The effect of O-fucosylation on the first EGF-like domain from human blood coagulation factor VII

The single EGF-like domain of mouse PAMR1 is modified by O-Glucose, O-Fucose and O-GlcNAc

Epidermal growth factor-like domains (EGF-LDs) of membrane and secreted proteins can be modified by N-glycans and/or potentially elongated O-linked monosaccharides such as O-glucose (O-Glc) found at two positions (O-Glc 1 and O-Glc2), O-fucose (O-Fuc) and O-N-acetylglucosamine (O-GlcNAc). The presence of three O-linked sugars within the same EGF-LD, such as in EGF-LD 20 of NOTCH1, has rarely been evidenced. We searched in KEGG GENES database to list mouse and human proteins with an EGF-LD sequence including one, two, three or four potential O-glycosylation consensus sites. Among the 129 murine retrieved proteins, most had predicted O-fucosylation and/or O-GlcNAcylation sites. Around 68% of EGF-LDs were subjected to only one O-linked sugar modification and near 5% to three modifications. Among these latter, we focused on the peptidase domain-containing protein associated with muscle regeneration 1 (PAMR1), having only one EGF-LD. To test the ability of this domain to be glycosylated, a correctly folded EGF-LD was produced in Escherichia coli periplasm, purified and subjected to in vitro incubations with the recombinant O-glycosyltransferases POGLUT1, POFUT1 and EOGT, adding O-Glc1, O-Fuc and O-GlcNAc, respectively. Using click chemistry and mass spectrometry, isolated PAMR1 EGF-LD was demonstrated to be modified by the three O-linked sugars. Their presence was individually confirmed on EGF-LD of full-length mouse recombinant PAMR1, with at least some molecules modified by both O-Glc1 and O-Fuc. Overall, these results are consistent with the presence of a triple O-glycosylated EGF-LD in mouse PAMR1.

Nucleocytoplasmic O-glycosylation in protists

O-Glycosylation is an increasingly recognized modification of intracellular proteins in all kingdoms of life, and its occurrence in protists has been investigated to understand its evolution and its roles in the virulence of unicellular pathogens. We focus here on two kinds of glycoregulation found in unicellular eukaryotes: one is a simple O-fucose modification of dozens if not hundreds of Ser/Thr-rich proteins, and the other a complex pentasaccharide devoted to a single protein associated with oxygen sensing and the assembly of polyubiquitin chains. These modifications are not required for life but contingently modulate biological processes in the social amoeba Dictyostelium and the human pathogen Toxoplasma gondii, and likely occur in diverse unicellular protists. O-Glycosylation that is co-localized in the cytoplasm allows for glycoregulation over the entire life of the protein, contrary to the secretory pathway where glycosylation usually occurs before its delivery to its site of function. Here, we interpret cellular roles of nucleocytoplasmic glycans in terms of current evidence for their effects on the conformation and dynamics of protist proteins, to serve as a guide for future studies to examine their broader significance.

Eptacog beta: a novel recombinant human factor VIIa for the treatment of hemophilia A and B with inhibitors

INTRODUCTION

Hemophilia A and B are X-linked recessive disorders caused by the deficiency of factor VIII or factor IX, respectively. Bleeding episodes are treated with factor replacement therapy. The most serious complication of this treatment is the development of inhibitors. In such patients, bypassing agents, such as activated recombinant human factor VII (rhFVIIa) or plasma-derived activated prothrombin complex concentrates, are administered to prevent or treat bleeding episodes. The high cost of the current bypassing agents limits their availability in emerging countries. Areas covered: Authors reviewed the published data on the development and clinical testing of eptacog beta, a new second-generation rhFVIIa produced in the milk of transgenic rabbits. The available data indicate that activated eptacog beta exhibits structural (N- and O- glycosylation), pharmacodynamic and pharmacokinetic characteristics similar to activated eptacog alfa, its main competitor, but binds slightly better to platelets and HUVEC, and it is safe and effective. Expert commentary: This critical review of available data on activated eptacog beta shows that it represents an alternative source of rhFVIIa at potentially lower cost with easily expandable manufacturing capacity that could contribute to cover the future patient needs.

