a signal-regulated switch at the 3'-end of mRNAs

The mRNA 3'-untranslated region (3'-UTR) modulates message stability, transport, intracellular location and translation. We have discovered a novel nuclear poly(A) polymerase termed Star-PAP (nuclear speckle targeted PIPKIα regulated-poly(A) polymerase) that couples with the transcriptional machinery and is regulated by the phosphoinositide lipid messenger phosphatidylinositol-4,5-bisphosphate (PI4,5P(2)), the central lipid in phosphoinositide signaling. PI4,5P(2) is generated primarily by type I phosphatidylinositol phosphate kinases (PIPKI). Phosphoinositides are present in the nucleus including at nuclear speckles compartments separate from known membrane structures. PIPKs regulate cellular functions by interacting with PI4,5P(2) effectors where PIPKs generate PI4,5P(2) that then modulates the activity of the associated effectors. Nuclear PIPKIα interacts with and regulates Star-PAP, and PI4,5P(2) specifically activates Star-PAP in a gene- and signaling-dependent manner. Importantly, other select signaling molecules integrated into the Star-PAP complex seem to regulate Star-PAP activities and processivities toward RNA substrates, and unique sequence elements around the Star-PAP binding sites within the 3'-UTR of target genes contribute to Star-PAP specificity for processing. Therefore, Star-PAP and its regulatory molecules form a signaling nexus at the 3'-end of target mRNAs to control the expression of select group of genes including the ones involved in stress responses.

CKI isoforms α and ε regulate Star-PAP target messages by controlling Star-PAP poly(A) polymerase activity and phosphoinositide stimulation

Star-PAP is a non-canonical, nuclear poly(A) polymerase (PAP) that is regulated by the lipid signaling molecule phosphatidylinositol 4,5 bisphosphate (PI4,5P(2)), and is required for the expression of a select set of mRNAs. It was previously reported that a PI4,5P(2) sensitive CKI isoform, CKIα associates with and phosphorylates Star-PAP in its catalytic domain. Here, we show that the oxidative stress-induced by tBHQ treatment stimulates the CKI mediated phosphorylation of Star-PAP, which is critical for both its polyadenylation activity and stimulation by PI4,5P(2). CKI activity was required for the expression and efficient 3'-end processing of its target mRNAs in vivo as well as the polyadenylation activity of Star-PAP in vitro. Specific CKI activity inhibitors (IC261 and CKI7) block in vivo Star-PAP activity, but the knockdown of CKIα did not equivalently inhibit the expression of Star-PAP targets. We show that in addition to CKIα, Star-PAP associates with another CKI isoform, CKIε in the Star-PAP complex that phosphorylates Star-PAP and complements the loss of CKIα. Knockdown of both CKI isoforms (α and ε) resulted in the loss of expression and the 3'-end processing of Star-PAP targets similar to the CKI activity inhibitors. Our results demonstrate that CKI isoforms α and ε modulate Star-PAP activity and regulates Star-PAP target messages.

kin-19/casein kinase Iα has dual functions in regulating asymmetric division and terminal differentiation in C. elegans epidermal stem cells

Casein Kinase I (CKI) is a conserved component of the Wnt signaling pathway, which regulates cell fate determination in metazoans. We show that post-embryonic asymmetric division and fate specification of C. elegans epidermal stem cells are controlled by a non-canonical Wnt/β-catenin signaling pathway, involving the β-catenins WRM-1 and SYS-1, and that C. elegans kin-19/CKIα functions in this pathway. Furthermore, we find that kin-19 is the only member of the Wnt asymmetry pathway that functions with, or in parallel to, the heterochronic temporal patterning pathway to control withdrawal from self-renewal and subsequent terminal differentiation of epidermal stem cells. We show that, except in the case of kin-19, the Wnt asymmetry pathway and the heterochronic pathway function separately and in parallel to control different aspects of epidermal stem cell fate specification. However, given the function of kin-19/CKIα in both pathways, and that CKI, Wnt signaling pathway and heterochronic pathway genes are widely conserved in animals, our findings suggest that CKIα may function as a regulatory hub through which asymmetric division and terminal differentiation are coordinated in adult stem cells of vertebrates.

