Identification of a rabphilin-3A-interacting protein as GTP cyclohydrolase I in PC12 cells

The regulation of vascular tetrahydrobiopterin bioavailability

6R l-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is an essential cofactor for several enzymes including phenylalanine hydroxylase and the nitric oxide synthases (NOS). Oral supplementation of BH4 has been successfully employed to treat subsets of patients with hyperphenylalaninaemia. More recently, research efforts have focussed on understanding whether BH4 supplementation may also be efficacious in cardiovascular disorders that are underpinned by reduced nitric oxide bioavailability. Whilst numerous preclinical and clinical studies have demonstrated a positive association between enhanced BH4 and vascular function, the efficacy of orally administered BH4 in human cardiovascular disease remains unclear. Furthermore, interventions that limit BH4 bioavailability may provide benefit in diseases where nitric oxide over production contributes to pathology. This review describes the pathways involved in BH4 bio-regulation and discusses other endogenous mechanisms that could be harnessed therapeutically to manipulate vascular BH4 levels.

The protein partners of GTP cyclohydrolase I in rat organs

Objective

GTP cyclohydrolase I (GCH1) is the rate-limiting enzyme for tetrahydrobiopterin biosynthesis and has been shown to be a promising therapeutic target in ischemic heart disease, hypertension, atherosclerosis and diabetes. The endogenous GCH1-interacting partners have not been identified. Here, we determined endogenous GCH1-interacting proteins in rat.

Methods and Results

A pulldown and proteomics approach were used to identify GCH1 interacting proteins in rat liver, brain, heart and kidney. We demonstrated that GCH1 interacts with at least 17 proteins including GTP cyclohydrolase I feedback regulatory protein (GFRP) in rat liver by affinity purification followed by proteomics and validated six protein partners in liver, brain, heart and kidney by immunoblotting. GCH1 interacts with GFRP and very long-chain specific acyl-CoA dehydrogenase in the liver, tubulin beta-2A chain in the liver and brain, DnaJ homolog subfamily A member 1 and fatty aldehyde dehydrogenase in the liver, heart and kidney and eukaryotic translation initiation factor 3 subunit I (EIF3I) in all organs tested. Furthermore, GCH1 associates with mitochondrial proteins and GCH1 itself locates in mitochondria.

Conclusion

GCH1 interacts with proteins in an organ dependant manner and EIF3I might be a general regulator of GCH1. Our finding indicates GCH1 might have broader functions beyond tetrahydrobiopterin biosynthesis.

Proteomic analysis of GTP cyclohydrolase 1 multiprotein complexes in cultured normal adult human astrocytes under both basal and cytokine-activated conditions

GTP cyclohydrolase 1 (GCH1) is the rate-limiting enzyme of a metabolic pathway synthesizing tetrahydrobiopterin (BH(4)), the cofactor dimerizing and activating inducible nitric oxide synthase (NOS-2). GCH1 protein expression and enzyme activity are minimal in cultured, phenotypically stable, untreated normal adult human astrocytes (NAHA), but are strongly induced, together with NOS-2, by a mixture of three proinflammatory cytokines (IL-1beta, TNF-alpha, and IFN-gamma--the CM-trio) released by microglia under brain-damaging conditions. The resulting hyper-production of NO severely harms neurons. In this study, using MALDI-TOF/MS, PMF, Western immunoblotting (WB), and antibody microarrays we identified several proteins coimmunoprecipitating with GCH1. Under basal conditions, GCH1 was associated with various adaptor/regulator molecules involved in G-protein-coupled receptors signalling, protein serine/threonine phosphatase 2Cbeta (PP2Cbeta), and serine-threonine kinases like Ca(2+) calmodulin kinases (CaMKs), casein kinases (CKs), cAMP-dependent kinases (PKAs), and mitogen-activated protein kinases (MAPKs). Exposure to the three cytokines' mixture (CM-trio) significantly changed, within the 48-72 h required for the induction and activation of GCH1, the levels and identities of some of the 0 h-associated proteins: after 72 h CK-IIalpha tended to dissociate from, whereas MAPK12 and JNK3 were strongly associated with fully active GCH1. These findings provide a first enticing glimpse into the intricate mechanisms regulating GCH1 activation by proinflammatory cytokines in NAHA, and may have therapeutic implications.

A yeast 2-hybrid analysis of human GTP cyclohydrolase I protein interactions

The yeast 2-hybrid system was used to identify protein domains involved in the oligomerization of human guanosine 5'-triphosphate (GTP) Cyclohydrolase I (GCH1) and the interaction of GCH1 with its regulatory partner, GCH1 feedback regulatory protein (GFRP). When interpreted within the structural framework derived from crystallography, our results indicate that the GCH1 N-terminal alpha-helices are not the only domains involved in the formation of dimers from monomers and also suggest an important role for the C-terminal alpha-helix in the assembly of dimers to form decamers. Moreover, a previously unknown role of the extended N-terminal alpha-helix in the interaction of GCH1 and GFRP was revealed. To discover novel GCH1 protein binding partners, we used the yeast 2-hybrid system to screen a human brain library with GCH1 N-terminal amino acids 1-96 as prey. This protruding extension of GCH1 contains two canonical Type-I Src homology-3 (SH3) ligand domains located within amino acids 1-42. Our screen yielded seven unique clones that were subsequently shown to require amino acids 1-42 for binding to GCH1. The interaction of one of these clones, Activator of Heat Shock 90 kDa Protein (Aha1), with GCH1 was validated by glutathione-s-transferase (GST) pull-down assay. Although the physiological relevance of the Aha1-GCH1 interaction requires further study, Aha1 may recruit GCH1 into the endothelial nitric oxide synthase/heat shock protein (eNOS/Hsp90) complex to support changes in endothelial nitric oxide production through the local synthesis of BH4.