Differences in N-glycosylation of recombinant human coagulation factor VII derived from BHK, CHO, and HEK293 cells

BACKGROUND &

METHODS

Recombinant factor VII (rFVII), the precursor molecule for recombinant activated FVII (rFVIIa), is, due to its need for complex post translational modifications, produced in mammalian cells. To evaluate the suitability of a human cell line in order to produce rFVII with post-translational modifications as close as possible to pdFVII, we compared the biochemical properties of rFVII synthesized in human embryonic kidney-derived (HEK)293 cells (HEK293rFVII) with those of rFVII expressed in Chinese hamster ovary (CHO, CHOrFVII) and baby hamster kidney (BHK, BHKrFVII) cells, and also with those of plasma derived FVII (pdFVII), using various analytical methods. rFVII was purified from selected production clones derived from BHK, CHO, and HEK293 cells after stable transfection, and rFVII isolates were analyzed for protein activity, impurities and post-translational modifications. RESULTS &

DISCUSSION

The analytical results showed no apparent gross differences between the various FVII proteins, except in their N-linked glycosylation pattern. Most N-glycans found on rFVII produced in HEK293 cells were not detected on rFVII from CHO and BHK cells, or, somewhat unexpectedly, on pdFVII; all other protein features were similar. HEK293rFVII glycans were mainly characterized by a higher structural variety and a lower degree of terminal sialylation, and a high amount of terminal N-acetyl galactosamines (GalNAc). All HEK293rFVII oligosaccharides contained one or more fucoses (Fuc), as well as hybrid and high mannose (Man) structures.

CONCLUSIONS

From all rFVII isolates investigated, CHOrFVII contained the highest degree of sialylation and no terminal GalNAc, and CHO cells were therefore assumed to be the best option for the production of rFVII.

Extension and validation of the GROMOS 53A6(GLYC) parameter set for glycoproteins

An extension of the GROMOS 53A6GLYC force field for carbohydrates to encompass glycoprotein linkages is presented. The set includes new atomic charges and incorporates adequate torsional potential parameters for N-, S-, C-, P-, and O-glycosydic linkages, offering compatibility with the GROMOS force field family for proteins. Validation included the description of glycosydic linkage geometries between amino acid and monosaccharide residues, comparison of NMR-derived protein-carbohydrate and carbohydrate-carbohydrate nuclear overhauser effect (NOE) signals for glycoproteins and the effects of glycosylation on protein flexibility and dynamics.

Roles of glycosylation in Notch signaling

Notch and the DSL Notch ligands Delta and Serrate/Jagged are glycoproteins with a single transmembrane domain. The extracellular domain (ECD) of both Notch receptors and Notch ligands contains numerous epidermal growth factor (EGF)-like repeats which are post-translationally modified by a variety of glycans. Inactivation of a subset of genes that encode glycosyltransferases which initiate and elongate these glycans inhibits Notch signaling. In the formation of developmental boundaries in Drosophila and mammals, in mouse T-cell and marginal zone B-cell development, and in co-culture Notch signaling assays, the regulation of Notch signaling by glycans is to date a cell-autonomous effect of the Notch-expressing cell. The regulation of Notch signaling by glycans represents a new paradigm of signal transduction. O-fucose glycans modulate the strength of Notch binding to DSL Notch ligands, while O-glucose glycans facilitate juxta-membrane cleavage of Notch, generating the substrate for intramembrane cleavage and Notch activation. Identifying precisely how the addition of particular sugars at specific locations on Notch modifies Notch signaling is a challenge for the future.

The influence of different glycosylation patterns on factor VII biological activity

In this study the bioactivity of three differently glycosylated blood coagulation factor VII (FVII) variants (human plasma FVII, recombinant human FVII produced in CHO and BHK cell cultures) were analyzed and compared. Surface plasmon resonance studies of FVII interaction with soluble and full length TF together with FVII autoactivation assays revealed that BHK-derived FVII has the highest bioactivity, while human plasma and CHO-derived FVII showed very similar bioactivity. The affinity of FVII variants to TF correlates with FVII autoactivation rates--the higher the affinity, the faster the autoactivation rate.

