Characterization of the interaction between RhoA and the amino-terminal region of PKN

Molecular insights into the regulatory landscape of PKC-related kinase-2 (PRK2/PKN2) using targeted small compounds

The PKC-related kinases (PRKs, also termed PKNs) are important in cell migration, cancer, hepatitis C infection, and nutrient sensing. They belong to a group of protein kinases called AGC kinases that share common features like a C-terminal extension to the catalytic domain comprising a hydrophobic motif. PRKs are regulated by N-terminal domains, a pseudosubstrate sequence, Rho-binding domains, and a C2 domain involved in inhibition and dimerization, while Rho and lipids are activators. We investigated the allosteric regulation of PRK2 and its interaction with its upstream kinase PDK1 using a chemical biology approach. We confirmed the phosphoinositide-dependent protein kinase 1 (PDK1)-interacting fragment (PIF)-mediated docking interaction of PRK2 with PDK1 and showed that this interaction can be modulated allosterically. We showed that the polypeptide PIFtide and a small compound binding to the PIF-pocket of PRK2 were allosteric activators, by displacing the pseudosubstrate PKL region from the active site. In addition, a small compound binding to the PIF-pocket allosterically inhibited the catalytic activity of PRK2. Together, we confirmed the docking interaction and allostery between PRK2 and PDK1 and described an allosteric communication between the PIF-pocket and the active site of PRK2, both modulating the conformation of the ATP-binding site and the pseudosubstrate PKL-binding site. Our study highlights the allosteric modulation of the activity and the conformation of PRK2 in addition to the existence of at least two different complexes between PRK2 and its upstream kinase PDK1. Finally, the study highlights the potential for developing allosteric drugs to modulate PRK2 kinase conformations and catalytic activity.

PKN1 kinase-negative knock-in mice develop splenomegaly and leukopenia at advanced age without obvious autoimmune-like phenotypes

Protein kinase N1 (PKN1) knockout (KO) mice spontaneously form germinal centers (GCs) and develop an autoimmune-like disease with age. Here, we investigated the function of PKN1 kinase activity in vivo using aged mice deficient in kinase activity resulting from the introduction of a point mutation (T778A) in the activation loop of the enzyme. PKN1[T778A] mice reached adulthood without external abnormalities; however, the average spleen size and weight of aged PKN1[T778A] mice increased significantly compared to aged wild type (WT) mice. Histologic examination and Southern blot analyses of spleens showed extramedullary hematopoiesis and/or lymphomagenesis in some cases, although without significantly different incidences between PKN1[T778A] and WT mice. Additionally, flow cytometry revealed increased numbers in B220⁺, CD3⁺, Gr1⁺ and CD193⁺ leukocytes in the spleen of aged PKN1[T778A] mice, whereas the number of lymphocytes, neutrophils, eosinophils, and monocytes was reduced in the peripheral blood, suggesting an advanced impairment of leukocyte trafficking with age. Moreover, aged PKN1[T778A] mice showed no obvious GC formation nor autoimmune-like phenotypes, such as glomerulonephritis or increased anti-dsDNA antibody titer, in peripheral blood. Our results showing phenotypic differences between aged Pkn1-KO and PKN1[T778A] mice may provide insight into the importance of PKN1-specific kinase-independent functions in vivo.

The protein kinase C super-family member PKN is regulated by mTOR and influences differentiation during prostate cancer progression

BACKGROUND

Phosphoinositide-3 (PI-3) kinase signaling has a pervasive role in cancer. One of the key effectors of PI-3 kinase signaling is AKT, a kinase that promotes growth and survival in a variety of cancers. Genetically engineered mouse models of prostate cancer have shown that AKT signaling is sufficient to induce prostatic epithelial neoplasia (PIN), but insufficient for progression to adenocarcinoma. This contrasts with the phenotype of mice with prostate-specific deletion of Pten, where excessive PI-3 kinase signaling induces both PIN and locally invasive carcinoma. We reasoned that additional PI-3 kinase effector kinases promote prostate cancer progression via activities that provide biological complementarity to AKT. We focused on the PKN kinase family members, which undergo activation in response to PI-3 kinase signaling, show expression changes in prostate cancer, and contribute to cell motility pathways in cancer cells.

METHODS

PKN kinase activity was measured by incorporation of ³² P into protein substrates. Phosphorylation of the turn-motif (TM) in PKN proteins by mTOR was analyzed using the TORC2-specific inhibitor torin and a PKN1 phospho-TM-specific antibody. Amino acid substitutions in the TM of PKN were engineered and assayed for effects on kinase activity. Cell motility-related functions and PKN localization was analyzed by depletion approaches and immunofluorescence microscopy, respectively. The contribution of PKN proteins to prostate tumorigenesis was characterized in several mouse models that express PKN transgenes. The requirement for PKN activity in prostate cancer initiated by loss of phosphatase and tensin homolog deleted on chromosome 10 (Pten), and the potential redundancy between PKN isoforms, was analyzed by prostate-specific deletion of Pkn1, Pkn2, and Pten.

RESULTS AND CONCLUSIONS

PKN1 and PKN2 contribute to motility pathways in human prostate cancer cells. PKN1 and PKN2 kinase activity is regulated by TORC2-dependent phosphorylation of the TM, which together with published data indicates that PKN proteins receive multiple PI-3 kinase-dependent inputs. Transgenic expression of active AKT and PKN1 is not sufficient for progression beyond PIN. Moreover, Pkn1 is not required for tumorigenesis initiated by loss of Pten. Triple knockout of Pten, Pkn1, and Pkn2 in mouse prostate results in squamous cell carcinoma, an uncommon but therapy-resistant form of prostate cancer.

Impaired lymphocyte trafficking in mice deficient in the kinase activity of PKN1

Knock-in mice lacking PKN1 kinase activity were generated by introducing a T778A point mutation in the catalytic domain. PKN1[T778A] mutant mice developed to adulthood without apparent external abnormalities, but exhibited lower T and B lymphocyte counts in the peripheral blood than those of wild-type (WT) mice. T and B cell development proceeded in an apparently normal fashion in bone marrow and thymus of PKN1[T778A] mice, however, the number of T and B cell counts were significantly higher in the lymph nodes and spleen of mutant mice in those of WT mice. After transfusion into WT recipients, EGFP-labelled PKN1[T778A] donor lymphocytes were significantly less abundant in the peripheral circulation and more abundant in the spleen and lymph nodes of recipient mice compared with EGFP-labelled WT donor lymphocytes, likely reflecting lymphocyte sequestration in the spleen and lymph nodes in a cell-autonomous fashion. PKN1[T778A] lymphocytes showed significantly lower chemotaxis towards chemokines and sphingosine 1-phosphate (S1P) than WT cells in vitro. The biggest migration defect was observed in response to S1P, which is essential for lymphocyte egress from secondary lymphoid organs. These results reveal a novel role of PKN1 in lymphocyte migration and localization.