The C2B domain of rabphilin directly interacts with SNAP-25 and regulates the docking step of dense core vesicle exocytosis in PC12 cells

Rabphilin is a membrane trafficking protein on secretory vesicles that consists of an N-terminal Rab-binding domain and C-terminal tandem C2 domains. The N-terminal part of rabphilin has recently been shown to function as an effector domain for both Rab27A and Rab3A in PC12 cells (Fukuda, M., Kanno, E., and Yamamoto, A. (2004) J. Biol. Chem. 279, 13065-13075), but the function of the C2 domains of rabphilin during secretory vesicle exocytosis is largely unknown. In this study we investigated the interaction between rabphilin and SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors, VAMP-2/synaptobrevin-2, syntaxin IA, and SNAP-25) and SNARE-associated proteins (Munc18-1 and Munc13-1) and found that the C2B domain of rabphilin, but not of other Rab27A-binding proteins with tandem C2 domains (i.e. Slp1-5), directly interacts with a plasma membrane protein, SNAP-25. The interaction between rabphilin and SNAP-25 occurs even in the absence of Ca(2+) (EC(50) = 0.817 microm SNAP-25), but 0.5 mm Ca(2+) increases the affinity for SNAP-25 2-fold (EC(50) = 0.405 microm SNAP-25) without changing the B(max) value (1.06 mol of SNAP-25/mol of rabphilin). Furthermore, vesicle dynamics were imaged by total internal reflection fluorescence microscopy in a single PC12 cell expressing a lumen-targeted pH-insensitive yellow fluorescent protein (Venus), neuropeptide Y-Venus. Expression of the wild-type rabphilin in PC12 cells significantly increased the number of docked vesicles to the plasma membrane without altering the kinetics of individual secretory events, whereas expression of the mutant rabphilin lacking the C2B domain, rabphilin-DeltaC2B, decreased the number of docked vesicle or fusing at the plasma membrane. These findings suggest that rabphilin is involved in the docking step of regulated exocytosis in PC12 cells, possibly through interaction between the C2B domain and SNAP-25.

cGMP inhibits GTP cyclohydrolase I activity and biosynthesis of tetrahydrobiopterin in human umbilical vein endothelial cells

Tetrahydrobiopterin (BH4) acts as an essential cofactor for the enzymatic activity of nitric oxide (NO) synthases. Biosynthesis of the cofactor BH4 starts from GTP and requires 3 enzymatic steps, which include GTP cyclohydrolase I (GCH I) catalysis of the first and rate-limiting step. In this study we examined the effects of cGMP on GCH I activity in human umbilical vein endothelial cells under inflammatory conditions. Exogenous application of the cGMP analogue 8-bromo-cGMP markedly inhibited GCH I activity in the short term, whereas an cAMP analogue had no effect on GCH I activity under the same condition. NO donors, NOR3 and sodium nitroprusside, elevated the intracellular cGMP level and reduced GCH I activity in the short term. This inhibition of GCH I activity was obliterated in the presence of an NO trapper carboxy-PTIO. NO donors had no effect on GCH I mRNA expression in the short term. Moreover, cycloheximide did not alter the inhibition by NO donors of GCH I activity. These findings suggest that stimulation of the cGMP signaling cascade down-regulates GCH I activity through post translational modification of the GCH I enzyme.

Cast: a novel protein of the cytomatrix at the active zone of synapses that forms a ternary complex with RIM1 and munc13-1

The cytomatrix at the active zone (CAZ) has been implicated in defining the site of Ca2+-dependent exocytosis of neurotransmitter. We have identified here a novel CAZ protein of approximately 120 kD from rat brain and named it CAST (CAZ-associated structural protein). CAST had no transmembrane segment, but had four coiled-coil domains and a putative COOH-terminal consensus motif for binding to PDZ domains. CAST was localized at the CAZ of conventional synapses of mouse brain. CAST bound directly RIM1 and indirectly Munc13-1, presumably through RIM1, forming a ternary complex. RIM1 and Munc13-1 are CAZ proteins implicated in Ca2+-dependent exocytosis of neurotansmitters. Bassoon, another CAZ protein, was also associated with this ternary complex. These results suggest that a network of protein-protein interactions among the CAZ proteins exists at the CAZ. At the early stages of synapse formation, CAST was expressed and partly colocalized with bassoon in the axon shaft and the growth cone. The vesicles immunoisolated by antibassoon antibody-coupled beads contained not only bassoon but also CAST and RIM1. These results suggest that these CAZ proteins are at least partly transported on the same vesicles during synapse formation.

Rabconnectin-3, a novel protein that binds both GDP/GTP exchange protein and GTPase-activating protein for Rab3 small G protein family

Rab3A, a member of the Rab3 small G protein family, regulates Ca(2+)-dependent exocytosis of neurotransmitter. The cyclical activation and inactivation of Rab3A are essential for the Rab3A action in exocytosis. GDP-Rab3A is activated to GTP-Rab3A by Rab3 GDP/GTP exchange protein (Rab3 GEP), and GTP-Rab3A is inactivated to GDP-Rab3A by Rab3 GTPase-activating protein (Rab3 GAP). It remains unknown how or in which step of the multiple exocytosis steps these regulators are activated and inactivated. We isolated here a novel protein that was co-immunoprecipitated with Rab3 GEP and GAP by their respective antibodies from the crude synaptic vesicle fraction of rat brain. The protein, named rabconnectin-3, bound both Rab3 GEP and GAP. The cDNA of rabconnectin-3 was cloned from a human cDNA library and its primary structure was determined. Human rabconnectin-3 consisted of 3,036 amino acids and showed a calculated M(r) of 339,753. It had 12 WD domains. Tissue and subcellular distribution analyses in rat indicated that rabconnectin-3 was abundantly expressed in the brain where it was enriched in the synaptic vesicle fraction. Immunofluorescence and immunoelectron microscopy revealed that rabconnectin-3 was concentrated on synaptic vesicles at synapses. These results indicate that rabconnectin-3 serves as a scaffold molecule for both Rab3 GEP and GAP on synaptic vesicles.

