Electron microscopy and hydrodynamic properties of blood clotting factor V and activation fragments of factor V with phospholipid vesicles

Restoring the procofactor state of factor Va-like variants by complementation with B-domain peptides

Coagulation factor V (FV) circulates as an inactive procofactor and is activated to FVa by proteolytic removal of a large inhibitory B-domain. Conserved basic and acidic sequences within the B-domain appear to play an important role in keeping FV as an inactive procofactor. Here, we utilized recombinant B-domain fragments to elucidate the mechanism of this FV autoinhibition. We show that a fragment encoding the basic region (BR) of the B-domain binds with high affinity to cofactor-like FV(a) variants that harbor an intact acidic region. Furthermore, the BR inhibits procoagulant function of the variants, thereby restoring the procofactor state. The BR competes with FXa for binding to FV(a), and limited proteolysis of the B-domain, specifically at Arg(1545), ablates BR binding to promote high affinity association between FVa and FXa. These results provide new insight into the mechanism by which the B-domain stabilizes FV as an inactive procofactor and reveal how limited proteolysis of FV progressively destabilizes key regulatory regions of the B-domain to produce an active form of the molecule.

A bipartite autoinhibitory region within the B-domain suppresses function in factor V

Activation of blood coagulation factor V (FV) is a key reaction of hemostasis. FV circulates in plasma as an inactive procofactor, and proteolytic removal of a large central B-domain converts it to an active cofactor (FVa) for factor Xa (FXa). Here we show that two short evolutionary conserved segments of the B-domain, together termed the procofactor regulatory region, serve an essential autoinhibitory function. This newly identified motif consists of a basic (963-1008) and an acidic (1493-1537) region and defines the minimal sequence requirements to maintain FV as a procofactor. Our data suggest that dismantling this autoinhibitory region via deletion or proteolysis is the driving force to unveil a high affinity binding site(s) for FXa. These findings document an unexpected sequence-specific role for the B-domain by negatively regulating FV function and preventing activity of the procofactor. These new mechanistic insights point to new ways in which the FV procofactor to cofactor transition could be modulated to alter hemostasis.

The molecular basis of factor V and VIII procofactor activation

Activation of precursor proteins by specific and limited proteolysis is a hallmark of the hemostatic process. The homologous coagulation factors (F)V and FVIII circulate in an inactive, quiescent state in blood. In this so-called procofactor state, these proteins have little, if any procoagulant activity and do not participate to any significant degree in their respective macromolecular enzymatic complexes. Thrombin is considered a key physiological activator, cleaving select peptide bonds in FV and FVIII which ultimately leads to appropriate structural changes that impart cofactor function. As the active cofactors (FVa and FVIIIa) have an enormous impact on thrombin and FXa generation, maintaining FV and FVIII as inactive procofactors undoubtedly plays an important regulatory role that has likely evolved to maintain normal hemostasis. Over the past three decades there has been widespread interest in studying the proteolytic events that lead to the activation of these proteins. While a great deal has been learned, mechanistic explanations as to how bond cleavage facilitates conversion to the active cofactor species remain incompletely understood. However, recent advances have been made detailing how thrombin recognizes FV and FVIII and also how the FV B-domain plays a dominant role in maintaining the procofactor state. Here we review our current understanding of the molecular process of procofactor activation with a particular emphasis on FV.

Inhibitory sequences within the B-domain stabilize circulating factor V in an inactive state

Blood coagulation factor V circulates as a procofactor with little or no procoagulant activity. It is activated to factor Va by thrombin following proteolytic removal of a large central B-domain. Although this reaction is well studied, the mechanism by which bond cleavage and B-domain release facilitate the transition to the active cofactor state has not been defined. Here we show that deletion or substitution of specific B-domain sequences drives the expression of procoagulant function without the need for proteolytic processing. Conversion to the constitutively active cofactor state is related, at least in part, to a cluster of amino acids that is highly basic and well conserved across the vertebrate lineage. Our findings demonstrate that discrete sequences in the B-domain serve to stabilize the inactive procofactor state, with proteolysis primarily functioning to remove these inhibitory constraints. These unexpected results provide new insight into the mechanism of factor V activation.

