Amino acids responsible for reduced affinities of vitamin K-dependent propeptides for the carboxylase

Vitamin K-dependent carboxylation of coagulation factors: insights from a cell-based functional study

Vitamin K-dependent carboxylation is a post-translational modification essential for the biological function of coagulation factors. Defects in carboxylation are mainly associated with bleeding disorders. With the discovery of new vitamin K-dependent proteins, the importance of carboxylation now encompasses vascular calcification, bone metabolism, and other important physiological processes. Our current knowledge of carboxylation, however, comes mainly from in vitro studies carried out under artificial conditions, which have a limited usefulness in understanding the carboxylation of vitamin K-dependent proteins in native conditions. Using a recently established mammalian cell-based assay, we studied the carboxylation of coagulation factors in a cellular environment. Our results show that the coagulation factor’s propeptide controls substrate binding and product releasing during carboxylation, and the propeptide of factor IX appears to have the optimal affinity for efficient carboxylation. Additionally, non-conserved residues in the propeptide play an important role in carboxylation. A cell-based functional study of naturally occurring mutations in the propeptide successfully interpreted the clinical phenotype of warfarin’s hypersensitivity during anticoagulation therapy in patients with these mutations. Unlike results obtained from in vitro studies, results from our cell-based study indicate that although the propeptide of osteocalcin cannot direct the carboxylation of the coagulation factor, it is required for the efficient carboxylation of osteocalcin. This suggests that the coagulation factors may have a different mechanism of carboxylation from osteocalcin. Together, results from this study provide insight into efficiently controlling one physiological process, such as coagulation without affecting the other, like bone metabolism.

Production of recombinant human factor IX by propeptide modification in Drosophila S2 cell line

OBJECTIVE

To compare the effect of pre-propeptide (pre-pro) of the human prothrombin (hPT), with both the native and an R-9N mutant forms of the human factor IX (hFIX) pre-pro on the hFIX carboxylation, in Drosophila cell.

RESULTS

The three different pre-pro sequences, equipped with Drosophila Kozak, were joined to the mature hFIX cDNA and were subjected to transient expression analysis of hFIX in the S2 Drosophila cells, compared to that of a native hFIX cDNA, with its native Kozak. Replacement of the hFIX pre-pro sequence with that of hPT increased the biological activity of hFIX, significantly. The highest total level of hFIX expression occurred for the native hFIX with the Drosophila Kozak. However, the hFIX secretion efficiency with this construct was less than that of the native hFIX with its native Kozak. The R-9N substitution, in the hFIX propeptide, with no apparent effect on the FIX γ-carboxylation, reduced the FIX expression efficiency.

CONCLUSION

Potential of the hPT pre-pro sequence for FIX expression in Drosophila cells, was confronted by γ-glutamyl carboxylase (GGCX) saturation in ER, besides the functional importance of -9 amino acid in propeptide is described; these are noteworthy for production of γ-carboxylated proteins.

Variants in FIX propeptide associated with vitamin K antagonist hypersensitivity: functional analysis and additional data confirming the common founder mutations

One of the most common and unwanted side effects during oral anticoagulant therapy (OAT) is bleeding complications. In rare cases, vitamin K antagonist (VKA)-related bleeding events are associated with mutations affecting the F9 propeptide at amino acid position 37 due to a substitution of alanine to either valine or threonine. Based on our actual cohort of 18 patients, we update the knowledge on this rare phenotype and its origin. A founder mutation for both variants was reconfirmed by haplotype analysis of intronic and extragenic short tandem repeat (STR) polymorphisms with a higher prevalence in Switzerland than in other regions of Europe. Screening of healthy individuals for the presence of these F9 gene mutations did not identify any of these variants, thus proving the rare occurrence of this genotype. Furthermore, both variants were expressed in vitro and warfarin dose responses were studied. Our warfarin dose response analysis confirmed higher sensitivity of both variants to warfarin with the effect being more apparent for Ala37Thr. Thus, although F9 propeptide mutation-associated hypersensitivity to VKA is a rare phenomenon, awareness towards this bleeding phenotype is important to identify patients at risk.

