Cloning, structural organization, and transcriptional activity of the rat vitamin K-dependent gamma-glutamyl carboxylase gene

Vitamin K-dependent gamma-glutamylcarboxylase in Atlantic salmon (Salmo salar L.)

Due to problems with bone deformities in farmed Atlantic salmon, there is a growing interest in the possible involvement of vitamin K in normal bone development, and sensitive biomarkers for evaluating vitamin K status are therefore needed. The vitamin K-dependent (VKD) enzyme gamma-glutamylcarboxylase (GGCX, EC 6.4.x.x) requires vitamin K as a cofactor for its post-translational modification of glutamic acid (Glu) residues to gamma-carboxyglutamic acid (Gla) residues in VKD proteins, and is required for their function in haemostasis and bone metabolism. The present study was designed to evaluate the enzyme assay for GGCX activity in isolated liver microsomes and its distribution in the tissues of Atlantic salmon. The effect of KH(2) and menadione on the GGCX activity in salmon liver was also compared. Results from the present study show a widespread tissue distribution and expression of GGCX in Atlantic salmon. The GGCX activity and ggcx expression in all bony tissues examined imply the presence of vitamin K, and suggest the involvement of vitamin K in bone metabolism of Atlantic salmon. We propose the GGCX assay as a sensitive marker for vitamin K status, and confirm that menadione does not work as a cofactor for GGCX in Atlantic salmon liver.

Exon 2 deletion splice variant of gamma-glutamyl carboxylase causes des-gamma-carboxy prothrombin production in hepatocellular carcinoma cell lines

Using GGCX gene-specific real-time PCR, exon 2 deletion splice variant of vitamin K-dependent gamma-glutamyl carboxylase (GGCX) mRNA was identified in HCC cell lines. Expressions of wild type and exon 2 deletion variant of GGCX were analyzed with relevance to DCP production in HCC cell lines. Hep3B, HepG2, HuH1, HuH7, and PLC/PRF/5 produced DCP, while SK-Hep-1, HLE, HLF, and JHH1 produced no detectable level of DCP. DCP-producing cells expressed exon 2 deletion variant of GGCX mRNA and protein, while DCP-negative cells expressed no detectable level of exon 2 deletion variant of GGCX. These results suggest that exon 2 deletion splice variant of GGCX causes dysfunction of GGCX enzyme activity resulting in DCP production in HCC cell lines.

Vitamin K-dependent gamma-glutamylcarboxylation: an ancient posttranslational modification

The vitamin K-dependent carboxylase carries out the posttranslational modification of specific glutamate residues in proteins to gamma-carboxy glutamic acid (Gla) in the presence of reduced vitamin K, molecular oxygen, and carbon dioxide. In the process, reduced vitamin K is converted to vitamin K epoxide, which is subsequently reduced to vitamin K, by vitamin K epoxide reductase (VKOR) for use in the carboxylation reaction. The modification has a wide range of physiological implications, including hemostasis, bone calcification, and signal transduction. The enzyme interacts with a high affinity gamma-carboxylation recognition sequence (gamma-CRS) of the substrate and carries out multiple modifications of the substrate before the product is released. This mechanism ensures complete carboxylation of the Gla domain of the coagulation factors, which is essential for their biological activity. gamma-Carboxylation, originally discovered in mammals, is widely distributed in the animal kingdom. It has been characterized in sea squirt (Ciona intestinalis), in flies (Drosophila melanogaster), and in marine snails (Conus textile), none of which have a blood coagulation system similar to mammals. The cone snails express a large array of gamma-carboxylated peptides that modulate the activity of ion channels. These findings have led to the suggestion that gamma-carboxylation is an extracellular posttranslational modification that antedates the divergence of molluscs, arthropods, and chordates. I will first summarize recent understanding of gamma-carboxylase and gamma-carboxylation gleaned from experiments using the mammalian enzyme, and then I will briefly describe the available information on gamma-carboxylation in D. melanogaster and C. textile.