Role of unusual O-glycans in intercellular signaling

In the last two decades, our knowledge of the role of glycans in development and signal transduction has expanded enormously. While most work has focused on the importance of N-linked or mucin-type O-linked glycosylation, recent work has highlighted the importance of several more unusual forms of glycosylation that are the focus of this review. In particular, the ability of O-fucose glycans on the epidermal growth factor-like (EGF) repeats of Notch to modulate signaling places glycosylation alongside phosphorylation as a means to modulate protein-protein interactions and their resultant downstream signals. The recent discovery that O-glucose modification of Notch EGF repeats is also required for Notch function has further expanded the range of glycosylation events capable of modulating Notch signaling. The prominent role of Notch during development and in later cell-fate decisions underscores the importance of these modifications in human biology. The role of glycans in intercellular signaling events is only beginning to be understood and appears ready to expand into new areas with the discovery that thrombospondin type 1 repeats are also modified with O-fucose glycans. Finally, a rare form of glycosylation called C-mannosylation modifies tryptophans in some signaling competent molecules and may be a further layer of complexity in the field. We will review each of these areas focusing on the glycan structures produced, the consequence of their presence, and the enzymes responsible.

Methods in enzymology: O-glycosylation of proteins

Cell surface and extracellular proteins are O-glycosylated, where the most abundant type of O-glycosylation in proteins is the GalNAc attachment to serine (Ser) or threonine (Thr) in the protein chain by an a-glycosidic linkage. Most eukaryotic nuclear and cytoplasmic proteins modified by a-linked O-GlcNAc to Ser or Thr exhibit reciprocal O-GlcNAc glycosylation and phosphorylation during the cell cycle, cell stimulation, and/or cell growth. Less-investigated types of O-glycosylation are O-fucosylation, O-mannosylation, and O-glucosylation, but they are functionally of high relevance for early stages of development and for vital physiological functions of proteins. Glycosaminoglycans are a-linked to proteoglycans via a xylose-containing tetrasaccharide, represented by linear chains of repetitive disaccharides modified by carboxylates and O- or/and N-linked sulfates. Analysis of O-glycosylation by mass spectrometry (MS) is a complex task due to the high structural diversity of glycan and protein factors. The parameters in structural analysis of O-glycans include determination of (i) O-glycosylation attachment sites in the protein sequence, (ii) the type of attached monosaccharide moiety, (iii) a core type in the case of GalNAc O-glycosylation, (iv) the type and size of the oligosaccharide portion, (v) carbohydrate branching patterns, (vi) the site of monosaccharide glycosidic linkages, (vii) the anomericity of glycosidic linkages, and (viii) covalent modifications of the sugar backbone chains by carbohydrate- and noncarbohydrate-type of substitutents. Classical and novel analytical strategies for identification and sequencing of O-glycans by MS are described. These include methods to analyze O-glycans after total or partial release from the parent protein by chemical or enzymatic approach or to analyze O-glycosylated peptides by mapping and sequencing from proteolytic mixtures. A recombination process of multiply charged glycopeptides with electrons by electron capture dissociation Fourier transform ion cyclotrone resonance (FTICR)-MS has been introduced and is instrumental for nonergodic polypeptide backbone cleavages without losses of labile glycan substituents. A method for O-glycoscreening under increased sensitivity and efficient sequencing as a combination of an on-line coupling of capillary electrophoresis separation, as well as an automated MS-tandem MS (MS/MS) switching under variable energy conditions collision-induced dissociation (CID) protocol, is beneficial for determination of O-acetylation and oversulfation (Bindila et al., 2004a; Zamfir et al., 2004a). O-glycomics by robotized chip-electrospray/ionization (ESI)-MS and MS/MS on the quadrupole time-of-flight (QTOF) and FTICR analyzers, accurate mass determination, and software for assignment of fragmentation spectra represent essentials for high-throughput (HTP) in serial screenings (Bindila et al., 2004b; Froesch et al., 2004; Vakhrushev et al., 2005). Dimerization of intact O-glycosylated proteins can be investigated by matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF)-MS after blotting.