RASSF1A Directly Antagonizes RhoA Activity through the Assembly of a Smurf1-Mediated Destruction Complex to Suppress Tumorigenesis

RASSF1A is a tumor suppressor implicated in many tumorigenic processes; however, the basis for its tumor suppressor functions are not fully understood. Here we show that RASSF1A is a novel antagonist of protumorigenic RhoA activity. Direct interaction between the C-terminal amino acids (256–277) of RASSF1A and active GTP-RhoA was critical for this antagonism. In addition, interaction between the N-terminal amino acids (69-82) of RASSF1A and the ubiquitin E3 ligase Smad ubiquitination regulatory factor 1 (Smurf1) disrupted GTPase activity by facilitating Smurf1-mediated ubiquitination of GTP-RhoA. We noted that the RhoA-binding domain of RASSF1A displayed high sequence homology with Rho-binding motifs in other RhoA effectors, such as Rhotekin. As predicted on this basis, RASSF1A competed with Rhotekin to bind RhoA and to block its activation. RASSF1A mutants unable to bind RhoA or Smurf1 failed to suppress RhoA-induced tumor cell proliferation, drug resistance, epithelial–mesenchymal transition, migration, invasion, and metastasis. Clinically, expression levels of RASSF1A and RhoA were inversely correlated in many types of primary and metastatic tumors and tumor cell lines. Collectively, our findings showed how RASSF1A may suppress tumorigenesis by intrinsically inhibiting the tumor-promoting activity of RhoA, thereby illuminating the potential mechanistic consequences of RASSF1A inactivation in many cancers. Cancer Res; 76(7); 1847–59. ©2016 AACR.

PKN3 is the major regulator of angiogenesis and tumor metastasis in mice

PKN, a conserved family member related to PKC, was the first protein kinase identified as a target of the small GTPase Rho. PKN is involved in various functions including cytoskeletal arrangement and cell adhesion. Furthermore, the enrichment of PKN3 mRNA in some cancer cell lines as well as its requirement in malignant prostate cell growth suggested its involvement in oncogenesis. Despite intensive research efforts, physiological as well as pathological roles of PKN3 in vivo remain elusive. Here, we generated mice with a targeted deletion of PKN3. The PKN3 knockout (KO) mice are viable and develop normally. However, the absence of PKN3 had an impact on angiogenesis as evidenced by marked suppressions of micro-vessel sprouting in ex vivo aortic ring assay and in vivo corneal pocket assay. Furthermore, the PKN3 KO mice exhibited an impaired lung metastasis of melanoma cells when administered from the tail vein. Importantly, PKN3 knock-down by small interfering RNA (siRNA) induced a glycosylation defect of cell-surface glycoproteins, including ICAM-1, integrin β1 and integrin α5 in HUVECs. Our data provide the first in vivo genetic demonstration that PKN3 plays critical roles in angiogenesis and tumor metastasis, and that defective maturation of cell surface glycoproteins might underlie these phenotypes.

Protein Kinase C-Related Kinase (PKN/PRK). Potential Key-Role for PKN1 in Protection of Hypoxic Neurons

Serine/threonine protein kinase C-related kinase (PKN/PRK) is a family of three isoenzymes (PKN1, PKN2, PKN3), which are widely distributed in eukaryotic organisms and share the same overall domain structure. The Nterminal region encompasses a conserved repeated domain, termed HR1a-c as well as a HR2/C2 domain. The serine/threonine kinase domain is found in the C-terminal region of the protein and shows high sequence homology to other members of the PKC superfamily.

In neurons, PKN1 is the most abundant isoform and has been implicated in a variety of functions including cytoskeletal organization and neuronal differentiation and its deregulation may contribute to neuropathological processes such as amyotrophic lateral sclerosis and Alzheimer’s disease. We have recently identified a candidate role of PKN1 in the regulation of neuroprotective processes during hypoxic stress. Our key findings were that: 1) the activity of PKN1 was significantly increased by hypoxia (1% O2) and neurotrophins (nerve growth factor and purine nucleosides); 2) Neuronal cells, deficient of PKN1 showed a decrease of cell viability and neurite formation along with a disturbance of the F-actinassociated cytoskeleton; 3) Purine nucleoside-mediated neuroprotection during hypoxia was severely hampered in PKN1 deficient neuronal cells, altogether suggesting a potentially critical role of PKN1 in neuroprotective processes.

This review gives an up-to-date overview of the PKN family with a special focus on the neuroprotective role of PKN1 in hypoxia.

Enzyme kinetics and distinct modulation of the protein kinase N family of kinases by lipid activators and small molecule inhibitors

The PKN (protein kinase N) family of Ser/Thr protein kinases regulates a diverse set of cellular functions, such as cell migration and cytoskeletal organization. Inhibition of tumour PKN activity has been explored as an oncology therapeutic approach, with a PKN3-targeted RNAi (RNA interference)-derived therapeutic agent in Phase I clinical trials. To better understand this important family of kinases, we performed detailed enzymatic characterization, determining the kinetic mechanism and lipid sensitivity of each PKN isoform using full-length enzymes and synthetic peptide substrate. Steady-state kinetic analysis revealed that PKN1–3 follows a sequential ordered Bi–Bi kinetic mechanism, where peptide substrate binding is preceded by ATP binding. This kinetic mechanism was confirmed by additional kinetic studies for product inhibition and affinity of small molecule inhibitors. The known lipid effector, arachidonic acid, increased the catalytic efficiency of each isoform, mainly through an increase in k_cat for PKN1 and PKN2, and a decrease in peptide K_M for PKN3. In addition, a number of PKN inhibitors with various degrees of isoform selectivity, including potent (K_i<10 nM) and selective PKN3 inhibitors, were identified by testing commercial libraries of small molecule kinase inhibitors. This study provides a kinetic framework and useful chemical probes for understanding PKN biology and the discovery of isoform-selective PKN-targeted inhibitors.

We conducted kinetic analysis of the relatively unexplored PKN family and effects of lipids, and identified potent inhibitors with various isoform selectivity. The kinetic mechanism, lipid activators and inhibitors could be useful for understanding PKN biology and developing PKN-targeted therapies.

Regulation of epithelial sodium transport via epithelial Na+ channel

Renal epithelial Na⁺ transport plays an important role in homeostasis of our body fluid content and blood pressure. Further, the Na⁺ transport in alveolar epithelial cells essentially controls the amount of alveolar fluid that should be kept at an appropriate level for normal gas exchange. The epithelial Na⁺ transport is generally mediated through two steps: (1) the entry step of Na⁺ via epithelial Na⁺ channel (ENaC) at the apical membrane and (2) the extrusion step of Na⁺ via the Na⁺, K⁺-ATPase at the basolateral membrane. In general, the Na⁺ entry via ENaC is the rate-limiting step. Therefore, the regulation of ENaC plays an essential role in control of blood pressure and normal gas exchange. In this paper, we discuss two major factors in ENaC regulation: (1) activity of individual ENaC and (2) number of ENaC located at the apical membrane.