Defective γ-Glutamyl Carboxylase Activity and Bleeding in Rambouillet Sheep

A flock of Rambouillet sheep was examined because of increased lamb mortality caused by ineffective hemostasis at parturition. Neonatal-affected lambs presented with inadequate hemostasis at the umbilicus, pale mucus membranes, and markedly prolonged activated clotting time. Affected lambs had consistently prolonged 1-stage prothrombin times and activated partial thromboplastin times that supported a defect in the common pathway or defects in both the intrinsic and extrinsic pathway of the coagulation cascade. Decreased activity of vitamin K-dependent procoagulant factors II, VII, IX, and X in male and female lambs suggested either a defect of the hepatic enzyme γ-glutamyl carboxylase, or vitamin K1 2,3 epoxide reductase. Affected lamb hepatic γ-glutamyl carboxylase activity was markedly decreased compared with that of age- and sex-matched control lambs, while vitamin K1 2,3 epoxide reductase and glucose-6-phosphatase activities were similar between an affected and normal lamb. Subcutaneous vitamin K1 supplementation did not increase vitamin K-dependent procoagulant factor activities in 3 lambs administered vitamin K1 daily. These data confirm defective γ-glutamyl carboxylase activity as the cause of impaired coagulation of sheep in this flock. This flock represents the only viable animal model of hereditarily defective γ-glutamyl carboxylase activity.

Effect of vitamin K-dependent protein precursor propeptide, vitamin K hydroquinone, and glutamate substrate binding on the structure and function of {gamma}-glutamyl carboxylase

The γ-glutamyl carboxylase utilizes four substrates to catalyze carboxylation of certain glutamic acid residues in vitamin K-dependent proteins. How the enzyme brings the substrates together to promote catalysis is an important question in understanding the structure and function of this enzyme. The propeptide is the primary binding site of the vitamin K-dependent proteins to carboxylase. It is also an effector of carboxylase activity. We tested the hypothesis that binding of substrates causes changes to the carboxylase and in turn to the substrate-enzyme interactions. In addition we investigated how the sequences of the propeptides affected the substrate-enzyme interaction. To study these questions we employed fluorescently labeled propeptides to measure affinity for the carboxylase. We also measured the ability of several propeptides to increase carboxylase catalytic activity. Finally we determined the effect of substrates: vitamin K hydroquinone, the pentapeptide FLEEL, and NaHCO(3), on the stability of the propeptide-carboxylase complexes. We found a wide variation in the propeptide affinities for carboxylase. In contrast, the propeptides tested had similar effects on carboxylase catalytic activity. FLEEL and vitamin K hydroquinone both stabilized the propeptide-carboxylase complex. The two together had a greater effect than either alone. We conclude that the effect of propeptide and substrates on carboxylase controls the order of substrate binding in such a way as to ensure efficient, specific carboxylation.

Pharmacogenomics in thrombosis

Inherited or acquired genetic abnormalities play a major role in thromboembolic complications. The goal of pharmacogenomics is to tailor medications to an individual's genetic makeup in order to improve the benefit-to-risk ratio. Significant findings have been documented showing the effect of certain genetic variations (e.g., in CYP2C9 and VKORC1) on the dose response to warfarin. Pharmacogenomic and genetic information is crucial to improving the efficacy and safety of pharmacotherapy and for the optimal management of thromboembolic disorders.