Glycosylation regulates Notch signalling

Intracellular post-translational modifications such as phosphorylation and ubiquitylation have been well studied for their roles in regulating diverse signalling pathways, but we are only just beginning to understand how differential glycosylation is used to regulate intercellular signalling. Recent studies make clear that extracellular post-translational modifications, in the form of glycosylation, are essential for the Notch signalling pathway, and that differences in the extent of glycosylation are a significant mechanism by which this pathway is regulated.

neurotic, a novel maternal neurogenic gene, encodes an O-fucosyltransferase that is essential for Notch-Delta interactions

Notch signalling, which is highly conserved from nematodes to mammals, plays crucial roles in many developmental processes. In the Drosophila embryo, deficiency in Notch signalling results in neural hyperplasia, commonly referred to as the neurogenic phenotype. We identify a novel maternal neurogenic gene, neurotic, and show that it is essential for Notch signalling. neurotic encodes a Drosophila homolog of mammalian GDP-fucose protein O-fucosyltransferase, which adds fucose sugar to epidermal growth factor-like repeats and is known to play a crucial role in Notch signalling. neurotic functions in a cell-autonomous manner, and genetic epistasis tests reveal that Neurotic is required for the activity of the full-length but not an activated form of Notch. Further, we show that neurotic is required for Fringe activity, which encodes a fucose-specific beta1, 3 N-acetylglucosaminyltransferase, previously shown to modulate Notch receptor activity. Finally, Neurotic is essential for the physical interaction of Notch with its ligand Delta, and for the ability of Fringe to modulate this interaction in Drosophila cultured cells. We present an unprecedented example of an absolute requirement of a protein glycosylation event for a ligand-receptor interaction. Our results suggest that O-fucosylation catalysed by Neurotic is also involved in the Fringe-independent activities of Notch and may provide a novel on-off mechanism that regulates ligand-receptor interactions.

Fucose: biosynthesis and biological function in mammals

Fucose is a deoxyhexose that is present in a wide variety of organisms. In mammals, fucose-containing glycans have important roles in blood transfusion reactions, selectin-mediated leukocyte-endothelial adhesion, host-microbe interactions, and numerous ontogenic events, including signaling events by the Notch receptor family. Alterations in the expression of fucosylated oligosaccharides have also been observed in several pathological processes, including cancer and atherosclerosis. Fucose deficiency is accompanied by a complex set of phenotypes both in humans with leukocyte adhesion deficiency type II (LAD II; also known as congenital disorder of glycosylation type IIc) and in a recently generated strain of mice with a conditional defect in fucosylated glycan expression. Fucosylated glycans are constructed by fucosyltransferases, which require the substrate GDP-fucose. Two pathways for the synthesis of GDP-fucose operate in mammalian cells, the GDP-mannose-dependent de novo pathway and the free fucose-dependent salvage pathway. In this review, we focus on the biological functions of mammalian fucosylated glycans and the biosynthetic processes leading to formation of the fucosylated glycan precursor GDP-fucose.

Regulation of notch signaling by o-linked fucose

Notch and its ligands are modified by a protein O-fucosyltransferase (OFUT1) that attaches fucose to a Serine or Threonine within EGF domains. By using RNAi to decrease Ofut1 expression in Drosophila, we demonstrate that O-linked fucose is positively required for Notch signaling, including both Fringe-dependent and Fringe-independent processes. The requirement for Ofut1 is cell autonomous, in the signal-receiving cell, and upstream of Notch activation. The transcription of Ofut1 is developmentally regulated, and surprisingly, overexpression of Ofut1 inhibits Notch signaling. Together, these results indicate that OFUT1 is a core component of the Notch pathway, which is required for the activation of Notch by its ligands, and whose regulation may contribute to the pattern of Notch activation during development.