Identification of an interaction between the TPalpha and TPbeta isoforms of the human thromboxane A2 receptor with protein kinase C-related kinase (PRK) 1: implications for prostate cancer

In humans, thromboxane (TX) A(2) signals through the TPα and TPβ isoforms of the TXA(2) receptor or TP. Here, the RhoA effector protein kinase C-related kinase (PRK) 1 was identified as an interactant of both TPα and ΤPβ involving common and unique sequences within their respective C-terminal (C)-tail domains and the kinase domain of PRK1 (PRK1(640-942)). Although the interaction with PRK1 is constitutive, agonist activation of TPα/TPβ did not regulate the complex per se but enhanced PRK1 activation leading to phosphorylation of its general substrate histone H1 in vitro. Altered PRK1 and TP expression and signaling are increasingly implicated in certain neoplasms, particularly in androgen-associated prostate carcinomas. Agonist activation of TPα/TPβ led to phosphorylation of histone H3 at Thr(11) (H3 Thr(11)), a previously recognized specific marker of androgen-induced chromatin remodeling, in the prostate LNCaP and PC-3 cell lines but not in primary vascular smooth muscle or endothelial cells. Moreover, this effect was augmented by dihydrotestosterone in androgen-responsive LNCaP but not in nonresponsive PC-3 cells. Furthermore, PRK1 was confirmed to constitutively interact with TPα/TPβ in both LNCaP and PC-3 cells, and targeted disruption of PRK1 impaired TPα/TPβ-mediated H3 Thr(11) phosphorylation in, and cell migration of, both prostate cell types. Collectively, considering the role of TXA(2) as a potent mediator of RhoA signaling, the identification of PRK1 as a bona fide interactant of TPα/TPβ, and leading to H3 Thr(11) phosphorylation to regulate cell migration, has broad functional significance such as within the vasculature and in neoplasms in which both PRK1 and the TPs are increasingly implicated.

Hypotonic swelling-induced activation of PKN1 mediates cell survival in cardiac myocytes

Hypotonic cell swelling in the myocardium is induced by pathological conditions, including ischemia-reperfusion, and affects the activities of ion transporters/channels and gene expression. However, the signaling mechanism activated by hypotonic stress (HS) is not fully understood in cardiac myocytes. A specialized protein kinase cascade, consisting of Pkc1 and MAPKs, is activated by HS in yeast. Here, we demonstrate that protein kinase N1 (PKN1), a serine/threonine protein kinase and a homolog of Pkc1, is activated by HS (67% osmolarity) within 5 min and reaches peak activity at 60 min in cardiac myocytes. Activation of PKN1 by HS was accompanied by Thr(774) phosphorylation and concomitant activation of PDK1, a potential upstream regulator of PKN1. HS also activated RhoA, thereby increasing interactions between PKN1 and RhoA. PP1 (10(-5) M), a selective Src family tyrosine kinase inhibitor, significantly suppressed HS-induced activation of RhoA and PKN1. Constitutively active PKN1 significantly increased the transcriptional activity of Elk1-GAL4, an effect that was inhibited by dominant negative MEK. Overexpression of PKN1 significantly increased ERK phosphorylation, whereas downregulation of PKN1 inhibited HS-induced ERK phosphorylation. Downregulation of PKN1 and inhibition of ERK by U-0126 both significantly inhibited the survival of cardiac myocytes in the presence of HS. These results suggest that a signaling cascade, consisting of Src, RhoA, PKN1, and ERK, is activated by HS, thereby promoting cardiac myocyte survival.

PKC and the control of localized signal dynamics

Networks of signal transducers determine the conversion of environmental cues into cellular actions. Among the main players in these networks are protein kinases, which can acutely and reversibly modify protein functions to influence cellular events. One group of kinases, the protein kinase C (PKC) family, have been increasingly implicated in the organization of signal propagation, particularly in the spatial distribution of signals. Examples of where and how various PKC isoforms direct this tier of signal organization are becoming more evident.

Evolution of domain combinations in protein kinases and its implications for functional diversity

Protein kinases phosphorylating Ser/Thr/Tyr residues in several cellular proteins exert tight control over their biological functions. They constitute the largest protein family in most eukaryotic species. Protein kinases classified based on sequence similarity in their catalytic domains, cluster into subfamilies, which share gross functional properties. Many protein kinases are associated or tethered covalently to domains that serve as adapter or regulatory modules, aiding substrate recruitment, specificity, and also serve as scaffolds. Hence the modular organisation of the protein kinases serves as guidelines to their functional and molecular properties. Analysis of genomic repertoires of protein kinases in eukaryotes have revealed wide spectrum of domain organisation across various subfamilies of kinases. Occurrence of organism-specific novel domain combinations suggests functional diversity achieved by protein kinases in order to regulate variety of biological processes. In addition, domain architecture of protein kinases revealed existence of hybrid protein kinase subfamilies and their emerging roles in the signaling of eukaryotic organisms. In this review we discuss the repertoire of non-kinase domains tethered to multi-domain kinases in the metazoans. Similarities and differences in the domain architectures of protein kinases in these organisms indicate conserved and unique features that are critical to functional specialization.

The C-terminus of PRK2/PKNgamma is required for optimal activation by RhoA in a GTP-dependent manner

PRK2/PKNgamma is a Rho effector and a member of the protein kinase C superfamily of serine/threonine kinases. Here, we explore the structure-function relationship between various motifs in the C-terminal half of PRK2 and its kinase activity and regulation. We report that two threonine residues at conserved phosphoacceptor position in the activation loop and the turn motif are essential for the catalytic activity of PRK2, but the phosphomimetic Asp-978 at hydrophobic motif is dispensable for kinase catalytic competence. Moreover, the PRK2-Delta958 mutant with the turn motif truncated still interacts with 3-phosphoinositide-dependent kinase-1 (PDK-1). Thus, both the intact hydrophobic motif and the turn motif in PRK2 are dispensable for the binding of PDK-1. We also found that while the last seven amino acid residues at the C-terminus of PRK2 are not required for the activation of the kinase by RhoA in vitro, however, the extreme C-terminal segment is critical for the full activation of PRK2 by RhoA in cells in a GTP-dependent manner. Our data suggest that the extreme C-terminus of PRK2 may represent a potential drug target for effector-specific pharmacological intervention of Rho-medicated biological processes.

A novel upstream regulator of WRKY53 transcription during leaf senescence in Arabidopsis thaliana

Arabidopsis WRKY proteins comprise a family of zinc finger-type transcription factors involved in the regulation of gene expression during pathogen defence, wounding, trichome development and senescence. To better understand the regulatory role of the senescence-related WRKY53 factor, we identified upstream regulatory factors using the yeast one-hybrid system. Among others, we identified a DNA-binding protein with a so far unknown function that contains a transcriptional activation domain and a kinase domain with similarities to HPT kinases. In vitro studies revealed that this activation domain protein (AD protein) can phosphorylate itself and that phosphorylation increases its DNA-binding activity to the WRKY53 promoter region. Using the yeast two-hybrid system, an interaction with proteins that were previously shown to bind to the WRKY53 promoter was tested. The AD protein interacted with MEKK1. The interaction with MEKK1 was confirmed in vivo by bimolecular fluorescence complementation (BiFC); however, the AD protein was not phosphorylated by MEKK1 in vitro and vice versa. This indicates that there may be competition between WRKY53 and AD protein for binding of MEKK1 at the WRKY53 promoter. Overexpression and knockout of the respective gene resulted in changes in transcription levels of WRKY53, indicating that AD protein is a positive regulator of WRKY53 expression. Expression of the AD protein gene can be induced by hydrogen peroxide treatment and reduced by jasmonic acid treatment, as previously shown for WRKY53.