Interaction of syntaxin with alpha-fodrin, a major component of the submembranous cytoskeleton

The soluble N-ethyl maleimide-sensitive factor attachment protein receptor machinery is involved in membrane docking and fusion. In this machinery, the syntaxin family is a central coordinator and participates in multiple protein-protein interactions. In this study we have shown that alpha-fodrin, nonerythroid spectrin, is a new binding partner of the syntaxin family. alpha-Fodrin bound to syntaxin-1a, -3, and -4, all of which are localized on the plasma membrane. Syntaxin-3 interacted with alpha-fodrin in dose-dependent and saturable manners but not with alpha-spectrin, erythroid spectrin. Syntaxin-3 interacted with alpha-fodrin through its C-terminal coiled-coil region. Binding of Munc18 or SNAP-25 to syntaxin-1a inhibited the interaction of alpha-fodrin with syntaxin-1a. Available evidence indicates that alpha-fodrin is implicated in exocytosis, but a precise mode of action of alpha-fodrin in exocytosis remains unclear. Our results suggest that alpha-fodrin regulates exocytosis through the interaction with members of the syntaxin family.

Crystal structure of rat GTP cyclohydrolase I feedback regulatory protein, GFRP

Tetrahydrobiopterin, the cofactor required for hydroxylation of aromatic amino acids regulates its own synthesis in mammals through feedback inhibition of GTP cyclohydrolase I. This mechanism is mediated by a regulatory subunit called GTP cyclohydrolase I feedback regulatory protein (GFRP). The 2.6 A resolution crystal structure of rat GFRP shows that the protein forms a pentamer. This indicates a model for the interaction of mammalian GTP cyclohydrolase I with its regulator, GFRP. Kinetic investigations of human GTP cyclohydrolase I in complex with rat and human GFRP showed similar regulatory effects of both GFRP proteins.

Semaphorin 4C, a transmembrane semaphorin, [corrected] associates with a neurite-outgrowth-related protein, SFAP75

Semaphorin 4C (S4C, previously called M-SemaF) was recently identified as a brain rich transmembrane member of semaphorin family of the vertebrate. In the cytoplasmic domain of S4C there is a proline-rich region suggesting that the cytoplasmic domain may play an important role in Sema4C function. In this study, we have identified the cytoplasmic domain (cd) of M-SemaF(S4C)-associating protein with a Mr of 75 kDa, named SFAP75, from mouse brain. SFAP75 turned out to be the same as the recently reported neurite-outgrowth-related protein named Norbin. Deletion mutants analyses of S4C and SFAP75 revealed that the membrane-proximal region of S4Ccd binds to the intermediate region of SFAP75. Western blot and immunohistochemical analyses with anti-Sema4C and anti-SFAP75 antibodies indicated that S4C and SFAP75 were specially enriched in the brain with a similar distribution pattern to each other. These results suggest that S4C interacts with SFAP75 and plays a role in neural function in brain.

Ankycorbin: a novel actin cytoskeleton-associated protein

BACKGROUND

Actin cytoskeleton structures are essential for a wide variety of cell functions, including cell shape change, cell motility, cell adhesion, cell polarity and cytokinesis. Many actin filament (F-actin)-binding proteins have been isolated and implicated in the maintenance and reorganization of actin cytoskeleton structures.

RESULTS

We purified here a novel protein with a molecular mass of about 125 kDa (p125) from rat liver. We cloned its cDNA from a mouse kidney cDNA library and determined its nucleotide and deduced amino acid sequences. p125 was a protein of 979 amino acids with a calculated Mr of 108 847. p125 contained six ankyrin repeats in the N-terminal region and a domain predicted to form a coiled-coil structure in the C-terminal region. We named p125 ankycorbin (ankyrin repeat- and coiled-coil structure-containing protein). Northern blot analysis indicated that ankycorbin was ubiquitously expressed in all the tissues examined. Immunofluorescence and immunoelectron microscope analyses revealed that ankycorbin was associated with the cortical actin cytoskeleton structures in terminal web and cell-cell adhesion sites and stress fibres. However, ankycorbin did not directly bind to F-actin as estimated by the F-actin co-sedimentation assay.

CONCLUSIONS

These results indicate that ankycorbin is indirectly associated with the actin cytoskeleton structures, presumably through an unidentified factor and suggest that it is involved in their maintenance and/or reorganization.

Functional interactions between GTP cyclohydrolase I and tyrosine hydroxylase in Drosophila

Tyrosine hydroxylase requires the regulatory cofactor, tetrahydrobiopterin, for catecholamine biosynthesis. Because guanosine triphosphate cyclohydrolase I is the rate limiting enzyme for the synthesis of this cofactor, it has a key role in catecholamine production. We show that GTP cyclohydrolase and tyrosine hydroxylase (TH) are co-localized in the Drosophila central nervous system. Mutations in the Punch locus, which encodes GTP cyclohydrolase, reduce TH activity; addition of cofactor to crude extracts could not fully rescue this activity in all mutant strains. The decrease in TH activity and the inability to increase it with added cofactor is not due to loss or decreased production of TH protein. We found that TH co-immunoprecipitated with GTP cyclohydrolase when wild type head extracts were incubated with anti-GTP cyclohydrolase antibody. We suggest that regulation of TH by its cofactor may require its association with GTP cyclohydrolase, and that the ability of GTP cyclohydrolase to associate with TH and its role in tetrahydrobiopterin synthesis may be separable functions of this enzyme. These results have important implications for understanding catecholamine-related neural diseases and designing strategies for gene therapy.

Serine 19 of human 6-pyruvoyltetrahydropterin synthase is phosphorylated by cGMP protein kinase II

6-Pyruvoyltetrahydropterin synthase (PTPS) participates in tetrahydrobiopterin cofactor biosynthesis. We previously identified in a PTPS-deficient patient an inactive PTPS allele with an Arg(16) to Cys codon mutation. Arg(16) is located in the protein surface exposed phosphorylation motif Arg(16)-Arg-Ile-Ser, with Ser(19) as the putative phosphorylation site for serine-threonine protein kinases. Purification of recombinant PTPS-S19A from bacterial cells resulted in an active enzyme (k(cat)/K(m) = 6.4 x 10(3) M(-1) s(-1)), which was similar to wild-type PTPS (k(cat)/K(m) = 4.1 x 10(3) M(-1) s(-1)). In assays with purified enzymes, wild-type but not PTPS-S19A was a specific substrate for the cGMP-dependent protein kinase (cGK) type I and II. Upon expression in COS-1 cells, PTPS-S19A was stable but not phosphorylated and had a reduced activity of approximately 33% in comparison to wild-type PTPS. Extracts from several human cell lines, including brain, contained a kinase that bound to and phosphorylated immobilized wild-type, but not mutant PTPS. Addition of cGMP stimulated phosphotransferase activity 2-fold. Extracts from transfected COS-1 cells overexpressing cGKII stimulated Ser(19) phosphorylation more than 100-fold, but only 4-fold from cGKI overexpressing cells. Moreover, fibroblast extracts from mice lacking cGKII exhibited significantly reduced phosphorylation of PTPS. These results suggest that Ser(19) of human PTPS may be a substrate for cGKII phosphorylation also in vivo, a modification that is essential for normal activity.