Phosphorylated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis

OBJECTIVE

TAR DNA-binding protein of 43kDa (TDP-43) is deposited as cytoplasmic and intranuclear inclusions in brains of patients with frontotemporal lobar degeneration with ubiquitinated inclusions (FTLD-U) and amyotrophic lateral sclerosis (ALS). Previous studies reported that abnormal phosphorylation takes place in deposited TDP-43. The aim of this study was to identify the phosphorylation sites and responsible kinases, and to clarify the pathological significance of phosphorylation of TDP-43.

METHODS

We generated multiple antibodies specific to phosphorylated TDP-43 by immunizing phosphopeptides of TDP-43, and analyzed FTLD-U and ALS brains by immunohistochemistry, immunoelectron microscopy, and immunoblots. In addition, we performed investigations aimed at identifying the responsible kinases, and we assessed the effects of phosphorylation on TDP-43 oligomerization and fibrillization.

RESULTS

We identified multiple phosphorylation sites in carboxyl-terminal regions of deposited TDP-43. Phosphorylation-specific antibodies stained more inclusions than antibodies to ubiquitin and, unlike existing commercially available anti-TDP-43 antibodies, did not stain normal nuclei. Ultrastructurally, these antibodies labeled abnormal fibers of 15nm diameter and on immunoblots recognized hyperphosphorylated TDP-43 at 45kDa, with additional 18 to 26kDa fragments in sarkosyl-insoluble fractions from FTLD-U and ALS brains. The phosphorylated epitopes were generated by casein kinase-1 and -2, and phosphorylation led to increased oligomerization and fibrillization of TDP-43.

INTERPRETATION

These results suggest that phosphorylated TDP-43 is a major component of the inclusions, and that abnormal phosphorylation of TDP-43 is a critical step in the pathogenesis of FTLD-U and ALS. Phosphorylation-specific antibodies will be powerful tools for the investigation of these disorders.

CKIalpha is associated with and phosphorylates star-PAP and is also required for expression of select star-PAP target messenger RNAs

We have recently identified Star-PAP, a nuclear poly(A) polymerase that associates with phosphatidylinositol-4-phosphate 5-kinase Ialpha (PIPKIalpha) and is required for the expression of a specific subset of mRNAs. Star-PAP activity is directly modulated by the PIPKIalpha product phosphatidylinositol 4,5-bisphosphate (PI-4,5-P(2)), linking nuclear phosphoinositide signaling to gene expression. Here, we show that PI-4,5-P(2)-dependent protein kinase activity is also a part of the Star-PAP protein complex. We identify the PI-4,5-P(2)-sensitive casein kinase Ialpha (CKIalpha) as a protein kinase responsible for this activity and further show that CKIalpha is capable of directly phosphorylating Star-PAP. Both CKIalpha and PIPKIalpha are required for the synthesis of some but not all Star-PAP target mRNA, and like Star-PAP, CKIalpha is associated with these messages in vivo. Taken together, these data indicate that CKIalpha, PIPKIalpha, and Star-PAP function together to modulate the production of specific Star-PAP messages. The Star-PAP complex therefore represents a location where multiple signaling pathways converge to regulate the expression of specific mRNAs.

The Saccharomyces cerevisiae 60 S ribosome biogenesis factor Tif6p is regulated by Hrr25p-mediated phosphorylation

The biosynthesis of 60 S ribosomal subunits in Saccharomyces cerevisiae requires Tif6p, the yeast homologue of mammalian eIF6. This protein is necessary for the formation of 60 S ribosomal subunits because it is essential for the processing of 35 S pre-rRNA to the mature 25 S and 5.8 S rRNAs. In the present work, using molecular genetic and biochemical analyses, we show that Hrr25p, an isoform of yeast casein kinase I, phosphorylates Tif6p both in vitro and in vivo. Tryptic phosphopeptide mapping of in vitro phosphorylated Tif6p by Hrr25p and (32)P-labeled Tif6p isolated from yeast cells followed by mass spectrometric analysis revealed that phosphorylation occurred on a single tryptic peptide at Ser-174. Sucrose gradient fractionation and coimmunoprecipitation experiments demonstrate that a small but significant fraction of Hrr25p is bound to 66 S preribosomal particles that also contain bound Tif6p. Depletion of Hrr25p from a conditional yeast mutant that fails to phosphorylate Tif6p was unable to process pre-rRNAs efficiently, resulting in significant reduction in the formation of 25 S rRNA. These results along with our previous observations that phosphorylatable Ser-174 is required for yeast cell growth and viability, suggest that Hrr25p-mediated phosphorylation of Tif6p plays a critical role in the biogenesis of 60 S ribosomal subunits in yeast cells.