Role of Rho-kinase in regulation of insulin action and glucose homeostasis

Accumulating evidence indicates an important role for serine phosphorylation of IRS-1 in the regulation of insulin action. Recent studies suggest that Rho-kinase (ROK) is a mediator of insulin signaling, via interaction with IRS-1. Here we show that insulin stimulation of glucose transport is impaired when ROK is chemically or biologically inhibited in cultured adipocytes and myotubes and in isolated soleus muscle ex vivo. Inactivation of ROK also reduces insulin-stimulated IRS-1 tyrosine phosphorylation and PI3K activity. Moreover, inhibition of ROK activity in mice causes insulin resistance by reducing insulin-stimulated glucose uptake in skeletal muscle in vivo. Mass spectrometry analysis identifies IRS-1 Ser632/635 as substrates of ROK in vitro, and mutation of these sites inhibits insulin signaling. These results strongly suggest that ROK regulates insulin-stimulated glucose transport in vitro and in vivo. Thus, ROK is an important regulator of insulin signaling and glucose metabolism.

Cell wall integrity signaling in Saccharomyces cerevisiae

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca(2+) signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

Phosphatidylinositol-4-phosphate 5-kinase regulates fission yeast cell integrity through a phospholipase C-mediated protein kinase C-independent pathway

Fission yeast its3-1 mutant is an allele of the essential gene its3+ that encodes a phosphatidylinositol-4-phosphate 5-kinase (PIP5K) that produces phosphatidylinositol 4,5-bisphosphate. We found that the its3-1 mutant is sensitive to micafungin, a (1,3)-beta-D-glucan synthase inhibitor, suggesting a cell wall integrity defect. Consistently, its3-1 mutation caused synthetic lethality with a (1,3)-beta-D-glucan synthase mutant, bgs1-i2, and its3-1 mutant cells showed aberrant localization of green fluorescent protein-Bgs1. Similar aberrant localization of green fluorescent protein-tagged Rgf1, a putative phosphatidylinositol 4,5-bisphosphate-binding guanine nucleotide exchange factor for Rho protein, in its3-1 mutants was observed, suggesting a defective Rgf1/Rho pathway. To unravel the molecular mechanism(s), putative downstream components of PIP5K signaling were analyzed. Unexpectedly, overexpression of phospholipase C (Plc1), but not that of protein kinase C (PKC; Pck1 and Pck2), suppressed the phenotypes of the its3-1 mutant. These findings indicate that PKCs are not involved in the suppression, and further analysis revealed that PKCs are not downstream of Plc1 in fission yeast. Also, the enzymatic activity of Plc1 is essential for the suppression of the phenotypes and for the viability of the its3-1 mutant. These findings suggest that Its3 PIP5K regulates cell integrity through a Plc1-mediated PKC-independent pathway, in addition to the Rho/PKC pathway.

Molecular analysis reveals localization of Saccharomyces cerevisiae protein kinase C to sites of polarized growth and Pkc1p targeting to the nucleus and mitotic spindle

The catalytic activity and intracellular localization of protein kinase C (PKC) are both highly regulated in vivo. This family of kinases contains conserved regulatory motifs, i.e., the C1, C2, and HR1 domains, which target PKC isoforms to specific subcellular compartments and restrict their activity spatially. Saccharomyces cerevisiae contains a single PKC isozyme, Pkc1p, which contains all of the regulatory motifs found in mammalian PKCs. Pkc1p localizes to sites of polarized growth, consistent with its main function in maintaining cell integrity. We dissected the molecular basis of Pkc1p localization by expressing each of its domains individually and in combinations as green fluorescent protein fusions. We find that the Rho1p-binding domains, HR1 and C1, are responsible for targeting Pkc1p to the bud tip and cell periphery, respectively. We demonstrate that Pkc1p activity is required for its normal localization to the bud neck, which also depends on the integrity of the septin ring. In addition, we show for the first time that yeast protein kinase C can accumulate in the nucleus, and we identify a nuclear exit signal as well as nuclear localization signals within the Pkc1p sequence. Thus, we propose that Pkc1p shuttles in and out of the nucleus and consequently has access to nuclear substrates. Surprisingly, we find that deletion of the HR1 domain results in Pkc1p localization to the mitotic spindle and that the C2 domain is responsible for this targeting. This novel nuclear and spindle localization of Pkc1p may provide a molecular explanation for previous observations that suggest a role for Pkc1p in regulating microtubule function.

Models of the cooperative mechanism for Rho effector recognition: implications for RhoA-mediated effector activation

Activated GTPases of the Rho family regulate a spectrum of functionally diverse downstream effectors, initiating a network of signal transduction pathways by interaction and activation of effector proteins. Although effectors are defined as proteins that selectively bind the GTP-bound state of the small GTPases, there have been also several indications for a nucleotide-independent binding mode. By characterizing the molecular mechanism of RhoA interaction with its effectors, we have determined the equilibrium dissociation constants of several Rho-binding domains of three different effector proteins (Rhotekin, ROCKI/ROK beta/p160ROCK, PRK1/PKNalpha where ROK is RhoA-binding kinase) for both RhoA.GDP and RhoA.GTP using fluorescence spectroscopy. In addition, we have identified two novel Rho-interacting domains in ROCKI, which bind RhoA with high affinity but not Cdc42 or Rac1. Our results, together with recent structural data, support the notion of multiple effector-binding sites in RhoA and strongly indicate a cooperative binding mechanism for PRK1 and ROCKI that may be the molecular basis of Rho-mediated effector activation.

Protein kinase PKN1 associates with TRAF2 and is involved in TRAF2-NF-κB signaling pathway

PKN1 is a fatty acid and Rho-activated serine/threonine protein kinase whose catalytic domain is highly homologous to protein kinase C (PKC) family. In yeast two-hybrid screening for PKN1 binding proteins, we identified tumor necrosis factor alpha (TNFalpha) receptor-associated factor 2 (TRAF2). TRAF2 is one of the major mediators of TNF receptor superfamily transducing TNF signal to various functional targets, including activation of NF-kappaB, JNK, and apoptosis. FLAG-tagged PKN1 was co-immunoprecipitated with endogenous TRAF2 from HEK293 cell lysate, and in vitro binding assay using the deletion mutants of TRAF2 showed that PKN1 directly binds to the TRAF domain of TRAF2. PKN1 has the TRAF2-binding consensus sequences PXQX (S/T) at amino acid residues 580-584 (PIQES), and P580AQ582A mutant was not co-immunoprecipitated with TRAF2. Furthermore, the reduced expression of PKN1 by RNA interference (RNAi) down-regulated TRAF2-induced NF-kappaB activation in HEK293T cells. These results suggest that PKN1 is involved in TRAF2-NF-kappaB signaling pathway.