Predicted solution structure of zymogen human coagulation FVII

A model solution structure for the complete tissue factor-free calcium ion-bound human zymogen FVII (residues 1-406) (FVII) has been constructed to study possible conformational changes associated with the activation process and tissue factor (TF) binding. The initial structure for the present model was constructed using the X-ray crystallographic structure of human coagulation FVIIa/TF complex bound with calcium ions (Banner et al., Nature 1996, 380, 41-46). This model was subsequently subjected to lengthy molecular dynamics simulations. The Amber force field in conjunction with the PME electrostatic summation method was employed. The estimated TF free solution structure was then compared with the currently available X-ray crystal structures of FVIIa (with or without TF, variable inhibitor bound) to estimate the restructuring of FVII due to TF binding and activation. The solution structure of the zymogen FVII in the absence of TF is predicted to be an extended domain structure similar to that of the TF-bound X-ray crystal structure. An additional extension of the serine protease (SP) domain of the zymogen above a reference lipid surface by approximately 7 A was in agreement with experiment. Significant Gla-EGF1 and EGF1-EGF2 interdomain motions in the zymogen were observed. Carbohydrate dimers attached to Ser-52 and Ser-60 did not cause restructuring in this domain. Minimal restructuring of the SP domain is found upon inference of the zymogen from the activated form. The catalytic triad residues maintain the H-bonded network while Lys-341 occupies the S1 specific site in the zymogen.

C-mannosylation and o-fucosylation of thrombospondin type 1 repeats

The final chemical structure of a newly synthesized protein is often only attained after further covalent modification. Ideally, a comprehensive proteome analysis includes this aspect, a task that is complicated by our incomplete knowledge of the range of possible modifications and often by the lack of suitable analysis methods. Here we present two recently discovered, unusual forms of protein glycosylation, i.e. C-mannosylation and O-fucosylation. Their analysis by a combined mass spectrometric approach is illustrated with peptides from the thrombospondin type 1 repeats (TSRs) of the recombinant axonal guidance protein F-spondin. Nano-electrospray ionization tandem-mass spectrometry of isolated peptides showed that eight of ten Trp residues in the TSRs of F-spondin are C-mannosylated. O-Fucosylation sites were determined by a recently established nano-electrospray ionization quadrupole time-of-flight tandem-mass spectrometry approach. Four of five TSRs carry the disaccharide Hex-dHex-O-Ser/Thr in close proximity to the C-mannosylation sites. In analogy to thrombospondin-1, we assume this to be Glc-Fuc-O-Ser/Thr. Our current knowledge of these glycosylations will be discussed.

Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1

Fringe modulates Notch signaling resulting in the establishment of compartmental boundaries in developing organisms. Fringe is a beta 3N-acetylglucosaminyltransferase (beta 3GlcNAcT) that transfers GlcNAc to O-fucose in epidermal growth factor-like repeats of Notch. Here we use five different Chinese hamster ovary cell glycosylation mutants to identify a key aspect of the mechanism of fringe action. Although the beta 3GlcNAcT activity of manic or lunatic fringe is shown to be necessary for inhibition of Jagged1-induced Notch signaling in a coculture assay, it is not sufficient. Fringe fails to inhibit Notch signaling if the disaccharide generated by fringe action, GlcNAc beta 3Fuc, is not elongated. The trisaccharide, Gal beta 4GlcNAc beta 3Fuc, is the minimal O-fucose glycan to support fringe modulation of Notch signaling. Of six beta 4galactosyltransferases (beta 4GalT) in Chinese hamster ovary cells, only beta 4GalT-1 is required to add Gal to GlcNAc beta 3Fuc, identifying beta 4GalT-1 as a new modulator of Notch signaling.