Regulation of protein kinase CK1alphaLS by dephosphorylation in response to hydrogen peroxide

Low levels of hydrogen peroxide (H(2)O(2)) are mitogenic to mammalian cells and stimulate the hyperphosphorylation of heterogeneous nuclear ribonucleoprotein C (hnRNP-C) by protein kinase CK1alpha. However, the mechanisms by which CK1alpha is regulated have been unclear. Here it is demonstrated that low levels of H(2)O(2) stimulate the rapid dephosphorylation of CK1alphaLS, a nuclear splice form of CK1alpha. Furthermore, it is demonstrated that either treatment of endothelial cells with H(2)O(2), or dephosphorylation of CK1alphaLS in vitro enhances the association of CK1alphaLS with hnRNP-C. In addition, dephosphorylation of CK1alphaLS in vitro enhances the kinase's ability to phosphorylate hnRNP-C. While CK1alpha appears to be present in all metazoans, analysis of CK1alpha genomic sequences from several species reveals that the alternatively spliced nuclear localizing L-insert is unique to vertebrates, as is the case for hnRNP-C. These observations indicate that CK1alphaLS and hnRNP-C represent conserved components of a vertebrate-specific H(2)O(2)-responsive nuclear signaling pathway.

The early response to DNA damage can lead to activation of alternative splicing activity resulting in CD44 splice pattern changes

Expression of the human papillomavirus 16 E6 oncogene interferes with several vital cellular processes, including the p53-dependent response to DNA damage. To assess the influence of E6 on the early response to DNA damage, we analyzed gene expression following mitomycin C-induced genotoxic stress in human E6-expressing U2OS cells (U2OSE64b) as well as in p53-expressing control cells (U2OSE6AS) by comparative global expression profiling. As expected, genes involved in p53-dependent pathways were activated in p53-expressing cells. In the U2OSE64b cells, however, a largely nonoverlapping group of genes was identified, including two splicing factors of the SR family. Immunoblot analysis revealed increased expression of several SR proteins during the early response to DNA damage, which was accompanied by activation of alternative splicing activity. Disruption of splicing activity by treatment with small interfering RNA directed against splicing factor SRp55 resulted in the increased viability of p53-deficient cells following DNA damage. To determine whether the transient activation of splicing activity was due to E6-mediated degradation of p53, or was due to some other activity of E6, we compared the early response of the p53 wild-type and p53-/- isogenic HCT116 cell lines, and found that the increase in splicing activity was observed only in the absence of p53. Finally, both the U2OSE64b and the p53-/- cells showed altered splicing patterns for the CD44 receptor. Together, these data show that cells lacking p53 can activate alternative splicing following DNA damage.

Regulation of p53-MDMX interaction by casein kinase 1 alpha

MDMX is a homolog of MDM2 that is critical for regulating p53 function during mouse development. MDMX degradation is regulated by MDM2-mediated ubiquitination. Whether there are other mechanisms of MDMX regulation is largely unknown. We found that MDMX binds to the casein kinase 1 alpha isoform (CK1alpha) and is phosphorylated by CK1alpha. Expression of CK1alpha stimulates the ability of MDMX to bind to p53 and inhibit p53 transcriptional function. Regulation of MDMX-p53 interaction requires CK1alpha binding to the central region of MDMX and phosphorylation of MDMX on serine 289. Inhibition of CK1alpha expression by isoform-specific small interfering RNA (siRNA) activates p53 and further enhances p53 activity after ionizing irradiation. CK1alpha siRNA also cooperates with DNA damage to induce apoptosis. These results suggest that CK1alpha is a functionally relevant MDMX-binding protein and plays an important role in regulating p53 activity in the absence or presence of stress.

