Smad8 mediates the signaling of the receptor serine kinase

Smad proteins are critical intracellular mediators of signaling by growth and differentiation factors of the transforming growth factor β superfamily. We have isolated a member of the Smad family, Smad8, from a rat brain cDNA library and biochemically and functionally characterized its ability to transduce signals from serine kinase receptors. In Xenopus embryo, Smad8 is able to transcriptionally activate a subset of mesoderm target genes similar to those induced by the receptor serine kinase, activin receptor-like kinase (ALK)-2. Smad8 can be specifically phosphorylated by a constitutively active ALK-2 but not the related receptor serine kinase, ALK-4. In response to signaling from ALK-2, Smad8 associates with a common regulatory molecule, Smad4, and this association leads to a synergistic effect on gene transcription. Furthermore, Smad8 is able to rescue the expression of mesoderm genes blocked by truncated ALK-2 in the embryo. These results indicate that Smad8 can function as a downstream signaling mediator of ALK-2.

Altered phosphorylation and intracellular distribution of a (CUG)n triplet repeat RNA-binding protein in patients with myotonic dystrophy and in myotonin protein kinase knockout mice

Myotonic dystrophy (DM) is associated with expansion of CTG repeats in the 3′-untranslated region of the myotonin protein kinase (DMPK) gene. The molecular mechanism whereby expansion of the (CUG)n repeats in the 3′-untranslated region of DMPK gene induces DM is unknown. We previously isolated a protein with specific binding to CUG repeat sequences (CUG-BP/hNab50) that possibly plays a role in mRNA processing and/or transport. Here we present evidence that the phosphorylation status and intracellular distribution of the RNA CUG-binding protein, identical to hNab50 protein (CUG-BP/hNab50), are altered in homozygous DM patient and that CUG-BP/hNab50 is a substrate for DMPK both in vivo and in vitro. Data from two biological systems with reduced levels of DMPK, homozygous DM patient and DMPK knockout mice, show that DMPK regulates both phosphorylation and intracellular localization of the CUG-BP/hNab50 protein. Decreased levels of DMPK observed in DM patients and DMPK knockout mice led to the elevation of the hypophosphorylated form of CUG-BP/hNab50. Nuclear concentration of the hypophosphorylated CUG-BP/hNab50 isoform is increased in DMPK knockout mice and in homozygous DM patient. DMPK also interacts with and phosphorylates CUG-BP/hNab50 protein in vitro. DMPK-mediated phosphorylation of CUG-BP/hNab50 results in dramatic reduction of the CUG-BP2, hypophosphorylated isoform, accumulation of which was observed in the nuclei of DMPK knockout mice. These data suggest a feedback mechanism whereby decreased levels of DMPK could alter phosphorylation status of CUG-BP/hNab50, thus facilitating nuclear localization of CUG-BP/hNab50. Our results suggest that DM pathophysiology could be, in part, a result of sequestration of CUG-BP/hNab50 and, in part, of lowered DMPK levels, which, in turn, affect processing and transport of specific subclass of mRNAs.

Structural basis for efficient phosphorylation of 3′-azidothymidine monophosphate by Escherichia coli thymidylate kinase

The crystal structures of Escherichia coli thymidylate kinase (TmpK) in complex with P1-(5′-adenosyl)-P5-(5′-thymidyl)pentaphosphate and P1-(5′-adenosyl)P5-[5′-(3′-azido-3′-deoxythymidine)] pentaphosphate have been solved to 2.0-Å and 2.2-Å resolution, respectively. The overall structure of the bacterial TmpK is very similar to that of yeast TmpK. In contrast to the human and yeast TmpKs, which phosphorylate 3′-azido-3′-deoxythymidine 5′-monophosphate (AZT-MP) at a 200-fold reduced turnover number (kcat) in comparison to the physiological substrate dTMP, reduction of kcat is only 2-fold for the bacterial enzyme. The different kinetic properties toward AZT-MP between the eukaryotic TmpKs and E. coli TmpK can be rationalized by the different ways in which these enzymes stabilize the presumed transition state and the different manner in which a carboxylic acid side chain in the P loop interacts with the deoxyribose of the monophosphate. Yeast TmpK interacts with the 3′-hydroxyl of dTMP through Asp-14 of the P loop in a bidentate manner: binding of AZT-MP results in a shift of the P loop to accommodate the larger substituent. In E. coli TmpK, the corresponding residue is Glu-12, and it interacts in a side-on fashion with the 3′-hydroxyl of dTMP. This different mode of interaction between the P loop carboxylic acid with the 3′ substituent of the monophosphate deoxyribose allows the accommodation of an azido group in the case of the E. coli enzyme without significant P loop movement. In addition, although the yeast enzyme uses Arg-15 (a glycine in E. coli) to stabilize the transition state, E. coli seems to use Arg-153 from a region termed Lid instead. Thus, the binding of AZT-MP to the yeast TmpK results in the shift of a catalytic residue, which is not the case for the bacterial kinase.