Modification of epidermal growth factor-like repeats with O-fucose. Molecular cloning and expression of a novel GDP-fucose protein O-fucosyltransferase

The O-fucose modification is found on epidermal growth factor-like repeats of a number of cell surface and secreted proteins. O-Fucose glycans play important roles in ligand-induced receptor signaling. For example, elongation of O-fucose on Notch by the beta1,3-N-acetylglucosaminyltransferase Fringe modulates the ability of Notch to respond to its ligands. The enzyme that adds O-fucose to epidermal growth factor-like repeats, GDP-fucose protein O-fucosyltransferase (O-FucT-1), was purified previously from Chinese hamster ovary (CHO) cells. Here we report the isolation of a cDNA that encodes human O-FucT-1. A probe deduced from N-terminal sequence analysis of purified CHO O-FucT-1 was used to screen a human heart cDNA library and expressed sequence tag and genomic data bases. The cDNA contains an open reading frame encoding a protein of 388 amino acids with a predicted N-terminal transmembrane sequence typical of a type II membrane orientation. Likewise, the mouse homolog obtained from an expressed sequence tag and 5'-rapid amplification of cDNA ends of a mouse liver cDNA library encodes a type II transmembrane protein of 393 amino acids with 90.4% identity to human O-FucT-1. Homologs were also found in Drosophila and Caenorhabditis elegans with 41.2 and 29.4% identity to human O-FucT-1, respectively. The human gene (POFUT1) is on chromosome 20 between PLAGL2 and KIF3B, near the centromere at 20p11. The mouse gene (Pofut1) maps near Plagl2 on a homologous region of mouse chromosome 2. POFUT1 gene transcripts were expressed in all tissues examined, consistent with the widespread localization of the modification. Expression of a soluble form of human O-FucT-1 in insect cells yielded a protein of the predicted molecular weight with O-FucT-1 kinetic and enzymatic properties similar to those of O-FucT-1 purified from CHO cells. The identification of the gene encoding protein O-fucosyltransferase I now makes possible mutational strategies to examine the functions of the unusual O-fucose post-translational modification.

Fucosylation of Cripto is required for its ability to facilitate nodal signaling

O-linked fucose modification is rare and has been shown to occur almost exclusively within epidermal growth factor (EGF)-like modules. We have found that the EGF-CFC family member human Cripto-1 (CR) is modified with fucose and through a combination of peptide mapping, mass spectrometry, and sequence analysis localized the site of attachment to Thr-88. The identification of a fucose modification on human CR within its EGF-like domain and the presence of a consensus fucosylation site within all EGF-CFC family members suggest that this is a biologically important modification in CR, which functionally distinguishes it from the EGF ligands that bind the type 1 erbB growth factor receptors. A single CR point mutation, Thr-88 --> Ala, results in a form of the protein that is not fucosylated and has substantially weaker activity in cell-based CR/Nodal signaling assays, indicating that fucosylation is functionally important for CR to facilitate Nodal signaling.

Fringe is a glycosyltransferase that modifies Notch

Notch receptors function in highly conserved intercellular signalling pathways that direct cell-fate decisions, proliferation and apoptosis in metazoans. Fringe proteins can positively and negatively modulate the ability of Notch ligands to activate the Notch receptor. Here we establish the biochemical mechanism of Fringe action. Drosophila and mammalian Fringe proteins possess a fucose-specific beta1,3 N-acetylglucosaminyltransferase activity that initiates elongation of O-linked fucose residues attached to epidermal growth factor-like sequence repeats of Notch. We obtained biological evidence that Fringe-dependent elongation of O-linked fucose on Notch modulates Notch signalling by using co-culture assays in mammalian cells and by expression of an enzymatically inactive Fringe mutant in Drosophila. The post-translational modification of Notch by Fringe represents a striking example of modulation of a signalling event by differential receptor glycosylation and identifies a mechanism that is likely to be relevant to other signalling pathways.