Rab11BP/Rabphilin-11, a downstream target of rab11 small G protein implicated in vesicle recycling

Rab11 small G protein has been implicated in vesicle recycling, but its upstream regulators or downstream targets have not yet been identified. We isolated here a downstream target of Rab11, named rabphilin-11, from bovine brain. Moreover, we isolated from a rat brain cDNA library its cDNA, which encoded a protein with a M(r) of 100,946 and 908 amino acids (aa). Rabphilin-11 bound GTP-Rab11 more preferentially than GDP-Rab11 at the N-terminal region and was specific for Rab11 and inactive for other Rab and Rho small G proteins. Both GTP-Rab11 and rabphilin-11 were colocalized at perinuclear regions, presumably the Golgi complex and recycling endosomes, in Madin-Darby canine kidney cells. In HeLa cells cultured on fibronectin, both the proteins were localized not only at perinuclear regions but also along microtubules, which were oriented toward membrane lamellipodia. Treatment of HeLa cells with nocodazole caused disruption of microtubules and dispersion of GTP-Rab11 and rabphilin-11. Overexpression of the C-terminal fragment of rabphilin-11 (aa 607-730), lacking the GTP-Rab11 binding domain, in HeLa cells reduced accumulation of transferrin at perinuclear regions and cell migration. Rabphilin-11 turned out to be a rat counterpart of recently reported bovine Rab11BP. These results indicate that rabphilin-11 is a downstream target of Rab11 which is involved in vesicle recycling.

Ponsin/SH3P12: an l-afadin- and vinculin-binding protein localized at cell-cell and cell-matrix adherens junctions

We recently isolated a novel actin filament (F-actin)-binding protein, afadin, that has two isoforms, l- and s-afadins. l-Afadin is ubiquitously expressed and specifically localized at zonula adherens (ZA) in epithelial cells and at cell-cell adherens junction (AJ) in nonepithelial cells, whereas s-afadin is abundantly expressed in neural tissue. l-Afadin has one PDZ domain, three proline-rich regions, and one F-actin-binding domain, whereas s-afadin lacks the third proline-rich region and the F-actin-binding domain. To understand the molecular mechanism of the specific localization of l-afadin at ZA in epithelial cells and at cell-cell AJ in nonepithelial cells, we attempted here to identify an l-afadin-binding protein(s) and isolated a protein, named ponsin. Ponsin had many splicing variants and the primary structures of two of them were determined. Both the two variants had three Src homology 3 (SH3) domains and turned out to be splicing variants of SH3P12. The third proline-rich region of l-afadin bound to the region of ponsin containing the second and third SH3 domains. Ponsin was ubiquitously expressed and localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells. Ponsin furthermore directly bound vinculin, an F-actin-binding protein localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells. Vinculin has one proline-rich region where two proline-rich sequences are located. The proline-rich region bound to the region of ponsin containing the first and second SH3 domains. l-Afadin and vinculin bound to ponsin in a competitive manner and these three proteins hardly formed a ternary complex. These results indicate that ponsin is an l-afadin- and vinculin-binding protein localized at ZA in epithelial cells, at cell-cell AJ in nonepithelial cells, and at cell-matrix AJ in both types of cells.

Regulation of pteridine-requiring enzymes by the cofactor tetrahydrobiopterin

Tetrahydrobiopterin (BH4) is synthesized from guanosine triphosphate (GTP) by GTP cyclohydrolase I (GCH), 6-pyruvoyltetrahydropterin synthase (PTS), and sepiapterin reductase (SPD). GCH is the rate-limiting enzyme. BH4 is a cofactor for three pteridine-requiring monooxygenases that hydroxylate aromatic L-amino acids, i.e., tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), and phenylalanine hydroxylase (PAH), as well as for nitric oxide synthase (NOS). The intracellular concentrations of BH4, which are mainly determined by GCH activity, may regulate the activity of TH (an enzyme-synthesizing catecholamines from tyrosine), TPH (an enzyme-synthesizing serotonin and melatonin from tryptophan), PAH (an enzyme required for complete degradation of phenylalanine to tyrosine, finally to CO2 + H2O), and also the activity of NOS (an enzyme forming NO from arginine), Dominantly inherited hereditary progressive dystonia (HPD), also termed DOPA-responsive dystonia (DRD) or Segawa's disease, is a dopamine deficiency in the nigrostriatal dopamine neurons, and is caused by mutations of one allele of the GCH gene. GCH activity and BH4 concentrations in HPD/DRD are estimated to be 2-20% of the normal value. By contrast, recessively inherited GCH deficiency is caused by mutations of both alleles of the GCH gene, and the GCH activity and BH4 concentrations are undetectable. The phenotypes of recessive GCH deficiency are severe and complex, such as hyperphenylalaninemia, muscle hypotonia, epilepsy, and fever episode, and may be caused by deficiencies of various neurotransmitters, including dopamine, norepinephrine, serotonin, and NO. The biosynthesis of dopamine, norepinephrine, epinephrine, serotonin, melatonin, and probably NO by individual pteridine-requiring enzymes may be differentially regulated by the intracellular concentration of BH4, which is mainly determined by GCH activity. Dopamine biosynthesis in different groups of dopamine neurons may be differentially regulated by TH activity, depending on intracellular BH4 concentrations and GCH activity. The nigrostriatal dopamine neurons may be most susceptible to a partial decrease in BH4, causing dopamine deficiency in the striatum and the HPD/DRD phenotype.