Vitamin K-dependent carboxylation

Vitamin K-dependent (VKD) protein carboxylation uses vitamin K epoxidation to convert Glus to carboxylated Glus (Glas), rendering VKD proteins active in physiologies that include hemostasis, apoptosis, bone mineralization, calcium homeostasis, growth control, and signal transduction. Clusters of Glus are modified by a processive carboxylase, generating a calcium-binding module that allows binding to either hydroxyapatite in the extracellular matrices or cell surfaces where anionic phospholipids become exposed, for example, during apoptosis or cell activation. Naturally occurring carboxylase mutations have been informative for function and are associated with bleeding complications and, surprisingly, a pseudoxanthoma elasticum (PXE)-like phenotype. A major advance in defining carboxylase function is the identification of the base that initiates carboxylation, which raises interesting possibilities for how vitamin K epoxidation is regulated by Glu substrate and carboxylase membrane topology. Vitamin K oxidoreductase (VKOR), the target of warfarin, generates the reduced vitamin K cofactor used by the carboxylase. Oxidation of active site thiols during vitamin K reduction inactivates VKOR, and activity is regenerated by an unknown reductase. The amounts of reduced vitamin K limit the capacity for carboxylation in cells, and overexpression of VKOR, but not carboxylase, improves carboxylation. However, the effect of VKOR overexpression is small, possibly because the reductase that regenerates VKOR activity is saturated. The review discusses these advances, as well as the potential impact of secretory components on carboxylation, which occurs during VKD protein secretion. Also discussed is the role of the carboxylase in mammals and lower organisms, including the bacterial pathogen Leptospira interrogans that has acquired a VKD carboxylase by horizontal transfer.

The vitamin K-dependent carboxylase

The vitamin K-dependent (VKD) carboxylase uses the oxygenation of vitamin K to convert glutamyl residues (Glus) to carboxylated Glus (Glas) in VKD proteins, rendering them active in a broad range of physiologies that include hemostasis, apoptosis, bone development, arterial calcification, signal transduction, and growth control. The carboxylase has a high-affinity site that selectively binds VKD proteins, usually through their propeptide, and also has a second low-affinity site of VKD protein interaction. Propeptide binding increases carboxylase affinity for the Glu substrate, and the coordinated binding of the VKD propeptide and Glu substrate increases carboxylase affinity for vitamin K and activity, possibly through a mechanism of substrate-assisted catalysis. Tethering of VKD proteins to the carboxylase allows clusters of Glus to be modified to Glas by a processive mechanism that becomes disrupted during warfarin therapy. Warfarin inhibits a vitamin K oxidoreductase that generates the reduced vitamin K cofactor required for continuous carboxylation and causes decreased carboxylase catalysis and increased dissociation of partially carboxylated, inactive VKD proteins. The availability of reduced vitamin K may also control carboxylation in r-VKD protein-expressing cells, where the amounts of reduced vitamin K are sufficient for full carboxylation of low, but not high, expression levels of VKD proteins, and where carboxylation is not improved by overexpression of r-carboxylase. This review discusses these recent advances in understanding the mechanism of carboxylation. Also covered is the identification of functional carboxylase residues, a brief description of the role of VKD proteins in mammalian and lower organisms, and the potential impact of quality control components on carboxylation, which occurs in the endoplasmic reticulum during the secretion of VKD proteins.

The vitamin K cycle

Post-translational modification of glutamate to gamma carboxyl glutamate is required for the activity of vitamin K-dependent proteins. Carboxylation is accomplished by the enzyme gamma glutamyl carboxylase (GGCX) which requires the propeptide-containing substrate and three co-substrates: reduced vitamin K, CO2, and O2. Most propeptides bind tightly to GGCX and all of the Glu residues that will be modified are modified during one binding event. Complete carboxylation is thus dependent upon the rate of carboxylation and the dissociation rate constant of the substrate from the GGCX enzyme. If the propeptide is released before carboxylation is complete, partially carboxylated vitamin K-dependent proteins are produced. The rate of carboxylation is mainly controlled by the level of reduced vitamin K available for the reactions while the dissociation rate constant is dependent upon both the propeptide and the Gla domain of the substrate. In addition, there are allosteric effects that increase the rate of dissociation of the fully carboxylated substrates. Carboxylation requires the abstraction of a proton from the 4-carbon of glutamate by reduced vitamin K and results in the conversion of vitamin K to vitamin K epoxide. The vitamin K epoxide must be recycled to vitamin K before it can be reused, a reaction catalyzed by the enzyme vitamin K epoxide reductase (VKOR). The gene for VKOR has recently been identified but the enzyme itself has not been purified to homogeneity. It appears, however, that most of the variability observed in patients response to warfarin may be attributed to variability in the VKOR gene.