A conserved region of human vitamin K-dependent carboxylase between residues 393 and 404 is important for its interaction with the glutamate substrate

Certain individuals with combined deficiencies of vitamin K-dependent proteins have a mutation, L394R, in their gamma-glutamyl carboxylase causing impaired glutamate binding. The sequence surrounding Leu394 is similar in all known carboxylases, suggesting that the region is functionally important. To test this hypothesis we made the following mutant enzymes: W390A, Y395A, S398A, W399A, and H404A. We purified the enzymes and corrected the activity measurements for active enzyme concentration. Carboxylases W390A, S398A, and H404A had activities similar to that of wild type; however, Y395A and W399A had lower activities than did wild type. In the following descriptions we include our previously reported results for L394R. Kinetic studies with the substrate FLEEL, revealed Km values of 0.5 (wild type), 6.5 (L394R), 15 (Y395A), and 24 (W399A) mm. The kcat values relative to wild type were 51% (L394R), 1% (Y395A), and 2% (W399A). The kcat/Km values were 24-fold (L394R) and >2000-fold lower for Y395A and W399A than for wild-type carboxylase. Inhibition of FLEEL carboxylation by the competitive inhibitor, Boc-mEEV, gave Ki values of 0.013 (wild type), 1.4 (L394R), 2.1 (Y395A), and >5 (W399A) mm. The Y395A propeptide affinity was similar to that of wild type, but those of L394R and W399A were 16-22-fold less than that of wild type. Results of kinetic studies with a propeptide-containing substrate were consistent with results of propeptide binding and FLEEL kinetics. Although propeptide and vitamin K binding in some mutants were affected, our data provide compelling evidence that glutamate recognition is the primary function of the conserved region around Leu394.

The vitamin K-dependent gamma-glutamyl carboxylase gene contains a TATA-less promoter with a novel upstream regulatory element

The expression of the vitamin K-dependent gamma-glutamyl carboxylase gene in liver is developmentally regulated. Since the gene product catalyzes an essential post-translational modification of the vitamin K-dependent blood coagulation proteins, the regulation of carboxylase expression is critical for hemostasis. We analyzed the activity of the rat carboxylase gene 5'-regulatory DNA sequences in rat hepatoma cell lines at different states of differentiation. These studies demonstrated that the 2.6-kb 5'-flanking sequence has differentiation-dependent transcriptional activity. Transient gene expression assays, examining the effects of nested deletions and site-directed mutagenesis of putative regulatory sequences, together with electrophoretic mobility shift assays (EMSAs) were used to identify sequences critical for the developmentally regulated transcription of the rat carboxylase gene. We identified a DNA sequence (-76 to -65; GTTCCGGCCTTC) not known to bind to transcription factors, yet which functions as an upstream promoter element. In vivo genomic DNA footprinting confirms the presence of nuclear protein-DNA interactions at this site in the endogenous carboxylase gene in differentiated hepatoma cells. Therefore, this DNA sequence has specific nuclear protein-binding activity and functional properties consistent with a regulatory element that plays a critical role in the developmental expression of the carboxylase gene, and hence the regulation of vitamin K-dependent blood coagulation protein synthesis.

Expression and characterization of recombinant vitamin K-dependent gamma-glutamyl carboxylase from an invertebrate, Conus textile