Protein kinase CK1alpha regulates mRNA binding by heterogeneous nuclear ribonucleoprotein C in response to physiologic levels of hydrogen peroxide

At low concentrations, hydrogen peroxide (H(2)O(2)) is a positive endogenous regulator of mammalian cell proliferation and survival; however, the signal transduction pathways involved in these processes are poorly understood. In primary human endothelial cells, low concentrations of H(2)O(2) stimulated the rapid phosphorylation of the acidic C-terminal domain (ACD) of heterogeneous nuclear ribonucleoprotein C (hnRNP-C), a nuclear restricted pre-mRNA-binding protein, at Ser(240) and at Ser(225)-Ser(228). A kinase activity was identified in mouse liver that phosphorylates the ACD of hnRNP-C at Ser(240) and at two sites at Ser(225)-Ser(228). The kinase was purified and identified by tandem mass spectrometry as protein kinase CK1alpha (formerly casein kinase 1alpha). Protein kinase CK1alpha immunoprecipitated from primary human endothelial cell nuclei also phosphorylated the ACD of hnRNP-C at these positions. Pretreatment of endothelial cells with the protein kinase CK1-specific inhibitor IC261 prevented the H(2)O(2)-stimulated phosphorylation of hnRNP-C. Utilizing phosphoserine-mimicking Ser-to-Glu point mutations, the effects of phosphorylation on hnRNP-C function were investigated by quantitative equilibrium fluorescence RNA binding analyses. Wild-type hnRNP-C1 and hnRNP-C1 modified at the basal sites of phosphorylation (S247E and S286E) both avidly bound RNA with similar binding constants. In contrast, hnRNP-C1 that was also modified at the CK1alpha phosphorylation sites exhibited a 14-500-fold decrease in binding affinity, demonstrating that CK1alpha-mediated phosphorylation modulates the mRNA binding ability of hnRNP-C.

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.

Casein Kinase 1α Interacts with Retinoid X Receptor and Interferes with Agonist-induced Apoptosis

Agonists of retinoid X receptors (RXRs), which include the natural 9-cis-retinoic acid and synthetic analogs, are potent inducers of growth arrest and apoptosis in some cancer cells. As such, they are being used in clinical trials for the treatment and prevention of solid tumors and are used to treat cutaneous T cell lymphoma. However, the molecular mechanisms that underlie the anti-cancer effects of RXR agonists remain unclear. Here, we show that a novel pro-apoptotic pathway that is induced by RXR agonist is negatively regulated by casein kinase 1alpha (CK1alpha). CK1alpha associates with RXR in an agonist-dependent manner and phosphorylates RXR. The ability of an RXR agonist to recruit CK1alpha to a complex with RXR in cells correlates inversely with its ability to inhibit growth. Remarkably, depletion of CK1alpha in resistant cells renders them susceptible to RXR agonist-induced growth inhibition and apoptosis. Our study shows that CK1alpha can promote cell survival by interfering with RXR agonist-induced apoptosis. Inhibition of CK1alpha may enhance the anti-cancer effects of RXR agonists.

Association of CPI-17 with protein kinase C and casein kinase I

The protein kinase C-potentiated inhibitor protein of 17kDa, called CPI-17, specifically inhibits myosin light chain phosphatase (MLCP). Phosphorylation of Thr-38 in vivo highly potentiates the ability of CPI-17 to inhibit MLCP. Thr-38 has been shown to be phosphorylated in vitro by a number of protein kinases including protein kinase C (PKC), Rho-associated coiled-coil kinase (ROCK), and protein kinase N (PKN). In this study we have focused on the association of protein kinases with CPI-17. Using affinity chromatography and Western blot analysis, we found interaction with all PKC isotypes and casein kinase I isoforms, CKIalpha and CKI. By contrast, ROCK and PKN did not associate with CPI-17, suggesting that PKC may be the relevant kinase that phosphorylates Thr-38 in vivo. CPI-17 interacted with the cysteine-rich domain of PKC and was phosphorylated by all PKC isotypes. We previously found that CPI-17 co-purified with casein kinase I in brain suggesting they are part of a complex and we now show that CPI-17 associates with the kinase domain of CKI isoforms.