Mammalian Notch1 is modified with two unusual forms of O-linked glycosylation found on epidermal growth factor-like modules

Notch is a large cell-surface receptor known to be an essential player in a wide variety of developmental cascades. Here we show that Notch1 endogenously expressed in Chinese hamster ovary cells is modified with O-linked fucose and O-linked glucose saccharides, two unusual forms of O-linked glycosylation found on epidermal growth factor-like (EGF) modules. Interestingly, both modifications occur as monosaccharide and oligosaccharide species. Through exoglycosidase digestions we determined that the O-linked fucose oligosaccharide is a tetrasaccharide with a structure identical to that found on human clotting factor IX: Sia-alpha2,3-Gal-beta1, 4-GlcNAc-beta1,3-Fuc-alpha1-O-Ser/Thr. The elongated form of O-linked glucose appears to be a trisaccharide. Notch1 is the first membrane-associated protein identified with either O-linked fucose or O-linked glucose modifications. It also represents the second protein discovered with an elongated form of O-linked fucose. The sites of glycosylation, which fall within the multiple EGF modules of Notch, are highly conserved across species and within Notch homologs. Since Notch is known to interact with its ligands through subsets of EGF modules, these results suggest that the O-linked carbohydrate modifications of these modules may influence receptor-ligand interactions.

Calcium-binding EGF-like modules in coagulation proteinases: function of the calcium ion in module interactions

Epidermal growth factor (EGF)-like modules are involved in protein-protein interactions and are found in numerous extracellular proteins and membrane proteins. Among these proteins are enzymes involved in blood coagulation, fibrinolysis and the complement system as well as matrix proteins and cell surface receptors such as the EGF precursor, the low density lipoprotein receptor and the developmentally important receptor, Notch. The coagulation enzymes, factors VII, IX and X and protein C, all have two EGF-like modules, whereas the cofactor of activated protein C, protein S, has four EGF-like modules in tandem. Certain of the cell surface receptors have numerous EGF modules in tandem. A subset of EGF modules bind one Ca(2+). The Ca(2+)-binding sequence motif is coupled to a sequence motif that brings about beta-hydroxylation of a particular Asp/Asn residue. Ca(2+)-binding to an EGF module is important to orient neighboring modules relative to each other in a manner that is required for biological activity. The Ca(2+) affinity of an EGF module is often influenced by its N-terminal neighbor, be it another EGF module or a module of another type. This can result in an increase in Ca(2+) affinity of several orders of magnitude. Point mutations in EGF modules that involve amino acids which are Ca(2+) ligands result in the biosynthesis of biologically inactive proteins. Such mutations have been identified, for instance, in factor IX, causing hemophilia B, in fibrillin, causing Marfan syndrome, and in the low density lipoprotein receptor, causing hypercholesterolemia. In this review the emphasis will be on the coagulation factors.

EGF-like module pair 3-4 in vitamin K-dependent protein S: modulation of calcium affinity of module 4 by module 3, and interaction with factor X

Calcium-binding epidermal growth factor (EGF)-like modules are found in numerous extracellular and membrane proteins involved in such diverse processes as blood coagulation, lipoprotein metabolism, determination of cell fate, and cell adhesion. Vitamin K-dependent protein S, a cofactor of the anticoagulant enzyme activated protein C, has four EGF-like modules in tandem with the three C-terminal modules each harbouring a Ca(2+)-binding consensus sequence. Recombinant fragments containing EGF modules 1-4 and 2-4 have two Ca(2+)-binding sites with dissociation constants ranging from 10(-8) to 10(-5) M. Module-module interactions that greatly influence the Ca(2+) affinity of individual modules have been identified. As a step towards an analysis of the structural basis of the high Ca(2+) affinity, we expressed the Ca(2+)-binding EGF pair 3-4 from human protein S. Correct folding was shown by (1)H NMR spectroscopy. Calcium-binding properties of the C-terminal module were determined by titration with chromophoric chelators; binding to the low-affinity N-terminal site was monitored by (1)H-(15)N NMR spectroscopy. At physiological pH and ionic strength, the dissociation constants for Ca(2+) binding were 1.0x10(-6) M and 4. 8x10(-3) M for modules 4 and 3, respectively, i.e. the calcium affinity of the C-terminal site was about 5000-fold higher than that of the N-terminal site. Moreover, the Ca(2+) affinity of EGF 4, in the pair 3-4, was about 9000-fold higher than that of synthetic EGF 4. The EGF modules in protein S are known to mediate the interaction with factor Xa. We have now found modules 3-4 to be involved in this interaction. However, the individual modules 3 and 4 manifested no measurable activity.