Precursors of Novel Gla-Containing Conotoxins Contain a Carboxy-Terminal Recognition Site That Directs γ-Carboxylation,

Vitamin K-dependent gamma-glutamyl carboxylase catalyzes the conversion of glutamyl residues to gamma-carboxyglutamate. Its substrates include vertebrate proteins involved in blood coagulation, bone mineralization, and signal transduction and invertebrate ion channel blockers known as conotoxins. Substrate recognition involves a recognition element, the gamma-carboxylation recognition site, typically located within a cleavable propeptide preceding the targeted glutamyl residues. We have purified two novel gamma-carboxyglutamate-containing conotoxins, Gla-TxX and Gla-TxXI, from the venom of Conus textile. Their cDNA-deduced precursors have a signal peptide but no apparent propeptide. Instead, they contain a C-terminal extension that directs gamma-carboxylation but is not found on the mature conotoxin. A synthetic 13-residue "postpeptide" from the Gla-TxXI precursor reduced the K(m) for the reaction of the Conus gamma-carboxylase with peptide substrates, including FLEEL and conantokin-G, by up to 440-fold, regardless of whether it was positioned at the N- or C-terminal end of the mature toxin. Comparison of the postpeptides to propeptides from other conotoxins suggested some common elements, and amino acid substitutions of these residues perturbed gamma-carboxylation of the Gla-TxXI peptide. The demonstration of a functional and transferable C-terminal postpeptide in these conotoxins indicates the presence of the gamma-carboxylation recognition site within the postpeptide and defines a novel precursor structure for vitamin K-dependent polypeptides. It also provides the first formal evidence to prove that gamma-carboxylation occurs as a post-translational rather than a cotranslational process.

Structural Evidence of a Fourth Gla Residue in Fish Osteocalcin: Biological Implications,

Osteocalcin is a small (45 amino acids) secreted protein found to accumulate in bone and dentin of many organisms by interacting with calcium and hydroxyapatite, through the presence of three gamma-carboxylated residues. In this work, we describe the first X-ray crystal structure for a nonmammalian osteocalcin, obtained at 1.4 A resolution, purified from the marine teleost fish Argyrosomus regius. The three-dimensional fit between the A. regius structure and that of the only other known X-ray structure, the porcine osteocalcin, revealed a superposition of the Calpha atoms of their metal chelating residues, Gla and Asp, showing that their spatial distribution is consistent with the interatomic distances of calcium cations in the hydroxyapatite crystals. In both structures, the protein forms a tight globular arrangement of their three alpha-helices while the remaining residues, at N- and C-terminal regions, have essentially no secondary structure characteristics. This study revealed the presence of a fourth gamma-carboxylation at Glu(25), not previously detected in the structure of the porcine osteocalcin or in any other of the sequentially characterized mammalian osteocalcins (human, cow, and rat). A confirmation of the fourth Gla residue in A. regius osteocalcin was achieved via LC-MS analysis. These four doubly charged residues are, together with Asp(24), concentrated in a common surface region located on the same side of the molecule. This further suggests that the known high affinity of osteocalcin for bone mineral may be derived from the clustering of calcium binding sites on this surface of the molecules.

Characteristics of recombinant W501S mutated human gamma-glutamyl carboxylase

A mutation (W501S) in the vitamin K-dependent gamma-glutamyl carboxylase (VKC) that leads to a congenital bleeding disorder was recently discovered in two patients. To characterize the enzyme defect, recombinant VKC-W501S was expressed in and purified from insect cells. The major effect of the mutation appears to be to decrease the affinity of the carboxylase for the propeptide of its substrates. This observation agrees with recent data that place part of the propeptide binding site within residues 495-513 of VKC. Additionally, we demonstrate that the affinity between descarboxy osteocalcin (d-OC) and VKC remains unaffected by the W501S mutation. This confirms earlier data that the high-affinity site for d-OC is not located on the propeptide binding domain of VKC. Two properties of the enzyme suggest an explanation for the observation that vitamin K supplementation ameliorates the effects of the mutation: (i) since full carboxylation requires the propeptide to remain bound to the enzyme sufficiently long for full carboxylation, a reduced affinity can cause its premature release before carboxylation is complete; (ii) propeptide binding results in a decrease of the KM for vitamin K hydroquinone in wild-type, but not in mutant carboxylase, resulting in increased vitamin K requirement of affected subjects.