Nexilin: a novel actin filament-binding protein localized at cell-matrix adherens junction

We isolated two novel actin filament (F-actin)-binding proteins from rat brain and rat 3Y1 fibroblast. They were splicing variants, and we named brain big one b-nexilin and fibroblast small one s-nexilin. b-Nexilin purified from rat brain was a protein of 656 amino acids (aa) with a calculated molecular weight of 78,392, whereas s-nexilin, encoded by the cDNA isolated from rat 3Y1 cells by the reverse transcriptase-PCR method, was a protein of 606 aa with a calculated molecular weight of 71,942. b-Nexilin had two F-actin- binding domains (ABDs) at the NH2-terminal and middle regions, whereas s-nexilin had one ABD at the middle region because 64 aa residues were deleted and 14 aa residues were inserted in the first NH2-terminal ABD of b-nexilin, and thereby the first ABD lost its activity. b- and s-nexilins bound along the sides of F-actin, but only b-nexilin showed F-actin cross-linking activity. b-Nexilin was mainly expressed in brain and testis, whereas s-nexilin was mainly expressed in testis, spleen, and fibroblasts, such as rat 3Y1 and mouse Swiss 3T3 cells, but neither b- nor s-nexilin was detected in liver, kidney, or cultured epithelial cells. An immunofluorescence microscopic study revealed that s-nexilin was colocalized with vinculin, talin, and paxillin at cell- matrix adherens junction (AJ) and focal contacts, but not at cell-cell AJ, in 3Y1 cells. Overexpressed b- and s-nexilins were localized at focal contacts but not at cell-cell AJ. These results indicate that nexilin is a novel F-actin-binding protein localized at cell-matrix AJ.

Protein kinase C phosphorylates and activates GTP cyclohydrolase I in rat renal mesangial cells

GTP cyclohydrolase I is the rate-limiting enzyme in the de novo synthesis pathway of tetrahydrobiopterin which is an essential cofactor for all NO synthase isoforms. The expression of GTP cyclohydrolase I is regulated on a transcriptional level by a variety of cytokines like interleukin 1beta or tumor necrosis factor alpha. The present paper reports that stimulation of protein kinase C by angiotensin II, platelet-derived growth factor BB or the phorbol ester 12-O-tetradecanoylphorbol-13-acetate triggers the phosphorylation and activation of GTP cyclohydrolase I. These data establish that in addition to transcriptional regulation, there is a prominent post-transcriptional modulation of enzyme activity.

Molecular cloning and characterization of the noncatalytic subunit of the Rab3 subfamily-specific GTPase-activating protein

We recently purified and characterized from rat brain a GTPase-activating protein (GAP) specific for the Rab3 small G protein subfamily implicated in Ca2+-dependent exocytosis. Rab3 GAP showed two bands with Mr of about 130,000 (p130) and 150,000 (p150) on SDS-polyacrylamide gel electrophoresis. p130, but not p150, showed the catalytic activity. Because p150 was likely the subunit of Rab3 GAP, here we cloned the cDNA of p150, determined its primary structure, and characterized it. The tissue and subcellular distribution patterns of p150 and p130 were similar, and both the proteins were enriched in the synaptic soluble fraction. p150 was co-immunoprecipitated with p130 from this fraction. Recombinant p150 formed a heterodimer with recombinant p130 as estimated by sucrose density gradient ultracentrifugation. Recombinant p150 neither showed the Rab3A GAP activity nor affected the activity of recombinant p130. When p150 and p130 were co-expressed in the cells, the subcellular localization of each protein did not change. These results indicate that p150 is the noncatalytic subunit of Rab3 GAP.

Isolation and characterization of cortactin isoforms and a novel cortactin-binding protein, CBP90

BACKGROUND

Cortactin is a major phosphotyrosyl protein in pp60v-src-transformed chicken embryo cells. Cortactin binds to actin filament (F-actin) through a unique region which consists of six tandem 37 amino acid repeats, named cortactin repeats. Furthermore cortactin has one src homology 3 (SH3) domain.

RESULTS

In this study we have isolated two new isoforms of cortactin from the rat brain using a polymerase chain reaction (PCR) method, and have named the original cortactin as cortactin-A and the newly isolated forms as cortactin-B and -C. Cortactin-A, -B and -C had six, five, and four cortactin repeats, respectively. All the isoforms were able to bind to F-actin, but only cortactin-A demonstrated an F-actin-crosslinking activity. In addition, cortactin-A was able to bind along the side of F-actin. Next, using a blot overlay assay with glutathione S-transferase (GST)-cortactin-A, we identified a cortactin-A-binding protein with an Mr of approximately 90 kDa in rat brain and named it CBP90 (cortactin-binding protein with an Mr of approximately 90 KDa). CBP90 was purified from rat brain and its cDNA was cloned from a rat brain cDNA library. The deduced amino acid sequence of CBP90 had no significant similarity to any other protein, but it had a proline-rich domain at the C-terminal region. CBP90 was able to bind to all the cortactin isoforms. A deletion mutant analysis of cortactin-A and CBP90 revealed that the SH3 domain of cortactin-A was able to bind to the proline-rich region of CBP90. A Western blot analysis with an anti-CBP90 antibody indicated that, among the rat tissues examined, CBP90 was exclusively expressed in brain. Furthermore, its subcellular distribution and developmental expression patterns were similar to those of cortactin.

CONCLUSION

These results suggest that cortactin interacts with CBP90 and plays a role in regulation of the actin cytoskeleton in brain.