Involvement of casein kinase Iepsilon in cytokine-induced granulocytic differentiation

Two closely related casein kinase I (CKI) isoforms, CKIdelta and CKIepsilon, are ubiquitously expressed in many human tissues, but their specific biologic function remains to be clarified. Here, we provide the first evidence that CKIepsilon is involved in hematopoietic cell differentiation. CKIepsilon, but not CKIdelta, was down-regulated along with human granulocytic differentiation. The specific down-regulation was observed in granulocyte colony-stimulating factor (G-CSF)-induced cell differentiation of murine interleukin-3 (IL-3)-dependent myeloid progenitor 32D cells. Introduction of wild-type (WT)-CKIepsilon into 32D cells inhibited the G-CSF-induced cell differentiation, whereas kinase-negative (KN)-CKIepsilon promoted the differentiation. Neither WT- nor KN-CKIepsilon affected IL-3-dependent cell growth. Moreover, introduction of WT- or KN-CKIdelta did not affect the cytokine-induced cell growth and differentiation. While G-CSF-induced activation of signal transducers and activators of transcription 3 (STAT3) was sustained by KN-CKIepsilon, STAT3 activation was attenuated by WT-CKIepsilon. This may be explained by the fact that the suppressor of cytokine signaling 3 (SOCS3) was stabilized by its physical association with CKIepsilon. Such stabilization by CKIepsilon was also seen in IL-3-induced beta-catenin. The stabilization of downstream components of cytokine and Wnt signaling by CKIepsilon might be critical for integration of several intracellular signaling pathways to a cell-specific biologic response in hematopoietic cell self-renewal.

Nuclear phosphoinositides and their functions

Phosphoinositides are minor components of biological membranes, which have emerged as essential regulators of a variety of cellular processes, both on the plasma membrane and on several intracellular organelles. The versatility of these lipids stems from their ability to function either as substrates for the generation of second messengers, as membrane-anchoring sites for cytosolic proteins or as regulators of the actin cytoskeleton. Despite a vast literature demonstrating the presence of phosphoinositides in the nucleus, only recently has the function(s) of the nuclear pool of these lipids and their soluble analogues, inositol polyphosphates, started to emerge. These compounds have been shown to serve as essential co-factors for several nuclear processes, including DNA repair, transcription regulation and RNA dynamics. In this light, phosphoinositides and inositol polyphosphates might represent high turnover activity switches for nuclear complexes responsible for these processes. The regulation of these large machineries would be linked to the phosphorylation state of the inositol ring and limited temporally and spatially based on the synthesis and degradation of these molecules.

DYRK1A accumulates in splicing speckles through a novel targeting signal and induces speckle disassembly

The protein kinase DYRK1A is distributed throughout the nucleoplasm, accumulating in speckle-like regions. We have found that this punctuated nuclear distribution is determined by the contribution of several elements. Although the nuclear import is mediated by two distinct nuclear localization signals, one at the N-terminus and the other located in the linker region, between subdomains X and XI of the catalytic domain, the accumulation in speckles that are SC35 positive depends on a sequence motif that is located C-terminal to the kinase domain and comprises a histidine tail. A similar sequence is also responsible for the targeting of cyclin T1. Therefore the histidine-rich region represents a novel splicing speckle targeting signal. Moreover, overexpression of DYRK1A induces speckle disassembly. Such disassembly is DYRK1A activity specific, since the overexpression of a DYRK1A kinase inactive mutant, the paralogous DYRK1B or a chimeric protein DYRK1B that has been directed to the speckles via the DYRK1A targeting signal, leaves the SC35 speckle pattern untouched. Thus DYRK1A protein kinase may play a role in regulating the biogenesis of the splicing speckle compartment.