Molecular dissection of the interaction between the small G proteins Rac1 and RhoA and protein kinase C-related kinase 1 (PRK1)

PRK1 is a serine/threonine kinase that belongs to the protein kinase C superfamily. It can be activated either by members of the Rho family of small G proteins, by proteolysis, or by interaction with lipids. Here we investigate the binding of PRK1 to RhoA and Rac1, two members of the Rho family. We demonstrate that PRK1 binds with a similar affinity to RhoA and Rac1. We present the solution structure of the second HR1 domain from the regulatory N-terminal region of PRK1, and we show that it forms an anti-parallel coiled-coil. In addition, we have used NMR to map the binding contacts of the HR1b domain with Rac1. These are compared with the contacts known to form between HR1a and RhoA. We have used mutagenesis to define the residues in Rac that are important for binding to HR1b. Surprisingly, as well as residues adjacent to Switch I, in Switch II, and in helix alpha5, it appears that the C-terminal stretch of basic amino acids in Rac is required for a high affinity interaction with HR1b.

Centrosomal proteins CG-NAP and kendrin provide microtubule nucleation sites by anchoring gamma-tubulin ring complex

Microtubule assembly is initiated by the gamma-tubulin ring complex (gamma-TuRC). In yeast, the microtubule is nucleated from gamma-TuRC anchored to the amino-terminus of the spindle pole body component Spc110p, which interacts with calmodulin (Cmd1p) at the carboxy-terminus. However, mammalian protein that anchors gamma-TuRC remains to be elucidated. A giant coiled-coil protein, CG-NAP (centrosome and Golgi localized PKN-associated protein), was localized to the centrosome via the carboxyl-terminal region. This region was found to interact with calmodulin by yeast two-hybrid screening, and it shares high homology with the carboxyl-terminal region of another centrosomal coiled-coil protein, kendrin. The amino-terminal region of either CG-NAP or kendrin indirectly associated with gamma-tubulin through binding with gamma-tubulin complex protein 2 (GCP2) and/or GCP3. Furthermore, endogenous CG-NAP and kendrin were coimmunoprecipitated with each other and with endogenous GCP2 and gamma-tubulin, suggesting that CG-NAP and kendrin form complexes and interact with gamma-TuRC in vivo. These proteins were localized to the center of microtubule asters nucleated from isolated centrosomes. Pretreatment of the centrosomes by antibody to CG-NAP or kendrin moderately inhibited the microtubule nucleation; moreover, the combination of these antibodies resulted in stronger inhibition. These results imply that CG-NAP and kendrin provide sites for microtubule nucleation in the mammalian centrosome by anchoring gamma-TuRC.

AKAP mediated signal transduction

Compartmentalization of cyclic AMP-dependent protein kinase (PKA) is achieved through association with A-kinase anchoring proteins (AKAPs). AKAPs are a group of structurally diverse proteins with the common function of binding to the regulatory subunit of PKA and confining the holoenzyme to discrete locations within the cell. This mode of regulation ensures that PKA is exposed to isolated cAMP gradients, which allows for efficient catalytic activation and accurate substrate selection. Several AKAPs coordinate multiple members of signaling cascades, effectively assembling upstream activators and downstream effectors within the same macromolecular complex. Consequently, AKAPs may serve as points of integration for numerous signaling pathways. This review details the most recent advances in our understanding of the various biological functions dependent upon AKAP-anchored signaling complexes.

PKN regulates phospholipase D1 through direct interaction

The association of phospholipase (PLD)-1 with protein kinase C-related protein kinases, PKNalpha and PKNbeta, was analyzed. PLD1 interacted with PKNalpha and PKNbeta in COS-7 cells transiently transfected with PLD1 and PKNalpha or PKNbeta expression constructs. The interactions between endogenous PLD1 and PKNalpha or PKNbeta were confirmed by co-immunoprecipitation from mammalian cells. In vitro binding studies using the deletion mutants of PLD1 indicated that PKNalpha directly bound to residues 228-598 of PLD1 and that PKNbeta interacted with residues 1-228 and 228-598 of PLD1. PKNalpha stimulated the activity of PLD1 in the presence of phosphatidylinositol 4,5-bisphosphate in vitro, whereas PKNbeta had a modest effect on the stimulation of PLD1 activity. The stimulation of PLD1 activity by PKNalpha was slightly enhanced by the addition of arachidonic acid. These results suggest that the PKN family functions as a novel intracellular player of PLD1 signaling pathway.

Phosphorylation of tau is regulated by PKN

For the phosphorylation state of microtubule-associated protein, tau plays a pivotal role in regulating microtubule networks in neurons. Tau promotes the assembly and stabilization of microtubules. The potential for tau to bind to microtubules is down-regulated after local phosphorylation. When we investigated the effects of PKN activation on tau phosphorylation, we found that PKN triggers disruption of the microtubule array both in vitro and in vivo and predominantly phosphorylates tau in microtubule binding domains (MBDs). PKN has a catalytic domain highly homologous to protein kinase C (PKC), a kinase that phosphorylates Ser-313 (= Ser-324, the number used in this study) in MBDs. Thus, we identified the phosphorylation sites of PKN and PKC subtypes (PKC-alpha, -betaI, -betaII, -gamma, -delta, -epsilon, -zeta, and -lambda) in MBDs. PKN phosphorylates Ser-258, Ser-320, and Ser-352, although all PKC subtypes phosphorylate Ser-258, Ser-293, Ser-324, and Ser-352. There is a PKN-specific phosphorylation site, Ser-320, in MBDs. HIA3, a novel phosphorylation-dependent antibody recognizing phosphorylated tau at Ser-320, showed immunoreactivity in Chinese hamster ovary cells expressing tau and the active form of PKN, but not in Chinese hamster ovary cells expressing tau and the inactive form of PKN. The immunoreactivity for phosphorylated tau at Ser-320 increased in the presence of a phosphatase inhibitor, FK506 treatment, which means that calcineurin (protein phosphatase 2B) may be involved in dephosphorylating tau at Ser-320 site. We also noted that PKN reduces the phosphorylation recognized by the phosphorylation-dependent antibodies AT8, AT180, and AT270 in vivo. Thus PKN serves as a regulator of microtubules by specific phosphorylation of tau, which leads to disruption of tubulin assembly.

PKN delays mitotic timing by inhibition of Cdc25C: Possible involvement of PKN in the regulation of cell division

The role of PKN, a fatty acid- and Rho small GTPase-activated protein kinase, in cell-cycle regulation was analyzed. Microinjection of the active form of PKN into a Xenopus embryo caused cleavage arrest, whereas normal cell division proceeded in the control embryo microinjected with buffer or the inactive form of PKN. Exogenous addition of the active form of PKN delayed mitotic timing in Xenopus egg cycling extracts judging by morphology of sperm nuclei and Cdc2/cyclin B histone H1 kinase activity. The kinase-negative form of PKN did not affect the timing, suggesting that delayed mitotic timing depends on the kinase activity of PKN. The dephosphorylation of Tyr-15 of Cdc2 was also delayed in correlation with Cdc2/cyclin B histone H1 kinase activation in extracts containing active PKN. The Cdc25C activity for the dephosphorylation of Tyr-15 in Cdc2 was suppressed by pretreatment with the active form of PKN. Furthermore, PKN efficiently phosphorylated Cdc25C in vitro, indicating that PKN directly inhibits Cdc25C activity by phosphorylation. These results suggest that PKN plays a significant role in the control of mitotic timing by inhibition of Cdc25C.