β2-Adrenergic receptor regulation by GIT1, a G protein-coupled receptor kinase-associated ADP ribosylation factor GTPase-activating protein

G protein-coupled receptor activation leads to the membrane recruitment and activation of G protein-coupled receptor kinases, which phosphorylate receptors and lead to their inactivation. We have identified a novel G protein-coupled receptor kinase-interacting protein, GIT1, that is a GTPase-activating protein (GAP) for the ADP ribosylation factor (ARF) family of small GTP-binding proteins. Overexpression of GIT1 leads to reduced β2-adrenergic receptor signaling and increased receptor phosphorylation, which result from reduced receptor internalization and resensitization. These cellular effects of GIT1 require its intact ARF GAP activity and do not reflect regulation of GRK kinase activity. These results suggest an essential role for ARF proteins in regulating β2-adrenergic receptor endocytosis. Moreover, they provide a mechanism for integration of receptor activation and endocytosis through regulation of ARF protein activation by GRK-mediated recruitment of the GIT1 ARF GAP to the plasma membrane.

An essential role of phosphatidylinositol 3-kinase in myogenic differentiation

The oncogene p3k, coding for a constitutively active form of phosphatidylinositol 3-kinase (PI 3-kinase; EC 2.7.1.137), strongly enhances myogenic differentiation in cultures of chicken-embryo myoblasts. It increases the size of the myotubes and induces elevated levels of the muscle-specific proteins MyoD, myosin heavy chain, creatine kinase, and desmin. Inhibition of PI 3-kinase activity with LY294002 or with dominant-negative mutants of PI 3-kinase interferes with myogenic differentiation and with the induction of muscle-specific genes. PI 3-kinase is therefore an upstream mediator for the expression of the muscle-specific genes and is both necessary and rate-limiting for the process of myogenesis.

Tumor necrosis factor-α induces adhesion molecule expression through the sphingosine kinase pathway

The signaling pathways that couple tumor necrosis factor-α (TNFα) receptors to functional, especially inflammatory, responses have remained elusive. We report here that TNFα induces endothelial cell activation, as measured by the expression of adhesion protein E-selectin and vascular adhesion molecule-1, through the sphingosine kinase (SKase) signaling pathway. Treatment of human umbilical vein endothelial cells with TNFα resulted in a rapid SKase activation and sphingosine 1-phosphate (S1P) generation. S1P, but not ceramide or sphingosine, was a potent dose-dependent stimulator of adhesion protein expression. S1P was able to mimic the effect of TNFα on endothelial cells leading to extracellular signal-regulated kinases and NF-κB activation, whereas ceramide or sphingosine was not. Furthermore, N,N-dimethylsphingosine, an inhibitor of SKase, profoundly inhibited TNFα-induced extracellular signal-regulated kinases and NF-κB activation and adhesion protein expression. Thus we demonstrate that the SKase pathway through the generation of S1P is critically involved in mediating TNFα-induced endothelial cell activation.

Effect of reduced myristoylated alanine-rich C kinase substrate expression on hippocampal mossy fiber development and spatial learning in mutant mice: Transgenic rescue and interactions with gene background

The myristoylated alanine-rich C kinase substrate (MARCKS) is a prominent protein kinase C (PKC) substrate in brain that is expressed highly in hippocampal granule cells and their axons, the mossy fibers. Here, we examined hippocampal infrapyramidal mossy fiber (IP-MF) limb length and spatial learning in heterozygous Macs mutant mice that exhibit an ≈50% reduction in MARCKS expression relative to wild-type controls. On a 129B6(N3) background, the Macs mutation produced IP-MF hyperplasia, a significant increase in hippocampal PKCɛ expression, and proficient spatial learning relative to wild-type controls. However, wild-type 129B6(N3) mice exhibited phenotypic characteristics resembling inbred 129Sv mice, including IP-MF hypoplasia relative to inbred C57BL/6J mice and impaired spatial-reversal learning, suggesting a significant contribution of 129Sv background genes to wild-type and possibly mutant phenotypes. Indeed, when these mice were backcrossed with inbred C57BL/6J mice for nine generations to reduce 129Sv background genes, the Macs mutation did not effect IP-MF length or hippocampal PKCɛ expression and impaired spatial learning relative to wild-type controls, which now showed proficient spatial learning. Moreover, in a different strain (B6SJL(N1), the Macs mutation also produced a significant impairment in spatial learning that was reversed by transgenic expression of MARCKS. Collectively, these data indicate that the heterozygous Macs mutation modifies the expression of linked 129Sv gene(s), affecting hippocampal mossy fiber development and spatial learning performance, and that MARCKS plays a significant role in spatial learning processes.