Centaurin-alpha 1 associates in vitro and in vivo with nucleolin

Centaurin-alpha(1) was originally described as a binding partner for phosphoinositides. In spite of the presence of a putative ADP-ribosylation factor (ARF) GTPase-activating protein (GAP) domain, no ARF-GAP activity has been attributed to centaurin-alpha(1) so far. Thus the function of this protein remains to be determined. In order to better understand its intracellular role, we aimed to identify centaurin-alpha(1) partners. Using affinity chromatography followed by mass spectrometry analysis, we identified several potential centaurin-alpha(1) protein partners. Nucleolin, a nucleolar protein involved in ribosome biosynthesis, was the main centaurin-alpha(1) interacting protein. The interaction between centaurin-alpha(1) and nucleolin was confirmed by Western blot analysis and GST pull down assays. Moreover, we have shown that ectopically expressed centaurin-alpha(1) associates in vivo with endogenous nucleolin in human embryonic kidney 293 cells. In addition, the association between nucleolin and centaurin-alpha(1) was disrupted by RNAse treatment, indicating that RNA integrity was necessary for their binding. This suggested that centaurin-alpha(1) was part of a ribonucleoprotein complex.

Identification of casein kinase Ialpha interacting protein partners

Casein kinase Ialpha (CKIalpha) belongs to a family of serine/threonine protein kinases involved in membrane trafficking, RNA processing, mitotic spindle formation and cell cycle progression. In this report, we identified several CKIalpha interacting proteins including RCC1, high mobility group proteins 1 and 2 (HMG1, HMG2), Erf, centaurin-alpha1, synaptotagmin IX and CPI-17 that were isolated from brain as CKIalpha co-purifying proteins. Actin, importin-alpha(1), importin-beta, PP2Ac, centaurin-alpha1, and HMG1 were identified by affinity chromatography using a peptide column comprising residues 214-233 of CKIalpha. We have previously shown that centaurin-alpha1 represents a CKIalpha partner both in vitro and in vivo. The nuclear protein regulator of chromosome condensation 1 (RCC1) is a guanosine nucleotide exchange factor for Ran which is involved in nuclear transport and mitotic spindle formation. Here we show that CKIalpha and RCC1 interact in brain and in cultured cells. However, the interaction does not involve residues 217-233 of CKIalpha which are proposed from X-ray structures to represent an anchoring site for CKI partners. Formation of the RCC1/CKIalpha complex is consistent with the association of the kinase with mitotic spindles. In conclusion, we have identified a number of novel CKIalpha protein partners and their relations to CKI are discussed.

Regulation of fibronectin alternative splicing by a basement membrane-like extracellular matrix

Hepatocytes are the source of plasma fibronectin (FN) which lacks the alternatively spliced EDI segment, distinctive of oncofetal FN. When hepatic or other epithelial cells are cultured on plastic, EDI inclusion is triggered. Here we report that EDI inclusion is inhibited when hepatic cells are cultured on a basement membrane-like extracellular matrix (ECM), demonstrating a new role for the ECM in the control of gene expression. The effect is duplicated by collagen IV and laminin but not by collagen I; is not observed with another alternatively spliced FN exon (EDII); and correlates with a decrease in cell proliferation, consistently with high EDI inclusion levels observed in many physiological and pathological proliferative processes.

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.

Phosphatidylinositol 3-kinase c2alpha contains a nuclear localization sequence and associates with nuclear speckles

Phosphoinositide 3-kinase C2alpha (PI3K-C2alpha) belongs to the class II phosphatidylinositol 3-kinases, which are defined by their in vitro usage of phosphatidylinositol and phosphatidylinositol 4-phosphate as substrates. All type II phosphatidylinositol 3-kinases contain at their C terminus a C2-like domain. Here we demonstrate that Homo sapiens phosphoinositide 3-kinase C2alpha (HsPI3K-C2alpha) has dual cellular localization present in the cytoplasm and in the nucleus. A distinct nuclear localization signal sequence was identified by expressing HsPI3K-C2alpha-green fluorescent protein fusion proteins in HeLa cells. The nuclear localization signal was mapped to a stretch of 11 amino acids (KRKTKISRKTR) located within C2-like domain of the kinase. In the cytoplasm and the nucleus HsPI3K-C2alpha associates with macromolecular complexes that are resistant to detergent extraction. Indirect immunofluorescence reveals that in the nucleus HsPI3K-C2alpha is enriched at distinct subnuclear domains known as nuclear speckles, which contain pre-mRNA processing factors and are functionally connected to RNA metabolism. Phosphorylation of HsPI3K-C2alpha is induced by inhibition of RNA polymerase II-dependent transcription and coincides with enlargement and rounding up of the nuclear speckles. The results suggest that phosphorylation of HsPI3K-C2alpha is inversely linked to mRNA transcription and supports the importance of phosphoinositides for nuclear activity.