Partial glycosylation of Asn2181 in human factor V as a cause of molecular and functional heterogeneity. Modulation of glycosylation efficiency by mutagenesis of the consensus sequence for N-linked glycosylation

Coagulation factor V (FV) circulates in two forms, FV1 and FV2, having slightly different molecular masses and phospholipid-binding properties. The aim was to determine whether this heterogeneity is due to the degree of glycosylation of Asn(2181). FVa1 and FVa2 were isolated and digested with endoglycosidase PNGase F. As judged by Western blotting, the FVa2 light chain contained two N-linked carbohydrates, whereas FVa1 contained three. Wild-type FV and three mutants, Asn(2181)Gln, Ser(2183)Thr, and Ser(2183)Ala, were expressed in COS1 cells, activated by thrombin, and analyzed by Western blotting. Wild-type FVa contained the 71 kDa-74 kDa doublet, whereas the Asn(2181)Gln and Ser(2183)Ala mutants contained only the 71 kDa light chain. In contrast, the Ser(2183)Thr mutant gave a 74 kDa light chain. This demonstrated that the third position in the Asn-X-Ser/Thr consensus affects glycosylation efficiency, Thr being associated with a higher degree of glycosylation than Ser. The Ser(2183)Thr mutant FVa was functionally indistinguishable from plasma-purified FVa1, whereas Asn(2181)Gln and Ser(2183)Ala mutants behaved like FVa2. Thus, the carbohydrate at Asn(2181) impaired the interaction between FVa and the phospholipid membrane, an interpretation consistent with a structural analysis of a three-dimensional model of the C2 domain and the position of a proposed phospholipid-binding site. In conclusion, we show that the FV1-FV2 heterogeneity is caused by differential glycosylation of Asn(2181) related to the presence of a Ser rather than a Thr at the third position in the consensus sequence of glycosylation.

The C-terminal region of the factor V B-domain is crucial for the anticoagulant activity of factor V

Factor V (FV) is recently shown to express anticoagulant activity. It functions as a synergistic cofactor with protein S to activated protein C (APC) in the degradation of factor VIIIa (FVIIIa). FV is composed of multiple domains, A1-A2-B-A3-C1-C2. Thrombin cleaves FV at Arg-709, Arg-1018, and Arg-1545 that leads to the generation of a procoagulant FV species which functions as a cofactor to factor Xa (FXa) in the activation of prothrombin to thrombin. During the activation process, the B-domain is released from the heavy (A1-A2) and light chains (A3-C1-C2) which constitute the active FV (FVa). To elucidate which effect the different thrombin cleavages in FV have on the ability of FV to express APC-cofactor activity, seven recombinant FV mutants containing all possible combinations of mutated and native thrombin cleavage sites were tested in a FVIIIa degradation assay. Thrombin cleavage at Arg-709 and/or Arg-1018 yielded FV molecules that were still able to function as APC cofactors, whereas cleavage at Arg-1545 led to a complete loss in APC-cofactor function. This suggests that the APC-cofactor function of FV depends on the B-domain remaining attached to the A3 domain. The importance of the FV B-domain for expression of APC-cofactor activity was further investigated using two B-domain deleted FV molecules, FV des-709-1545 (with the whole B-domain deleted) and FV des-709-1476 (with amino acids 710-1476 of the B-domain being removed). FV des-709-1476 expressed APC-cofactor activity, whereas the FV des-709-1545 was completely devoid of such activity. Thus, the C-terminal part of the B-domain (residues 1477-1545) was crucial for the APC-cofactor function. FV and factor VIII (FVIII) are homologous proteins having similar domain organization. A FV/FVIII chimera, harboring the B-domain from FVIII (FVBVIII) instead of the FV B-domain did not work as an APC cofactor, further illustrating the importance of the FV B-domain for the APC-cofactor function.