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 putative vitamin K-dependent gamma-glutamyl carboxylase internal propeptide appears to be the propeptide binding site

The vitamin K-dependent gamma-glutamyl carboxylase binds an 18-amino acid sequence usually attached as a propeptide to its substrates. Price and Williamson (Protein Sci. (1993) 2, 1997-1998) noticed that residues 495-513 of the carboxylase shares similarity with the propeptide. They suggested that this internal propeptide could bind intramolecularly to the propeptide binding site of carboxylase, thereby preventing carboxylation of substrates lacking a propeptide recognition sequence. To test Price's hypothesis, we created nine mutant enzyme species that have single or double mutations within this putative internal propeptide. The apparent K(d) values of these mutant enzymes for human factor IX propeptide varied from 0.5- to 287-fold when compared with that of wild type enzyme. These results are consistent with the internal propeptide hypothesis but could also be explained by these residues participating in propeptide binding site per se. To distinguish between the two alternative hypotheses, we measured the dissociation rates of propeptides from each of the mutant enzymes. Changes in an internal propeptide should not affect the dissociation rates, but changes to a propeptide binding site may affect the dissociation rate. We found that dissociation rates varied in a manner consistent with the apparent K(d) values measured above. Furthermore, kinetic studies using propeptide-containing substrates demonstrated a correlation between the affinity for propeptide and V(max). Taken together, our results indicated that these mutations affected the propeptide binding site rather than a competitive inhibitory internal propeptide sequence. These results agree with our previous observations, indicating that residues in this region are involved in propeptide binding.

Gene mutations and three-dimensional structural analysis in 13 families with severe factor X deficiency

Factor X (FX) deficiency is a rare autosomal recessive disorder. The phenotype and genotype of 15 Iranian patients with FX deficiency from 13 unrelated families with a high frequency of consanguinity were analysed. Five different assays identified four patients from three families with a discrepancy between low-FX coagulant activity (FX:C) and higher-FX antigen (FX:Ag) (a type II deficiency). The remaining 11 patients had parallel reductions of FX:C and FX:Ag (a type I deficiency). Nine different homozygous candidate mutations were identified, of which eight were novel. The four type II cases were associated with an Arg(-1)Thr missense mutation in the prepropeptide: Arg(-1) is highly conserved in all vitamin K-dependent proteins. Four type I mutations (Gly78Asp, Cys81Tyr, Gly94Arg and Asp95Glu) were localized to the EGF-1 and EGF-2 domains, for which molecular views showed that the protein folding would be disrupted. The type I mutation Gly222Asp was localized in the catalytic domain of FX, and is sufficiently close to the Asp-His-Ser catalytic triad to disrupt its correct protein folding. The two type I splice site mutations were IVS1+3, A-->T and IVS2-3, T-->G. These novel homozygous FX mutations were consistent with their phenotypes and agree with experimental data from knockout mice, indicating that FX is an essential protein for survival.

Characteristics and composition of the vitamin K-dependent gamma-glutamyl carboxylase-binding domain on osteocalcin

Two different sites on vitamin K-dependent gamma-glutamyl carboxylase (VKC) are involved in enzyme-substrate interaction: the propeptide-binding site required for high-affinity substrate binding and the active site for glutamate carboxylation. Synthetic descarboxy osteocalcin (d-OC) is a low-K(m) substrate for the VKC, but unique since it possesses a high-affinity recognition site for the VKC, distinct from the propeptide which is essential as a binding site for VKC. However, the exact location and composition of this VKC-recognition domain on d-OC has remained unclear until now. Using a stereospecific substrate analogue [t-butyloxycarbonyl-(2S,4S)-4-methylglutamic acid-Glu-Val (S-MeTPT)] we demonstrate in this paper that the high affinity of d-OC for VKC cannot be explained by a direct interaction with either the active site or with the propeptide-binding site on VKC. It is shown using various synthetic peptides derived from d-OC that there are two domains on d-OC necessary for recognition: one located between residues 1 and 12 and a second between residues 26 and 39, i.e. at the C-terminal side of the gamma-carboxyglutamate (Gla) domain. Both internal sequences contribute substantially to the efficiency of carboxylation. On the basis of these data we postulate the presence of a second high-affinity substrate-binding site on VKC capable of specifically binding d-OC, which is the first vitamin K-dependent substrate of which the VKC binding domain is interrupted by the Gla domain.