The marine snail Conus is the sole invertebrate wherein both the vitamin K-dependent carboxylase and its product, gamma-carboxyglutamic acid, have been identified. To examine its biosynthesis of gamma-carboxyglutamic acid, we studied the carboxylase from Conus venom ducts. The carboxylase cDNA from Conus textile has an ORF that encodes a 811-amino-acid protein which exhibits sequence similarity to the vertebrate carboxylases, with 41% identity and approximately 60% sequence similarity to the bovine carboxylase. Expression of this cDNA in COS cells or insect cells yielded vitamin K-dependent carboxylase activity and vitamin K-dependent epoxidase activity. The recombinant carboxylase has a molecular mass of approximately 130 kDa. The recombinant Conus carboxylase carboxylated Phe-Leu-Glu-Glu-Leu and the 28-residue peptides based on residues -18 to +10 of human proprothrombin and proFactor IX with Km values of 420 micro m, 1.7 micro m and 6 micro m, respectively; the Km for vitamin K is 52 micro m. The Km values for peptides based on the sequence of the conotoxin epsilon-TxIX and two precursor analogs containing 12 or 29 amino acids of the propeptide region are 565 micro m, 75 micro m and 74 micro m, respectively. The recombinant Conus carboxylase, in the absence of endogenous substrates, is stimulated up to fivefold by vertebrate propeptides but not by Conus propeptides. These results suggest two propeptide-binding sites in the carboxylase, one that binds the Conus and vertebrate propeptides and is required for substrate binding, and the other that binds only the vertebrate propeptide and is required for enzyme stimulation. The marked functional and structural similarities between the Conus carboxylase and vertebrate vitamin K-dependent gamma-carboxylases argue for conservation of a vitamin K-dependent carboxylase across animal species and the importance of gamma-carboxyglutamic acid synthesis in diverse biological systems.

gamma -Glutamyl carboxylation: An extracellular posttranslational modification that antedates the divergence of molluscs, arthropods, and chordates

The posttranslational gamma-carboxylation of glutamate residues in secreted proteins to gamma-carboxyglutamate is carried out by the vitamin K-dependent enzyme gamma-glutamyl carboxylase. gamma-Carboxylation has long been thought to be a biochemical specialization of vertebrates, essential for blood clotting. Recently, a gamma-carboxylase was shown to be expressed in Drosophila, although its function remains undefined in this organism. We have characterized both cDNA and genomic clones for the gamma-glutamyl carboxylase from the marine mollusc, Conus, the only nonvertebrate organism for which gamma-carboxyglutamate-containing proteins have been biochemically and physiologically characterized. The predicted amino acid sequence has a high degree of sequence similarity to the Drosophila and vertebrate enzymes. Although gamma-carboxylases are highly conserved, the Conus and mammalian enzymes have divergent substrate specificity. There are striking parallels in the gene organization of Conus and human gamma-carboxylases. Of the 10 Conus introns identified, 8 are in precisely the same position as the corresponding introns in the human enzyme. This remarkable conservation of intron/exon boundaries reveals that an intron-rich gamma-carboxylase was present early in the evolution of the animal phyla; although specialized adaptations in mammals and molluscs that require this extracellular modification have been identified, the ancestral function(s) and wider biological roles of gamma-carboxylation still need to be defined. The data raise the possibility that most introns in the genes of both mammals and molluscs antedate the divergence of these phyla.

Identification of sequences within the gamma-carboxylase that represent a novel contact site with vitamin K-dependent proteins and that are required for activity

The vitamin K-dependent (VKD) carboxylase converts clusters of Glu residues to gamma-carboxylated Glu residues (Glas) in VKD proteins, which is required for their activity. VKD precursors are targeted to the carboxylase by their carboxylase recognition site, which in most cases is a propeptide. We have identified a second tethering site for carboxylase and VKD proteins that is required for carboxylase activity, called the vitamin K-dependent protein site of interaction (VKS). Several VKD proteins specifically bound an immobilized peptide comprising amino acids 343-355 of the human carboxylase (CVYKRSRGKSGQK) but not a scrambled peptide containing the same residues in a different order. Association with the 343-355 peptide was independent of propeptide binding, because the VKD proteins lacked the propeptide and because the 343-355 peptide did not disrupt association of a propeptide factor IX-carboxylase complex. Analysis with peptides that overlapped amino acids 343-355 indicated that the 343-345 CVY residues were necessary but not sufficient for prothrombin binding. Ionic interactions were also suggested because peptide-VKD protein binding could be disrupted by changes in ionic strength or pH. Mutagenesis of Cys(343) to Ser and Tyr(345) to Phe resulted in 7-11-fold decreases in vitamin K epoxidation and peptide (EEL) substrate and carboxylase carboxylation, and kinetic analysis showed 5-6-fold increases in K(m) values for the Glu substrate. These results suggest that Cys(343) and Tyr(345) are near the catalytic center and affect the active site conformation required for correct positioning of the Glu substrate. The 343-355 VKS peptide had a higher affinity for carboxylated prothrombin (K(d) = 5 microm) than uncarboxylated prothrombin (K(d) = 60 microm), and the basic VKS region may also facilitate exiting of the Gla product from the catalytic center by ionic attraction. Tethering of VKD proteins to the carboxylase via the propeptide-binding site and the VKS region has important implications for the mechanism of VKD protein carboxylation, and a model is proposed for how the carboxylase VKS region may be required for efficient and processive VKD protein carboxylation.