Structural investigation of the A domains of human blood coagulation factor V by molecular modeling

Factor V (FV) is a large (2,196 amino acids) nonenzymatic cofactor in the coagulation cascade with a domain organization (A1-A2-B-A3-C1-C2) similar to the one of factor VIII (FVIII). FV is activated to factor Va (FVa) by thrombin, which cleaves away the B domain leaving a heterodimeric structure composed of a heavy chain (A1-A2) and a light chain (A3-C1-C2). Activated protein C (APC), together with its cofactor protein S (PS), inhibits the coagulation cascade via limited proteolysis of FVa and FVIIIa (APC cleaves FVa at residues R306, R506, and R679). The A domains of FV and FVIII share important sequence identity with the plasma copper-binding protein ceruloplasmin (CP). The X-ray structure of CP and theoretical models for FVIII have been recently reported. This information allowed us to build a theoretical model (994 residues) for the A domains of human FV/FVa (residues 1-656 and 1546-1883). Structural analysis of the FV model indicates that: (a) the three A domains are arranged in a triangular fashion as in the case of CP and the organization of these domains should remain essentially the same before and after activation; (b) a Type II copper ion is located at the A1-A3 interface; (c) residues R306 and R506 (cleavage sites for APC) are both solvent exposed; (d) residues 1667-1765 within the A3 domain, expected to interact with the membrane, are essentially buried; (e) APC does not bind to FVa residues 1865-1874. Several other features of factor V/Va, like the R506Q and A221V mutations; factor Xa (FXa) and human neutrophil elastase (HNE) cleavages; protein S, prothrombin and FXa binding, are also investigated.

Structure of membrane-bound human factor Va

Coagulation factor Va is an essential cofactor which combines with the serine protease factor Xa on a phospholipid surface to form the prothrombinase complex. In the present study, the structure of factor Va interacting with lipid surfaces containing phosphatidylserine was studied by electron microscopy. Two-dimensional crystals of factor Va were obtained on planar lipid films under quasi-physiological conditions. The two-dimensional projected structure of factor Va was calculated at a resolution of 2 nm, revealing dimers of factor Va arranged on the surface lattice with the symmetry of the plane group p2. Average unit cell dimensions are a = 14.4 nm, b = 8.8 nm, gamma = 107 degrees. Each factor Va molecule presents two distinct domains of protein density consisting of one small domain, of 3 nm in diameter, connected to a larger domain of about 6 nm x 4.5 nm. The projected structure of factor Va covers an area equivalent to about fifty phospholipid molecules. In addition, edge-on views of factor Va molecules bound to liposomes reveal a globular structure connected through a thin stem to the liposome surface. A three-dimensional model of membrane-bound factor Va is proposed.

Factor V: a prototype pro-cofactor for vitamin K-dependent enzyme complexes in blood clotting

The relative abundance of factor V, factor X and prothrombin has enabled detailed analyses of the prothrombinase complex. Determination of the primary structure for factor V has provided the basis for examination of structure-function relationships. The imminent in vitro expression of recombinant factor V will provide the opportunity for site-specific mutagenesis and a verification of these structure-function relationships. A comparison of the physical properties and primary structures for factors V and VIII has revealed extensive similarities in these two cofactor proteins. This observation indicates that a direct application of the technology developed for the analysis of prothrombinase will lead to an equal understanding of the factor Xase complex. Whether similar relationships exist for other blood coagulation enzyme complexes remains to be determined.