Phosphorylation of GTP cyclohydrolase I and modulation of its activity in rodent mast cells. GTP cyclohydrolase I hyperphosphorylation is coupled to high affinity IgE receptor signaling and involves protein kinase C

GTP cyclohydrolase I controls the de novo pathway for the synthesis of tetrahydrobiopterin, which is the essential cofactor for tryptophan 5-monooxygenase and thus, for serotonin production. In mouse bone marrow-derived mast cells, the kit ligand selectively up-regulates GTP cyclohydrolase I activity (Ziegler, I., Hültner, L. , Egger, D., Kempkes, B., Mailhammer, R., Gillis, S., and Rödl, W. (1993) J. Biol. Chem. 268, 12544-12551). Immunoblot analysis now confirms that this long term enhancement is caused by increased expression of the enzyme. Furthermore we show that GTP cyclohydrolase I is subject to modification at the post-translational level. In vivo labeling with [32P]orthophosphate demonstrates that in primary mast cells and in transfected RBL-2H3 cells overexpressing GTP cyclohydrolase I, the enzyme exists in a phosphorylated form. Antigen binding to the high affinity receptor for IgE triggers an additional and transient phosphorylation of GTP cyclohydrolase I with a concomitant rise in its activity, and in consequence, cellular tetrahydrobiopterin levels increase. These events culminate 8 min after stimulation and can be mimicked by phorbol ester. The hyperphosphorylation is greatly reduced by the protein kinase C inhibitor Ro-31-8220. In vitro phosphorylation studies indicate that GTP cyclohydrolase I is a substrate for both casein kinase II and protein kinase C.

Rat GTP cyclohydrolase I is a homodecameric protein complex containing high-affinity calcium-binding sites

Recombinant rat liver GTP cyclohydrolase I has been prepared by heterologous gene expression in Escherichia coli and characterized by biochemical and biophysical methods. Correlation averaged electron micrograph images of preferentially oriented enzyme particles revealed a fivefold rotational symmetry of the doughnut-shaped views with an average particle diameter of 10 nm. Analytical ultracentrifugation and quantitative scanning transmission electron microscopy yielded average molecular masses of 270 kDa and 275 kDa, respectively. Like the Escherichia coli homolog, these findings suggest that the active enzyme forms a homodecameric protein complex consisting of two fivefold symmetric pentameric rings associated face-to-face. Examination of the amino acid sequence combined with calcium-binding experiments and mutational analysis revealed a high-affinity, EF-hand-like calcium-binding loop motif in eukaryotic enzyme species, which is absent in bacteria. Intrinsic fluorescence measurements yielded an approximate dissociation constant of 10 nM for calcium and no significant binding of magnesium. Interestingly, a loss of calcium-binding capacity observed for two rationally designed mutations within the presumed calcium-binding loop of the rat GTP cyclohydrolase I yielded a 45% decrease in enzyme activity. This finding suggests that failure of calcium binding may be the consequence of a mutation recently identified in the causative GTP cyclohydrolase I gene of patients suffering from dopa responsive dystonia.

Tomosyn: a syntaxin-1-binding protein that forms a novel complex in the neurotransmitter release process

Syntaxin-1 is a component of the synaptic vesicle docking and/or membrane fusion soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) complex (7S and 20S complexes) in nerve terminals. Syntaxin-1 also forms a heterodimer with Munc18/n-Sec1/rbSec1 in a complex that is distinct from the 7S and 20S complexes. In this report, we identify a novel syntaxin-1-binding protein, tomosyn, that is capable of dissociating Munc18 from syntaxin-1 and forming a novel 10S complex with syntaxin-1, soluble N-etyhlmaleimide-sensitive factor attachment (SNAP) 25, and synaptotagmin. The 130 kDa isoform of tomosyn is specifically expressed in brain, where its distribution partly overlaps with that of syntaxin-1 in nerve terminals. High level expression of either syntaxin-1 or tomosyn results in a specific reduction in Ca2+-dependent exocytosis from PC12 cells. These results suggest that tomosyn is an important component in the neurotransmitter release process where it may stimulate SNARE complex formation.

Neurabin-II/spinophilin. An actin filament-binding protein with one pdz domain localized at cadherin-based cell-cell adhesion sites

In a preceding paper, we reported a novel actin filament (F-actin)-binding protein, named neurabin, which was specifically expressed in neural tissue and implicated in neurite formation. We purified from rat brain another F-actin-binding protein, which had a domain organization similar to that of neurabin but was ubiquitously expressed, and named it neurabin-II. The original neurabin, renamed neurabin-I, had 1095 amino acids and a calculated Mr of 122,729, whereas neurabin-II had 817 amino acids and a calculated Mr of 89, 642. Both neurabin-I and -II had one F-actin-binding domain at the N-terminal region, one PDZ domain at the middle region, a domain known to interact with transmembrane proteins, and domains predicted to form coiled-coil structures at the C-terminal region. Both neurabin-I and -II bound along the sides of F-actin and showed F-actin-cross-linking activity. The subcellular distribution analysis indicated that neurabin-II was enriched at the postsynaptic density fraction in rat brain and the adherens junction fraction in rat liver. Immunofluorescence microscopic analysis revealed that neurabin-II was highly concentrated at the synapse in primary cultured rat hippocampal neurons and at the cadherin-based cell-cell adhesion sites in Madin-Darby canine kidney cells. Neurabin-II turned out to be the same as a recently reported protein phosphatase 1-binding protein named spinophilin. These results suggest that neurabin-II/spinophilin plays an important role in linking the actin cytoskeleton to the plasma membrane.

Interactions of drebrin and gephyrin with profilin

Profilin is an actin monomer-binding protein which stimulates actin polymerization. Recent studies have revealed that profilin interacts with VASP, Mena, Bnilp, Bnrlp, and mDia, all of which have the proline-rich domain. Here, we isolated three profilin-binding proteins from rat brain cytosol by glutathione S-transferase-profilin affinity column chromatography and identified them as Mena, drebrin, and gephyrin. These proteins had a proline-rich domain and directly interacted with profilin.

Neurabin: a novel neural tissue-specific actin filament-binding protein involved in neurite formation

We purified from rat brain a novel actin filament (F-actin)-binding protein of approximately 180 kD (p180), which was specifically expressed in neural tissue. We named p180 neurabin (neural tissue-specific F-actin- binding protein). We moreover cloned the cDNA of neurabin from a rat brain cDNA library and characterized native and recombinant proteins. Neurabin was a protein of 1,095 amino acids with a calculated molecular mass of 122,729. Neurabin had one F-actin-binding domain at the NH2-terminal region, one PSD-95, DlgA, ZO-1-like domain at the middle region, a domain known to interact with transmembrane proteins, and domains predicted to form coiled-coil structures at the COOH-terminal region. Neurabin bound along the sides of F-actin and showed F-actin-cross-linking activity. Immunofluorescence microscopic analysis revealed that neurabin was highly concentrated in the synapse of the developed neurons. Neurabin was also concentrated in the lamellipodia of the growth cone during the development of neurons. Moreover, a study on suppression of endogenous neurabin in primary cultured rat hippocampal neurons by treatment with an antisense oligonucleotide showed that neurabin was involved in the neurite formation. Neurabin is a candidate for key molecules in the synapse formation and function.