PKN delays mitotic timing by inhibition of Cdc25C: possible involvement of PKN in the regulation of cell division

The role of PKN, a fatty acid- and Rho small GTPase-activated protein kinase, in cell-cycle regulation was analyzed. Microinjection of the active form of PKN into a Xenopus embryo caused cleavage arrest, whereas normal cell division proceeded in the control embryo microinjected with buffer or the inactive form of PKN. Exogenous addition of the active form of PKN delayed mitotic timing in Xenopus egg cycling extracts judging by morphology of sperm nuclei and Cdc2/cyclin B histone H1 kinase activity. The kinase-negative form of PKN did not affect the timing, suggesting that delayed mitotic timing depends on the kinase activity of PKN. The dephosphorylation of Tyr-15 of Cdc2 was also delayed in correlation with Cdc2/cyclin B histone H1 kinase activation in extracts containing active PKN. The Cdc25C activity for the dephosphorylation of Tyr-15 in Cdc2 was suppressed by pretreatment with the active form of PKN. Furthermore, PKN efficiently phosphorylated Cdc25C in vitro, indicating that PKN directly inhibits Cdc25C activity by phosphorylation. These results suggest that PKN plays a significant role in the control of mitotic timing by inhibition of Cdc25C.

PKN binds and phosphorylates human papillomavirus E6 oncoprotein

The high risk human papillomaviruses (HPVs) are associated with carcinomas of cervix and other genital tumors. Previous studies have identified two viral oncoproteins E6 and E7, which are expressed in the majority of HPV-associated carcinomas. The ability of high risk HPV E6 protein to immortalize human mammary epithelial cells has provided a single gene model to study the mechanisms of E6-induced oncogenic transformation. In recent years, it has become clear that in addition to E6-induced degradation of p53 tumor suppressor protein, other targets of E6 are required for mammary epithelial cells immortalization. Using the yeast two-hybrid system, we have identified a novel interaction of HPV16 E6 with protein kinase PKN, a fatty acid- and Rho small G protein-activated serine/threonine kinase with a catalytic domain highly homologous to protein kinase C. We demonstrate direct binding of high risk HPV E6 proteins to PKN in wheat-germ lysate in vitro and in 293T cells in vivo. Importantly, E6 proteins of high risk HPVs but not low risk HPVs were able to bind PKN. Furthermore, all the immortalization-competent and many immortalization-non-competent E6 mutants bind PKN. These data suggest that binding to PKN may be required but not sufficient for immortalizing normal mammary epithelial cells. Finally, we show that PKN phosphorylates E6, demonstrating for the first time that HPV E6 is a phosphoprotein. Our finding suggests a novel link between HPV E6 mediated oncogenesis and regulation of a well known phosphorylation cascade.

Rho GTPase control of protein kinase C-related protein kinase activation by 3-phosphoinositide-dependent protein kinase

The protein kinase C-related protein kinases (PRKs) have been shown to be under the control of the Rho GTPases and influenced by autophosphorylation. In analyzing the relationship between these inputs, it is shown that activation in vitro and in vivo involves the activation loop phosphorylation of PRK1/2 by 3-phosphoinositide-dependent protein kinase-1 (PDK1). Rho overexpression in cultured cells is shown to increase the activation loop phosphorylation of endogenous PRKs and is demonstrated to influence this process by controlling the ability of PRKs to bind to PDK1. The interaction of PRK1/2 with PDK1 is shown to be dependent upon Rho. Direct demonstration of ternary (Rho.PRK.PDK1) complex formation in situ is provided by the observation that PDK1 is recruited to RhoB-containing endosomes only if PRK is coexpressed. Furthermore, this in vivo complex is maintained after phosphoinositide 3-kinase inhibition. The control of PRKs by PDK1 thus evidences a novel strategy of substrate-directed control involving GTPases.

Interaction of PKN with a neuron-specific basic helix-loop-helix transcription factor, NDRF/NeuroD2

By the yeast two-hybrid screening of a human brain cDNA library with the amino-terminal regulatory region of PKN as a bait, a clone encoding a neuron-specific basic Helix-Loop-Helix (bHLH) transcription factor, NDRF/NeuroD2 was isolated. NDRF/NeuroD2 was co-precipitated with PKN from the lysate of COS-7 cells transfected with both expression constructs for NDRF/NeuroD2 and PKN. In vitro binding studies using the deletion mutants of NDRF/NeuroD2 synthesized in a rabbit reticulocyte lysate indicated that the internal region containing the bHLH domain of NDRF/NeuroD2 was necessary and sufficient for the interaction with PKN. In addition, recombinant NDRF/NeuroD2 purified from Escherichia coli could bind PKN, suggesting the direct interaction between NDRF/NeuroD2 and PKN. Transient transfection assays using P19 cells revealed that expression of NDRF/NeuroD2 increased the transactivation of the rat insulin promoter element 3 (RIPE3) enhancer up to approximately 12-fold and that co-expression of catalytically active form of PKN, but not kinase-deficient derivative, resulted in a further threefold increase of NDRF/NeuroD2-mediated transcription. These findings suggest that PKN may contribute to transcriptional responses through the post-translational modification of the NDRF/NeuroD2-dependent transcriptional machinery.

The structural basis of Rho effector recognition revealed by the crystal structure of human RhoA complexed with the effector domain of PKN/PRK1

The small G protein Rho has emerged as a key regulator of cellular events involving cytoskeletal reorganization. Here we report the 2.2 A crystal structure of RhoA bound to an effector domain of protein kinase PKN/PRK1. The structure reveals the antiparallel coiled-coil finger (ACC finger) fold of the effector domain that binds to the Rho specificity-determining regions containing switch I, beta strands B2 and B3, and the C-terminal alpha helix A5, predominantly by specific hydrogen bonds. The ACC finger fold is distinct from those for other small G proteins and provides evidence for the diverse ways of effector recognition. Sequence analysis based on the structure suggests that the ACC finger fold is widespread in Rho effector proteins.

Identification and characterization of PKNbeta, a novel isoform of protein kinase PKN: expression and arachidonic acid dependency are different from those of PKNalpha

The cDNA clone encoding a novel isoform of protein kinase PKN, termed PKNbeta, was isolated from a HeLa cDNA library. PKNbeta had high sequence homology with PKNalpha, originally isolated PKN, especially in the repeats of charged amino acid-rich region with leucine-zipper like sequences (CZ region/HR1), in the carboxyl-terminal catalytic domain, and in approximately 130 amino acid stretch (D region/HR2), located between CZ region/HR1 and the catalytic domain. However, the amino acid sequence of PKNbeta differed from that of PKNalpha in the region immediately amino-terminal to the catalytic domain, which contained two distinct proline-rich sequences consistent with the class II consensus sequence, PXXPXR, for binding to SH3 domain. Distribution of PKNbeta differed from that of PKNalpha in the following two respects: (1) Northern blotting indicated that PKNbeta mRNA could not be detected in human adult tissues, but was expressed abundantly in human cancer cell lines; (2) immunochemical analysis indicated that PKNbeta localized in nucleus and perinuclear Golgi apparatus, and was almost absent in cytoplasmic region in NIH3T3 cells. Recombinant PKNbeta expressed in COS7 cells displayed autophosphorylation and peptide kinase activity, but was found to be significantly less responsive to arachidonic acid than PKNalpha. The identification of this novel isoform underscores the diversity of PKN signaling pathway.