A conserved motif within the vitamin K-dependent carboxylase gene is widely distributed across animal phyla

The vitamin K-dependent gamma-glutamyl carboxylase catalyzes the posttranslational conversion of glutamic acid to gamma-carboxyglutamic acid, an amino acid critical to the function of the vitamin K-dependent blood coagulation proteins. Given the functional similarity of mammalian vitamin K-dependent carboxylases and the vitamin K-dependent carboxylase from Conus textile, a marine invertebrate, we hypothesized that structurally conserved regions would identify sequences critical to this common functionality. Furthermore, we examined the diversity of animal species that maintain vitamin K-dependent carboxylation to generate gamma-carboxyglutamic acid. We have cloned carboxylase homologs in full-length or partial form from the beluga whale (Delphinapterus leucas), toadfish (Opsanus tau), chicken (Gallus gallus), hagfish (Myxine glutinosa), horseshoe crab (Limulus polyphemus), and cone snail (Conus textile) to compare these structures to the known bovine, human, rat, and mouse cDNA sequences. Comparison of the predicted amino acid sequences identified a nearly perfectly conserved 38-amino acid residue region in all of these putative carboxylases. In addition, this amino acid motif is also present in the Drosophila genome and identified a Drosophila homolog of the gamma-carboxylase. Assay of hagfish liver demonstrated vitamin K-dependent carboxylase activity in this hemichordate. These results demonstrate the broad distribution of the vitamin K-dependent carboxylase gene, including a highly conserved motif that is likely critical for enzyme function. The vitamin K-dependent biosynthesis of gamma-carboxyglutamic acid appears to be a highly conserved function in the animal kingdom.

Expression and characterization of the naturally occurring mutation L394R in human gamma-glutamyl carboxylase

Patients with mutation L394R in gamma-glutamyl carboxylase have a severe bleeding disorder because of decreased biological activities of all vitamin K-dependent coagulation proteins. Vitamin K administration partially corrects this deficiency. To characterize L394R, we purified recombinant mutant L394R and wild-type carboxylase expressed in baculovirus-infected insect cells. By kinetic studies, we analyzed the catalytic activity of mutant L394R and its binding to factor IX's propeptide and vitamin KH(2). Mutant L394R differs from its wild-type counterpart as follows: 1) 110-fold higher K(i) for Boc-mEEV, an active site-specific, competitive inhibitor of FLEEL; 2) 30-fold lower V(max)/K(m) toward the substrate FLEEL in the presence of the propeptide; 3) severely reduced activity toward FLEEL carboxylation in the absence of the propeptide; 4) 7-fold decreased affinity for the propeptide; 5) 9-fold higher K(m) for FIXproGla, a substrate containing the propeptide and the Gla domain of human factor IX; and 6) 5-fold higher K(m) for vitamin KH(2). The primary defect in mutant L394R appears to be in its glutamate-binding site. To a lesser degree, the propeptide and KH(2) binding properties are altered in the L394R mutant. Compared with its wild-type counterpart, the L394R mutant shows an augmented activation of FLEEL carboxylation by the propeptide.