Afadin: A novel actin filament-binding protein with one PDZ domain localized at cadherin-based cell-to-cell adherens junction

A novel actin filament (F-actin)-binding protein with a molecular mass of approximately 205 kD (p205), which was concentrated at cadherin-based cell-to-cell adherens junction (AJ), was isolated and characterized. p205 was purified from rat brain and its cDNA was cloned from a rat brain cDNA library. p205 was a protein of 1,829 amino acids (aa) with a calculated molecular mass of 207,667 kD. p205 had one F-actin-binding domain at 1,631-1,829 aa residues and one PDZ domain at 1,016- 1,100 aa residues, a domain known to interact with transmembrane proteins. p205 was copurified from rat brain with another protein with a molecular mass of 190 kD (p190). p190 was a protein of 1,663 aa with a calculated molecular mass of 188,971 kD. p190 was a splicing variant of p205 having one PDZ domain at 1,009-1,093 aa residues but lacking the F-actin-binding domain. Homology search analysis revealed that the aa sequence of p190 showed 90% identity over the entire sequence with the product of the AF-6 gene, which was found to be fused to the ALL-1 gene, known to be involved in acute leukemia. p190 is likely to be a rat counterpart of human AF-6 protein. p205 bound along the sides of F-actin but hardly showed the F-actin-cross-linking activity. Northern and Western blot analyses showed that p205 was ubiquitously expressed in all the rat tissues examined, whereas p190 was specifically expressed in brain. Immunofluorescence and immunoelectron microscopic studies revealed that p205 was concentrated at cadherin-based cell-to-cell AJ of various tissues. We named p205 l-afadin (a large splicing variant of AF-6 protein localized at adherens junction) and p190 s-afadin (a small splicing variant of l-afadin). These results suggest that l-afadin serves as a linker of the actin cytoskeleton to the plasma membrane at cell-to-cell AJ.

Isolation and characterization of a GTPase activating protein specific for the Rab3 subfamily of small G proteins

The Rab small G protein family, consisting of nearly 30 members, is implicated in intracellular vesicle trafficking. They cycle between the GDP-bound and GTP-bound forms, and the latter is converted to the former by the action of a GTPase activating protein (GAP). No GAP specific for each Rab family member or Rab subfamily has been isolated in mammal. Here we purified a GAP with Rab3A as a substrate from rat brain. The purified protein was specifically active on the Rab3 subfamily members (Rab3A, -B, -C, and -D). Of this subfamily, Rab3A and -C are implicated in Ca2+-dependent exocytosis, particularly in neurotransmitter release. This GAP, named Rab3 GAP, was active on the lipid-modified form, but not on the lipid-unmodified form. Rab3 GAP showed a minimum molecular mass of about 130 kDa on SDS-polyacrylamide gel electrophoresis. We cloned its cDNA from a human brain cDNA library, and the isolated cDNA encoded a protein with a Mr of 110,521 and 981 amino acids, which showed no homology to any known protein. The recombinant protein exhibited GAP activity toward the Rab3 subfamily members, and the catalytic domain was located at the C-terminal region. Northern blot analysis indicated that Rab3 GAP was ubiquitously expressed.

Isolation and characterization of a GDP/GTP exchange protein specific for the Rab3 subfamily small G proteins

The Rab small G protein family, consisting of nearly 30 members, is implicated in intracellular vesicle trafficking. They cycle between the GDP-bound inactive and GTP-bound active forms, and the former is converted to the latter by the action of a GDP/GTP exchange protein (GEP). No GEP specific for each Rab family member or Rab subfamily has been isolated. Here we purified a GEP from rat brain with lipid-modified Rab3A as a substrate. The purified protein was specifically active on Rab3A, Rab3C, and Rab3D of the Rab3 subfamily. Of these subfamily members, Rab3A and Rab3C are implicated in Ca2+-dependent exocytosis, particularly in neurotransmitter release. This GEP (Rab3 GEP) was active on the lipid-modified form, but not on the lipid-unmodified form. Rab3 GEP showed a minimum molecular mass of about 200 kDa on SDS-polyacrylamide gel electrophoresis. We cloned its cDNA from a rat brain cDNA library and determined its primary structure. The isolated cDNA encoded a protein with a Mr of 177,982 and 1,602 amino acids, which showed no homology to any known protein. The recombinant protein exhibited GEP activity toward Rab3A, Rab3C, and Rab3D. Northern blot and Western blot analyses indicated that Rab3 GEP was expressed in all the rat tissues examined with the highest expression in brain.

Physical and functional interaction of rabphilin-3A with alpha-actinin

Rabphilin-3A is a downstream target molecule of Rab3A small GTP-binding protein and implicated in Ca2+-dependent neurotransmitter release. Here we have isolated a rabphilin-3A-interacting molecule from a human brain cDNA library by the yeast two-hybrid method and identified it to be alpha-actinin, known to cross-link actin filaments into a bundle. alpha-Actinin interacts with the N-terminal region of rabphilin-3A, with which GTP-Rab3A interacts, and this interaction stimulates the activity of alpha-actinin to cross-link actin filaments into a bundle. The interaction of rabphilin-3A with alpha-actinin is inhibited by guanosine 5'-(3-O-thio)triphosphate-Rab3A. These results suggest that the Rab3A-rabphilin-3A system regulates the alpha-actinin-regulated reorganization of actin filaments. It has been shown that reorganization of actin filaments is also involved in Ca2+-dependent exocytosis. Therefore, rabphilin-3A may serve as a linker for Rab3A and cytoskeleton.