Four casein kinase I isoforms are differentially partitioned between nucleus and cytoplasm

The casein kinase I (CKI) family consists of at least seven vertebrate genes, some of which can be alternatively spliced. Previously, we have studied the four splice variants of the chicken CKIalpha gene. The four proteins differ only by the presence or absence of two peptides, a 28-amino-acid "L" insert in the catalytic domain and a 12-amino-acid "S" insert near the extreme C-terminus. Here cells were transfected with DNA encoding all four isoforms fused to the green fluorescent protein (GFP) and the localization of each protein was examined. We noted that the L insert includes the sequence PVGKRKR, which has the characteristics of a nuclear localization signal (NLS), and we show that the CKIalphaL and CKIalphaLS isoforms which contain this sequence are targeted to the nucleus, where a fraction becomes associated with nuclear speckles. In contrast the two isoforms lacking the L insert remain predominantly cytoplasmic. Mutation of the first lysine in the putative NLS to asparagine prevented the nuclear entry of GFP-CKIalphaL. Therefore different CKIalpha isoforms are targeted to different cellular compartments in a fashion modulated by alternate transcription and in these locations presumably phosphorylate and regulate different cellular substrates.

Casein kinase I associates with members of the centaurin-alpha family of phosphatidylinositol 3,4,5-trisphosphate-binding proteins

Mammalian casein kinases I (CKI) belong to a family of serine/threonine protein kinases involved in diverse cellular processes including cell cycle progression, membrane trafficking, circadian rhythms, and Wnt signaling. Here we show that CKIalpha co-purifies with centaurin-alpha(1) in brain and that they interact in vitro and form a complex in cells. In addition, we show that the association is direct and occurs through the kinase domain of CKI within a loop comprising residues 217-233. These residues are well conserved in all members of the CKI family, and we show that centaurin-alpha(1) associates in vitro with all mammalian CKI isoforms. To date, CKIalpha represents the first protein partner identified for centaurin-alpha(1). However, our data suggest that centaurin-alpha(1) is not a substrate for CKIalpha and has no effect on CKIalpha activity. Centaurin-alpha(1) has been identified as a phosphatidylinositol 3,4,5-trisphosphate-binding protein. Centaurin-alpha(1) contains a cysteine-rich domain that is shared by members of a newly identified family of ADP-ribosylation factor guanosine trisphosphatase-activating proteins. These proteins are involved in membrane trafficking and actin cytoskeleton rearrangement, thus supporting a role for CKIalpha in these biological events.

Casein kinase I is anchored on axonemal doublet microtubules and regulates flagellar dynein phosphorylation and activity

Flagellar dynein activity is regulated by phosphorylation. One critical phosphoprotein substrate in Chlamydomonas is the 138-kDa intermediate chain (IC138) of the inner arm dyneins (Habermacher, G., and Sale, W. S. (1997) J. Cell Biol. 136, 167-176). In this study, several approaches were used to determine that casein kinase I (CKI) is physically anchored in the flagellar axoneme and regulates IC138 phosphorylation and dynein activity. First, using a videomicroscopic motility assay, selective CKI inhibitors rescued dynein-driven microtubule sliding in axonemes isolated from paralyzed flagellar mutants lacking radial spokes. Rescue of dynein activity failed in axonemes isolated from these mutant cells lacking IC138. Second, CKI was unequivocally identified in salt extracts from isolated axonemes, whereas casein kinase II was excluded from the flagellar compartment. Third, Western blots indicate that within flagella, CKI is anchored exclusively to the axoneme. Analysis of multiple Chlamydomonas motility mutants suggests that the axonemal CKI is located on the outer doublet microtubules. Finally, CKI inhibitors that rescued dynein activity blocked phosphorylation of IC138. We propose that CKI is anchored on the outer doublet microtubules in position to regulate flagellar dynein.