A novel fluorescence assay to study propeptide interaction with gamma-glutamyl carboxylase

The vitamin K-dependent gamma-glutamyl carboxylase catalyzes the posttranslational modification of select glutamate residues of its vitamin K-dependent substrates to gamma-carboxyglutamate. In this report, we describe a new fluorescence assay that is sensitive and specific for the propeptide binding site of active carboxylase. We employed the assay to make three important observations: (1) A tight binding fluorescein-labeled consensus propeptide can be used to quantify the active fraction of the enzyme. (2) The off-rate for a fluorescein-labeled factor IX propeptide was 3000-fold slower than the rate of carboxylation, a difference that may explain how carboxylase can carry out multiple carboxylations of a substrate during the same binding event. (3) We show evidence that substrate binding to the active site modifies the propeptide binding site of carboxylase. The significant (9-fold) differences in off-rates for the propeptide in the presence and absence of its co-substrates may represent a release mechanism for macromolecular substrates from the enzyme. Additionally, sedimentation velocity and equilibrium experiments indicate a monomeric association of enzyme with propeptide. Furthermore, the carboxylase preparation is monodisperse in the buffer used for our studies.

Enhanced gamma-carboxylation of recombinant factor X using a chimeric construct containing the prothrombin propeptide

Factor Xa is the serine protease component of prothrombinase, the enzymatic complex responsible for thrombin generation. Production of recombinant factor X/Xa has proven to be difficult because of inefficient gamma-carboxylation, a critical post-translational modification. The affinities of the vitamin K-dependent propeptides for the gamma-carboxylase vary over 2 logs, with the propeptide of factor X having the highest affinity followed by the propeptides of factor VII, protein S, factor IX, protein C, and prothrombin [Stanley, T. B. (1999) J. Biol. Chem. 274, 16940-16944]. On the basis of this observation, it was hypothesized that exchanging the propeptide of factor X with one that binds the gamma-carboxylase with a reduced affinity would enhance gamma-carboxylation by allowing greater substrate turnover. A chimeric cDNA consisting of the human prothrombin signal sequence and propeptide followed by mature human factor X was generated and stably transfected into HEK 293 cells, and modified factor X was purified from conditioned medium. The results indicate that on average 85% of the total factor X produced with the prothrombin propeptide was fully gamma-carboxylated, representing a substantial improvement over a system that employs the native factor X propeptide, with which on average only 32% of the protein is fully gamma-carboxylated. These results indicate that the affinity of the gamma-carboxylase for the propeptide greatly influences the extent of gamma-carboxylation. It was also observed that regardless of which propeptide sequence is directing gamma-carboxylation (factor X or prothrombin), two pools of factor X are secreted; one is uncarboxylated and a second is fully gamma-carboxylated, supporting the notion that the gamma-carboxylase is a processive enzyme.

The spasmodic peptide defines a new conotoxin superfamily

We purified and characterized a peptide from the venom of Conus textile that makes normal mice assume the phenotype of a well-known mutant, the spasmodic mouse. This "spasmodic" peptide has 27 amino acids, including two gamma-carboxyglutamate (Gla) residues. A cDNA clone encoding the precursor for the peptide was identified; a gamma-carboxylation recognition signal sequence (gamma-CRS) is present in the -1 --> -20 region of the peptide precursor. Both the gamma-CRS and the position of the Gla residues in the mature toxin are notably different from other Gla-containing conopeptides. The spasmodic peptide has a novel disulfide framework and distinct signal sequence which together define a new P-superfamily of conopeptides. A cDNA encoding another member of the P-superfamily was identified from a different species, Conus gloriamaris.