Warfarin and the biochemistry of the vitamin K dependent proteins

We have reviewed the biochemistry of the normal and warfarinized vitamin K-dependent coagulation proteins, both individually and as members of their respective macromolecular enzyme complexes. Much has been learned in the past 10-15 years about the molecular mechanisms of coagulation. However, we have only scratched the surface and are beset by a number of interesting challenges. Clearly, we must begin to dissect the other vitamin K-dependent macromolecular enzyme complexes as has been done for prothrombinase. Prothrombin differs in fundamental ways from the other vitamin K-dependent proteins and we can anticipate significant differences as well as similarities in the other complexes. The presence of potentially functional domains in the non-catalytic portions of the vitamin K-dependent proteins and their similarities within this family of proteins should prove a fruitful area of future research. Finally, the evaluation of warfarinized vitamin K-dependent proteins, both in fundamental studies with purified systems and using uniquely engineered immunoassays, should shed light on basic molecular mechanisms and modes of monitoring warfarin therapy.

A domain of membrane-bound blood coagulation factor Va is located far from the phospholipid surface. A fluorescence energy transfer measurement

The larger subunit of blood coagulation factor Va was covalently labeled with iodoacetamido derivatives of fluorescein and rhodamine without loss of functional activity, as measured by either the one-stage clotting assay or the ability to accelerate prothrombin activation in a purified system. The spectral properties of the dyes were not altered by the presence or absence of the smaller subunit of factor Va, Ca2+, prothrombin, factor Xa, or phosphatidylcholine/phosphatidylserine (PC/PS, 4:1) vesicles. When fluorescein-labeled protein (factor VaF) was titrated with PC/PS vesicles containing either octadecylrhodamine or 5-(N-hexadecanoylamino)eosin, fluorescence energy transfer was observed between the protein-bound donor dyes and the acceptor dyes at the outer surface of the phospholipid bilayer. The extent of energy transfer correlated directly with the extent of protein binding to the vesicles monitored by light scattering. The distance of closest approach between the fluorescein on factor Va and the bilayer surface averaged 90 A for the two different acceptors. Association of factor VaF with factor Xa on the phospholipid surface reduced this separation by 7 A, but association with prothrombin did not alter the distance between the labeled domain on factor VaF and the surface. The efficiency of diffusion-enhanced energy transfer between rhodamine-labeled factor Va and terbium dipicolinate entrapped inside PC/PS vesicles was less than 0.01, consistent with the location of the dye far above the inner surface of the vesicle. Thus, a domain of membrane-bound factor Va is located a minimum of 90 A above the phospholipid surface.(ABSTRACT TRUNCATED AT 250 WORDS)

Components of purified sarcolemma from porcine skeletal muscle

Sarcolemmal membranes were isolated from porcine skeletal muscle by modifications of a LiBr-extraction technique. Latency determinations of acetylcholinesterase, ouabain-sensitive p-nitrophenylphosphatase, [3H]ouabain binding, and (Na+ + K+)-ATPase activities indicated that 65-76% of the membranes were sealed inside-out vesicles. The preparations were enriched in cholesterol and phospholipid, and demonstrated adenylate cyclase activity and both cAMP and cGMP phosphodiesterase activities. An indication of the purity of this fraction was that the Ca2+-ATPase activity (0.13 mumol Pi mg-1 min-1 at 37 degrees C) was 3.8% of that of porcine skeletal muscle sarcoplasmic reticulum preparations. Pertussis toxin specifically catalyzed the ADP-ribosylation of a Mr 41,000 sarcolemmal protein, indicating the presence of the inhibitory guanine nucleotide regulatory protein of adenylate cyclase, Ni. An endogenous ADP-ribosyltransferase activity, with several membrane protein substrates, was also demonstrated. The addition of exogenous cAMP-dependent protein kinase or calmodulin promoted the phosphorylation of a number of sarcolemmal proteins. The calmodulin-dependent phosphorylation exhibited an approximate K 1/2 for Ca2+ of 0.5 microM, and an approximate K 1/2 for calmodulin of 0.1 microM. 125I-Calmodulin affinity labeling of the sarcolemma, using dithiobis(succinimidyl propionate), demonstrated the presence of Mr 160,000 and 280,000 calmodulin-binding components in these membranes. These results demonstrate that this porcine preparation will be valuable in the study of skeletal muscle sarcolemmal ion transport, protein and hormonal receptors, and protein kinase-catalyzed phosphorylation.