Tetrahydrobiopterin biosynthesis in C6 glioma cells: induction of GTP cyclohydrolase I gene expression by lipopolysaccharide and cytokine treatment

The possibility that 5,6,7,8-tetrahydrobiopterin (BH4) biosynthesis is stimulated in glial cells by treatment with lipopolysaccharide (LPS) and tumor necrosis factor (TNF-alpha) was examined in the astrocyte-derived C6 glioma cell line. Under basal culture conditions BH4 levels were found to be at the limit of detection. Concurrent treatment with 10 micrograms/ml LPS and 50 ng/ml TNF-alpha caused a time-dependent 13-fold increase in the levels of BH4. This treatment paradigm also induced nitric oxide synthase activity, as evidenced by increased levels of nitrite, an oxidized metabolite of NO, in the culture medium. LPS and TNF-alpha treatment led to a 25-fold increase in GTPCH enzyme activity, the first and rate-limiting enzyme in BH4 synthesis, and a corresponding 23-fold increase in GTPCH protein levels. Northern blot analysis showed that increased levels of GTPCH mRNA preceded changes in GTPCH protein, GTPCH enzyme activity and BH4 levels and reached a maximal of 44-fold that was sustained for at least 48 h. These results demonstrate that LPS and TNF-alpha stimulate de-novo BH4 biosynthesis and suggest that C6 cells offer a model system for studying the molecular events that control the induction of GTPCH gene expression and BH4 synthesis in glial cells.

Interleukin 1 beta and cAMP trigger the expression of GTP cyclohydrolase I in rat renal mesangial cells

Endogenous synthesis of tetrahydrobiopterin (BH4) is an important requirement for cytokine-stimulated nitric oxide (NO) production in mesangial cells. We have shown that inducible NO synthase is expressed in mesangial cells in response to two principal classes of activating signals, inflammatory cytokines such as interleukin 1 beta (IL-1 beta) and agents that elevate cellular levels of cAMP [Kunz, Mühl, Walker and Pfeilschifter (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 5387-5391]. In the present paper we demonstrate that IL-1 beta and cAMP similarly increase the steady-state mRNA levels of GTP cyclohydrolase I (EC 3,5,4,16), the rate-limiting enzyme in BH4 biosynthesis, as measured by a sensitive and quantitative nuclease protection assay. Stimulation of cells with a combination of IL-1 beta plus cAMP revealed an additive induction profile of GTP cyclohydrolase I mRNA. Message stability studies established that GTP cyclohydrolase I mRNA induced by cAMP has a longer half-life than the IL-1 beta-induced message. Moreover, cAMP exposure markedly prolonged the half-life of GTP cyclohydrolase I mRNA, from 1.5 to 3.4 h. In a next step we generated a rabbit polyclonal antibody against rat GTP cyclohydrolase I expressed in Escherichia coli and demonstrated that IL-1 beta and cAMP elevated GTP cyclohydrolase I protein levels in mesangial cells. Furthermore, IL-1 beta and cAMP led to a marked increase in GTP cyclohydrolase I activity and to increased accumulation of biopterin in mesangial cells. Combinations of IL-1 beta and cAMP resulted in a synergistic stimulation of GTP cyclohydrolase I activity. This may suggest that, in addition to transcriptional and post-transcriptional regulation, there is a prominent post-translational modulation of enzyme activity.

Distinct functional properties of Rab3A and Rab3B in PC12 neuroendocrine cells

Rab3A and Rab3B are highly homologous monomeric GTPases that are putative regulators of exocytosis in those tissues in which they are expressed. We have characterized and directly compared the targeting and functional properties of these isoforms in PC12 neuroendocrine cells. Rab3A and Rab3B both targeted to norepinephrine (NE)-containing large dense core vesicles (LDCVs) when stably expressed in PC12 cells, as determined by immunofluorescence and membrane fractionation. Both Rab3 isoforms also bound to recombinant rabphilin-3A in a GTP-dependent manner. The membrane association of rabphilin-3A was modestly enhanced in Rab3B-expressing PC12 cells relative to Rab3A-overexpressing cells. In addition, overexpression of Rab3A modestly inhibited Ca2+-evoked NE release, whereas Rab3B and a GTP binding mutant (Rab3B N135I) markedly stimulated the efficiency of [3H]NE secretion by PC12 cells (i.e. secretion normalized to total cell radioactivity). Expression of Rab3B and Rab3B N135I increased not only the efficiency of NE secretion but also the accumulation of [3H]NE into LDCVs (i.e. the secretory cargo available for secretion). Neither of these effects was attributable to changes in the numbers of LDCVs nor the docking of LDCVs at the plasma membrane. Our results indicate that Rab3A and Rab3B have similar membrane targeting properties and are capable of interacting with the same putative downstream effector; i.e. rabphilin-3A. However, these isoforms are functionally distinct monomeric GTPases with Rab3B stimulating a late step in Ca2+-evoked secretion when expressed in PC12 cells.

Structural and functional consequences of mutations in 6-pyruvoyltetrahydropterin synthase causing hyperphenylalaninemia in humans. Phosphorylation is a requirement for in vivo activity

Four naturally occurring mutants with single amino acid alterations in human 6-pyruvoyltetrahydropterin synthase (PTPS) were overexpressed and characterized in vitro. The corresponding DNA mutations were found in patients with hyperphenylalaninemia and monoamine neurotransmitter insufficiency due to lack of the tetrahydrobiopterin biosynthetic enzyme PTPS. To predict the structure of the mutant enzymes, computer modeling was performed based on the solved three-dimensional structure of the homohexameric rat enzyme. One mutant (delta V57) is incorrectly folded and thus unstable in vitro and in vivo, while a second mutant (P87L) has substantial activity but enhanced sensitivity to local unfolding. Two other mutants, R16C and R25Q, form stable homomultimers and exhibit significant activity in vitro but no activity in COS-1 cells. In vivo 32P labeling showed that wild-type PTPS, P87L, and R25Q are phosphorylated, while R16C is not modified. This strongly suggests that the serine 19 within the consensus sequence for various kinases, RXXS, is the site of modification. Our results demonstrate that PTPS undergoes protein phosphorylation and requires additional, not yet identified post-translational modification(s) for its in vivo function.