Biochemical and crystallographic characterization of a Rho effector domain of the protein serine/threonine kinase N in a complex with RhoA

The effector domain of human protein serine/threonine kinase N (PKN), an effector protein for the small GTP-binding protein Rho, was expressed and purified for protein characterization and crystallization in a complex form with human RhoA. In solution, RhoA binds to the PKN effector domain with 1:2 stoichiometry in a GTP-dependent manner. The obtained complex crystals diffract to 2.2 A resolution.

Characterization of a novel giant scaffolding protein, CG-NAP, that anchors multiple signaling enzymes to centrosome and the golgi apparatus

A novel 450-kDa coiled-coil protein, CG-NAP (centrosome and Golgi localized PKN-associated protein), was identified as a protein that interacted with the regulatory region of the protein kinase PKN, having a catalytic domain homologous to that of protein kinase C. CG-NAP contains two sets of putative RII (regulatory subunit of protein kinase A)-binding motif. Indeed, CG-NAP tightly bound to RIIalpha in HeLa cells. Furthermore, CG-NAP was coimmunoprecipitated with the catalytic subunit of protein phosphatase 2A (PP2A), when one of the B subunit of PP2A (PR130) was exogenously expressed in COS7 cells. CG-NAP also interacted with the catalytic subunit of protein phosphatase 1 in HeLa cells. Immunofluorescence analysis of HeLa cells revealed that CG-NAP was localized to centrosome throughout the cell cycle, the midbody at telophase, and the Golgi apparatus at interphase, where a certain population of PKN and RIIalpha were found to be accumulated. These data indicate that CG-NAP serves as a novel scaffolding protein that assembles several protein kinases and phosphatases on centrosome and the Golgi apparatus, where physiological events, such as cell cycle progression and intracellular membrane traffic, may be regulated by phosphorylation state of specific protein substrates.

Interaction of the small G protein RhoA with the C terminus of human phospholipase D1

Mammalian phosphatidylcholine-specific phospholipase D1 (PLD1) is a signal transduction-activated enzyme thought to function in multiple cell biological settings including the regulation of membrane vesicular trafficking. PLD1 is activated by the small G proteins, ADP-ribosylation factor (ARF) and RhoA, and by protein kinase C-alpha (PKC-alpha). This stimulation has been proposed to involve direct interaction and to take place at a distinct site in PLD1 for each activator. In the present study, we employed the yeast two-hybrid system to attempt to identify these sites. Successful interaction of ARF and PKC-alpha with PLD1 was not achieved, but a C-terminal fragment of human PLD1 (denoted "D4") interacted with the active mutant of RhoA, RhoAVal-14. Deletion of the CAAX box from RhoAVal-14 decreased the strength of the interaction, suggesting that lipid modification of RhoA is important for efficient binding to PLD1. The specificity of the interaction was validated by showing that the PLD1 D4 fragment interacts with glutathione S-transferase-RhoA in vitro in a GTP-dependent manner and that it associates with RhoAVal-14 in COS-7 cells, whereas the N-terminal two-thirds of PLD1 does not. Finally, we show that recombinant D4 peptide inhibits RhoA-stimulated PLD1 activation but not ARF- or PKC-alpha-stimulated PLD1 activation. These results conclusively demonstrate that the C-terminal region of PLD1 contains the RhoA-binding site and suggest that the ARF and PKC interactions occur elsewhere in the protein.

Loop 6 of RhoA confers specificity for effector binding, stress fiber formation, and cellular transformation

Rho family GTPases regulate multiple cellular processes, including cytoskeletal organization, gene expression, and transformation. These effects are achieved through the interaction of GTP-bound proteins with various downstream targets. A series of RhoA/Rac1 and Rho/Ras chimeras was generated to map the domain(s) of RhoA involved in its association with two classes of effector kinase, represented by PRK2 and ROCK-I. Although the switch 1 domain was required for effector binding, the N terminus of Rho (residues 1-75) was interchangeable with that of Rac. This suggested that the region of Rho that confers effector binding specificity lay further C-terminal. Subsequent studies indicated that the "insert domain"(residues 123-137), a region unique to Rho family GTPases, is not the specificity determinant. However, a determinant for effector binding was identified between Rho residues 75-92. Rac to Rho point mutations (V85D or A88D) within loop 6 of Rac promoted its association with PRK2 and ROCK, whereas the reciprocal Rho(D87V/D90A) double mutant significantly reduced effector binding capacity. In vivo studies showed that microinjection of Rac(Q6IL/V85D/A88D) but not Rac(Q6IL) induced stress fiber formation in LLC-PK epithelial cells, suggesting that loop 6 residues conferred the ability of Rac to activate ROCK. On the other hand, the reciprocal Rho (Q6IL/D87V/D90A) mutant was defective in its ability to transform NIH 3T3 cells. These data suggest that although Rho effectors can utilize a Rho or Rac switch 1 domain to sense the GTP-bound state of Rho, unique residues within loop 6 are essential for determining both effector binding specificity and cellular function.

A protein kinase, PKN, accumulates in Alzheimer neurofibrillary tangles and associated endoplasmic reticulum-derived vesicles and phosphorylates tau protein

A possible role for a protein kinase, PKN, a fatty acid-activated serine/threonine kinase with a catalytic domain homologous to the protein kinase C family and a direct target for Rho, was investigated in the pathology of Alzheimer's disease (AD) using a sensitive immunocytochemistry on postmortem human brain tissues and a kinase assay for human tau protein. The present study provides evidences by light, electron, and confocal laser microscopy that in control human brains, PKN is enriched in neurons, where the kinase is concentrated in a subset of endoplasmic reticulum (ER) and ER-derived vesicles localized to the apical compartment of juxtanuclear cytoplasm, as well as late endosomes, multivesicular bodies, Golgi bodies, secretary vesicles, and nuclei. In AD-affected neurons, PKN was redistributed to the cortical cytoplasm and neurites and was closely associated with neurofibrillary tangles (NFTs) and their major constituent, abnormally modified tau. PKN was also found in degenerative neurites within senile plaques. In addition, we report that human tau protein is directly phosphorylated by PKN both in vitro and in vivo. Thus, our results suggest a specific role for PKN in NFT formation and neurodegeneration in AD damaged neurons.

Proteolytic activation of PKN by caspase-3 or related protease during apoptosis

PKN, a fatty acid- and Rho-activated serine/threonine kinase having a catalytic domain highly homologous to protein kinase C (PKC), was cleaved at specific sites in apoptotic Jurkat and U937 cells on Fas ligation and treatment with staurosporin or etoposide, respectively. The cleavage of PKN occurred with a time course similar to that of PKCdelta, a known caspase substrate. This proteolysis was inhibited by a caspase inhibitor, acetyl-Asp-Glu-Val-Asp-aldehyde. The cleavage fragments were generated in vitro from PKN by treatment with recombinant caspase-3. Site-directed mutagenesis of specific aspartate residues prevented the appearance of these fragments. These results indicate that PKN is cleaved by caspase-3 or related protease during apoptosis. The major proteolysis took place between the amino-terminal regulatory domain and the carboxyl-terminal catalytic domain, and it generated a constitutively active kinase fragment. The cleavage of PKN may contribute to signal transduction, eventually leading to apoptosis.