Inorganic polyphosphate kinase is required to stimulate protein degradation and for adaptation to amino acid starvation in Escherichia coli

Inorganic polyphosphate (polyP) kinase was studied for its roles in physiological responses to nutritional deprivation in Escherichia coli. A mutant lacking polyP kinase exhibited an extended lag phase of growth, when shifted from a rich to a minimal medium (nutritional downshift). Supplementation of amino acids to the minimal medium abolished the extended growth lag of the mutant. Levels of the stringent response factor, guanosine 5′-diphosphate 3′-diphosphate, increased in response to the nutritional downshift, but, unlike in the wild type, the levels were sustained in the mutant. These results suggested that the mutant was impaired in the induction of amino acid biosynthetic enzymes. The expression of an amino acid biosynthetic gene, hisG, was examined by using a transcriptional lacZ fusion. Although the mutant did not express the fusion in response to the nutritional downshift, Northern blot analysis revealed a significant increase of hisG-lacZ mRNA. Amino acids generated by intracellular protein degradation are very important for the synthesis of enzymes at the onset of starvation. In the wild type, the rate of protein degradation increased in response to the nutritional downshift whereas it did not in the mutant. Supplementation of amino acids at low concentrations to the minimal medium enabled the mutant to express the hisG-lacZ fusion. Thus, the impaired regulation of protein degradation results in the adaptation defect, suggesting that polyP kinase is required to stimulate protein degradation.

The polo-box-dependent induction of ectopic septal structures by a mammalian polo kinase, Plk, in Saccharomyces cerevisiae

Members of the polo subfamily of protein kinases play pivotal roles in cell-cycle control and proliferation. In addition to a high degree of sequence similarity in the kinase domain, polo kinases contain a strikingly conserved motif termed “polo-box” in the noncatalytic C-terminal domain. We have previously shown that the mammalian polo-like kinase Plk is a functional homolog of Saccharomyces cerevisiae Cdc5. Here, we show that, in a polo-box- and kinase activity-dependent manner, ectopic expression of Plk in budding yeast can induce a class of cells with abnormally elongated buds. In addition to localization at spindle poles and cytokinetic neck filaments, Plk induces and localizes to ectopic septin ring structures within the elongated buds. In contrast, mutations in the polo-box abolish both localization to, and induction of, septal structures. Consistent with the polo-box-dependent subcellular localization, the C-terminal domain of Plk, but not its polo-box mutant, is sufficient for subcellular localization. Our data suggest that Plk may contribute a signal to initiate or promote cytokinetic event(s) and that an intact polo-box is required for regulation of these cellular processes.

Mammalian Cdk5 is a functional homologue of the budding yeast Pho85 cyclin-dependent protein kinase

Mammalian Cdk5 is a member of the cyclin-dependent kinase family that is activated by a neuron-specific regulator, p35, to regulate neuronal migration and neurite outgrowth. p35/Cdk5 kinase colocalizes with and regulates the activity of the Pak1 kinase in neuronal growth cones and likely impacts on actin cytoskeletal dynamics through Pak1. Here, we describe a functional homologue of Cdk5 in budding yeast, Pho85. Like Cdk5, Pho85 has been implicated in actin cytoskeleton regulation through phosphorylation of an actin-regulatory protein. Overexpression of CDK5 in yeast cells complemented most phenotypes associated with pho85Δ, including defects in the repression of acid phosphatase expression, sensitivity to salt, and a G1 progression defect. Consistent with the functional complementation, Cdk5 associated with and was activated by the Pho85 cyclins Pho80 and Pcl2 in yeast cells. In a reciprocal series of experiments, we found that Pho85 associated with the Cdk5 activators p35 and p25 to form an active kinase complex in mammalian and insect cells, supporting our hypothesis that Pho85 and Cdk5 are functionally related. Our results suggest the existence of a functionally conserved pathway involving Cdks and actin-regulatory proteins that promotes reorganization of the actin cytoskeleton in response to regulatory signals.

A cytosolic activity distinct from Crm1 mediates nuclear export of protein kinase inhibitor in permeabilized cells

The leucine-rich nuclear export signal (NES) is used by a variety of proteins to facilitate their delivery from the nucleus to the cytoplasm. One of the best-studied examples, protein kinase inhibitor (PKI), binds to the catalytic subunit of protein kinase A in the nucleus and mediates its rapid export to the cytoplasm. We developed a permeabilized cell assay that reconstitutes nuclear export mediated by PKI, and we used it to characterize the cytosolic factors required for this process. The two-step assay involves an import phase and an export phase, and quantitation is achieved by digital fluorescence microscopy. During the import phase, a fluorescent derivative of streptavidin is imported into the nuclei of digitonin-permeabilized HeLa cells. During the export phase, biotinylated PKI diffuses into the nucleus, binds to fluorescent streptavidin, and mediates export of the complex to the cytoplasm. Nuclear export of the PKI complex is cytosol dependent and can be stimulated by addition of the purified NES receptor, Crm1. HeLa cell cytosol treated with N-ethylmaleimide (NEM) or phenyl-Sepharose to inactivate or deplete Crm1, respectively, is still fully active in the PKI export assay. Significantly, the export activity can be depleted from cytosol by preadsorption with a protein conjugate that contains a functional NES. These data indicate that cytosol contains an export activity that is distinct from Crm1 and is likely to correspond to an NES receptor.

Mitogen-activated protein kinase and neural specification in Xenopus

We have investigated the activity and function of mitogen-activated protein kinase (MAPK) during neural specification in Xenopus. Ectodermal MAPK activity increased between late blastula and midgastrula stages. At midgastrula, MAPK activity in both newly induced neural ectoderm and ectoderm overexpressing the anterior neural inducer noggin was 5-fold higher than in uninduced ectoderm. Overexpression of MAPK phosphatase-1 (MKP-1) in ectoderm inhibited MAPK activity and prevented neurectoderm-specific gene expression when the ectoderm was recombined with dorsal mesoderm or treated with fibroblast growth factor (FGF). Neurectoderm-specific gene expression was observed, however, in ectoderm overexpressing both noggin and MKP-1. To evaluate the role of MAPK in posterior regionalization, ectodermal isolates were treated with increasing concentrations of FGF and assayed for MAPK activity and neurectoderm-specific gene expression. Although induction of posterior neural ectoderm by FGF was accompanied by an elevation of MAPK activity, relative MAPK activity associated with posterior neural fate was no higher than that of ectoderm specified to adopt an anterior neural fate. Thus, increasingly posterior neural fates are not correlated with quantitative increases in MAPK activity. Because MAPK has been shown to down-regulate Smad1, MAPK may disrupt bone morphogenetic protein 4 (BMP-4) signaling during neural specification. Our results suggest that MAPK plays an essential role in the establishment of neural fate in vivo.

The Akt kinase: Molecular determinants of oncogenicity

The serine-threonine kinase Akt is a downstream target of phosphoinositide 3-kinase (PI 3-kinase); it is activated by the phosphoinositide 3-phosphate-dependent kinases PDK1 and PDK2. Certain mutated forms of Akt induce oncogenic transformation in chicken embryo fibroblast cultures and hemangiosarcomas in young chickens. This ability to transform cells depends on localization of Akt at the plasma membrane and on the kinase activity of Akt. A transdominant negative form of Akt interferes with oncogenic transformation induced by the p3k oncogene, which codes for an activated form of PI 3-kinase. Akt is therefore an essential mediator of p3k-induced oncogenicity.

Metabotropic glutamate receptor-initiated translocation of protein kinase p90rsk to polyribosomes: A possible factor regulating synaptic protein synthesis

Maintenance of lasting synaptic efficacy changes requires protein synthesis. We report here a mechanism that might influence translation control at the level of the single synapse. Stimulation of metabotropic glutamate receptors in hippocampal slices induces a rapid protein kinase C-dependent translocation of multifunction kinase p90rsk to polyribosomes; concomitantly, there is enhanced phosphorylation of at least six polyribosome binding proteins. Among the polyribosome bound proteins are the p90rsk-activating kinase ERK-2 and a known p90rsk substrate, glycogen synthase kinase 3β, which regulates translation efficiency via eukaryotic initiation factor 2B. Thus metabotropic glutamate receptor stimulation could induce synaptic activity-dependent translation via translocation of p90rsk to ribosomes.

Identification of a new class of protein kinases represented by eukaryotic elongation factor-2 kinase

The several hundred members of the eukaryotic protein kinase superfamily characterized to date share a similar catalytic domain structure, consisting of 12 conserved subdomains. Here we report the existence and wide occurrence in eukaryotes of a protein kinase with a completely different structure. We cloned and sequenced the human, mouse, rat, and Caenorhabditis elegans eukaryotic elongation factor-2 kinase (eEF-2 kinase) and found that with the exception of the ATP-binding site, they do not contain any sequence motifs characteristic of the eukaryotic protein kinase superfamily. Comparison of different eEF-2 kinase sequences reveals a highly conserved region of ≈200 amino acids which was found to be homologous to the catalytic domain of the recently described myosin heavy chain kinase A (MHCK A) from Dictyostelium. This suggests that eEF-2 kinase and MHCK A are members of a new class of protein kinases with a novel catalytic domain structure.