PKN interacts with a paraneoplastic cerebellar degeneration-associated antigen, which is a potential transcription factor

PKN is a fatty acid-activated serine/threonine protein kinase, having a catalytic domain homologous to protein kinase C family. PKN has been recently reported to interact with a small GTP-binding protein Rho and cytoskeletal proteins such as neurofilament and alpha-actinin. To identify the new components of the PKN-signaling pathway, the yeast two-hybrid system was employed. Using the amino-terminal regulatory domain of PKN as a bait, cDNA encoding a neural antigen PCD17, which is recognized by characteristic antibodies of patients with paraneoplastic cerebellar degeneration, was isolated from a human brain cDNA library. The interaction between PKN and PCD17 was also determined by the in vitro binding analysis. PCD17 was coimmunoprecipitated with PKN from the lysate of COS7 cells transfected with both expression constructs for PKN and the amino-terminal region of PCD17. PCD17 was phosphorylated by PKN, and the extent of this phosphorylation was enhanced by addition of 40 microM arachidonic acid. The amino-terminal region of PCD17 could form a homodimer in vitro, and PCD17 fused to the Gal4 DNA binding domain showed the transcriptional transactivation of the chloramphenicol acetyltransferase reporter gene linked to 5 Gal4 binding sites and minimal promoter in rat C6 glioma cells. These results suggest the participation of PCD17 in gene expression and lead to a clue for elucidating the PKN signaling pathway from the cytosol to the nucleus.

The extended protein kinase C superfamily

Members of the mammalian protein kinase C (PKC) superfamily play key regulatory roles in a multitude of cellular processes, ranging from control of fundamental cell autonomous activities, such as proliferation, to more organismal functions, such as memory. However, understanding of mammalian PKC signalling systems is complicated by the large number of family members. Significant progress has been made through studies based on comparative analysis, which have defined a number of regulatory elements in PKCs which confer specific location and activation signals to each isotype. Further studies on simple organisms have shown that PKC signalling paradigms are conserved through evolution from yeast to humans, underscoring the importance of this family in cellular signalling and giving novel insights into PKC function in complex mammalian systems.

Evidence for distinct mechanisms of transition state stabilization of GTPases by fluoride

GTPase-activating proteins (GAPs) function by stabilizing the GTPase transition state. This has been most clearly demonstrated by the formation of a high-affinity complex between various GAPs and GDP-bound GTPases in the presence of aluminum tetrafluoride, which can mimic the gamma-phosphate of GTP. Herein, we report that p190 RhoGAP forms a high-affinity complex with Rho GTPases in the presence of fluoride ions, suggesting that p190 also functions to stabilize the GTPase transition state. However, this Rho-p190 complex does not require aluminum ions or even guanine nucleotide, indicating a distinct role for fluoride that is not consistent with the gamma-phosphate-mimicking hypothesis. These results indicate that it is necessary to reconsider the assumed role of fluoride in stabilizing a variety of other GTPase-GAP interactions where the requirement for aluminum or guanine nucleotide has not yet been addressed.

PRK1 is targeted to endosomes by the small GTPase, RhoB

RhoB has been shown to be an endosomal GTPase both by immunocytochemistry and electron microscopy, however, its role in endocytosis is unknown. Elucidation of the cellular roles of other members of this superfamily of signaling proteins has come with the identification of their downstream partners. We show here that the recently isolated serine/threonine kinase PRK1 is targeted to the endosomal compartment by RhoB. This is established both through immunofluorescence and cell fractionation. PRK1 is shown to interact with activated RhoB in cells and is localized to endosomes through its Rho-binding HR1 domain. Translocation of PRK1 to the endosomal compartment by RhoB is accompanied by a shift in the electrophoretic mobility of the kinase indicative of an accompanying activation.

Phosphorylation of protein kinase C-related kinase PRK2 during meiotic maturation of starfish oocytes

The resumption of meiosis in the developing starfish oocyte is the result of intracellular signaling events initiated by 1-methyladenine stimulation. One of the earliest detectable kinase activities during meiotic maturation of starfish oocytes is a protein kinase C or PKC-like activity. In this study, several isoforms of protein kinase C were cloned from the oocyte; however, the most abundant PKC-like maternal transcript corresponds to protein kinase C-related kinase 2 (PRK2). PRK2 is expressed in the immature oocyte and at least until germinal vesicle breakdown. Subcellular localization of PRK2 revealed a cytoplasmic distribution in the immature oocyte, which, during meiotic maturation, remained in the cytoplasm but also localized to the disintegrating germinal vesicle. Significantly, PRK2 is phosphorylated in vivo in response to 1-methyladenine which precedes MPF activation, making PRK2 a candidate regulator of early signaling events of meiotic maturation.

Molecular cloning and characterization of a novel mitochondrial phosphoprotein, MIPP65, from rat liver

A novel 65-kDa protein (designated MIPP65), which was phosphorylated by PKN in vitro in a manner highly dependent on arachidonic acid, was partially purified from the heat-stable proteins extracted from a 30,000g precipitate of rat liver. The cDNA clones were obtained by polymerase chain reaction using oligonucleotides based on partial amino acid sequences. The complete amino acid sequence deduced from the cDNAs contained two homologous regions with the mitochondrial NADH-ubiquinone oxidoreductase 9-kDa subunit precursor at the amino- and carboxyl-termini, whereas the central region was not related to any known proteins and contained a serine cluster. Northern blotting and immunoblotting analyses indicated that MIPP65 was expressed ubiquitously in rat tissues. Immunofluorescence analysis of the endogenous MIPP65 using polyclonal antiserum against MIPP65 showed a predominantly mitochondrial localization in C6 glioma cells. The recombinant MIPP65 expressed in COS7 cells showed a similar pattern of localization to that in C6 glioma cells. On the other hand, deletion of the amino-terminal region of MIPP65 abrogated such localization, indicating that the amino-terminal region contained a mitochondrial-targeting signal. From [32P]orthophosphate-labeled C6 glioma cells, the endogenous MIPP65 could be immunoprecipitated as a phosphoprotein with antiserum against MIPP65. These results suggest that MIPP65 is a novel mitochondrial phosphoprotein that is a candidate substrate for PKN.

Domain-specific phosphorylation of vimentin and glial fibrillary acidic protein by PKN

PKN is a serine/threonine protein kinase with a catalytic domain homologous to the protein kinase C family and unique N-terminal leucine zipper-like sequences. Using analyses with the yeast two-hybrid system and in vitro binding assay, we found that the regulatory domain of PKN interacted with vimentin. We then examined whether PKN would phosphorylate vimentin in vitro. Vimentin proved to be an excellent substrate for PKN, and the phosphorylation of vimentin by PKN occurred in the head domain with the result of a nearly complete inhibition of its filament formation in vitro. Similar results were also obtained with another type III intermediate filament protein, glial fibrillary acidic protein (GFAP). These results raise the possibility that PKN may regulate filament structures of vimentin and GFAP by domain-specific phosphorylation.