Contact with the N termini in the central E domain enhances the reactivities of the distal D domains of fibrin to factor XIIIa

Temporal and spatial regulation of protein cross-linking by the pre-assembled substrates of a Bacillus subtilis spore coat transglutaminase

In many cases protein assemblies are stabilized by covalent bonds, one example of which is the formation of intra- or intermolecular ε-(γ-glutamyl)lysil cross-links catalyzed by transglutaminases (TGases). Because of the potential for unwanted cross-linking reactions, the activities of many TGases have been shown to be tightly controlled. Bacterial endospores are highly resilient cells in part because they are surrounded by a complex protein coat. Proteins in the coat that surrounds Bacillus subtilis endospores are crosslinked by a TGase (Tgl). Unlike other TGases, however, Tgl is produced in an active form, and efficiently catalyzes amine incorporation and protein cross-linking in vitro with no known additional requirements. The absence of regulatory factors raises questions as to how the activity of Tgl is controlled during spore coat assembly. Here, we show that substrates assembled onto the spore coat prior to Tgl production govern the localization of Tgl to the surface of the developing spore. We also show that Tgl residues important for substrate recognition are crucial for its localization. We identified the glutamyl (Q) and lysil (K) substrate docking sites and we show that residues on the Q side of Tgl are more important for the assembly of Tgl than those on the K side. Thus, the first step in the reaction cycle, the interaction with Q-substrates and formation of an acyl-enzyme intermediate, is also the determinant step in the localization of Tgl. Consistent with the idea that Tg exerts a “spotwelding” activity, cross-linking pre-formed assemblies, we show that C30 is an oblong hexamer in solution that is cross-linked in vitro into high molecular weight forms. Moreover, during the reaction, Tgl becomes part of the cross-linked products. We suggest that the dependency of Tgl on its substrates is used to accurately control the time, location and extent of the enzyme´s activity, directed at the covalent fortification of pre-assembled complexes at the surface of the developing spore.

The orderly recruitment of proteins during the assembly of complex macromolecular structures poses challenges throughout cell biology. During endospore development in the bacterium Bacillus subtilis at least 80 proteins synthesized in the mother cell are assembled around the developing spore to form a protective coat. Regulation of coat gene expression has been described in detail but it is unknown how the information encoded by the structures of the proteins guide their assembly. We have examined the assembly of a transglutaminase, Tgl, which introduces ε-(γ-glutamyl)lysil cross-links in coat protein substrates. We describe with molecular detail a substrate-driven assembly model that directs the enzyme to the locations of its substrates where, as we suggest, it exerts a “spotwelding” activity to fortify pre-assembled complexes. The catalytic cysteine, located in a tunnel that spans the Tgl structure, first forms an acyl enzyme intermediate with a glutamine (Q) donor substrate. Then, it engages a lysine (K) donor substrate to form the cross-linked product. We have identified the Q and K acceptor ends of the Tgl tunnel, and we show that substitutions in substrate recognition residues at the Q side impair assembly more strongly than at the K side. Thus, assembly of Tgl parallels its catalytic cycle, directing the enzyme to the pre-formed complexes that are to be cross-linked.

The interaction between fibrinogen and zymogen FXIII-A2B2 is mediated by fibrinogen residues γ390-396 and the FXIII-B subunits

Coagulation transglutaminase factor XIII (FXIII) exists in circulation as heterotetrameric proenzyme FXIII-A₂B₂ Effectively all FXIII-A₂B₂ circulates bound to fibrinogen, and excess FXIII-B₂ circulates in plasma. The motifs that mediate interaction of FXIII-A₂B₂ with fibrinogen have been elusive. We recently detected reduced binding of FXIII-A₂B₂ to murine fibrinogen that has γ-chain residues 390-396 mutated to alanines (Fibγ^390-396A). Here, we evaluated binding features using human components, including recombinant fibrinogen variants, FXIII-A₂B₂, and isolated FXIII-A₂ and -B₂ homodimers. FXIII-A₂B₂ coprecipitated with wild-type (γA/γA), alternatively-spliced (γ'/γ'), and αC-truncated (Aα251) fibrinogens, whereas coprecipitation with human Fibγ^390-396A was reduced by 75% (P <0001). Surface plasmon resonance showed γA/γA, γ'/γ', and Aα251 fibrinogens bound FXIII-A₂B₂ with high affinity (nanomolar); however, Fibγ^390-396A did not bind FXIII-A₂B₂ These data indicate fibrinogen residues γ390-396 comprise the major binding motif for FXIII-A₂B₂ Compared with γA/γA clots, FXIII-A₂B₂ activation peptide release was 2.7-fold slower in Fibγ^390-396A clots (P < .02). Conversely, activation of recombinant FXIII-A₂ (lacking FXIII-B₂) was similar in γA/γA and Fibγ^390-396A clots, suggesting fibrinogen residues γ390-396 accelerate FXIII-A₂B₂ activation in a FXIII-B₂-dependent mechanism. Recombinant FXIII-B₂ bound γA/γA, γ'/γ', and Aα251 with similar affinities as FXIII-A₂B₂, but did not bind or coprecipitate with Fibγ^390-396A FXIII-B₂ also coprecipitated with fibrinogen from FXIII-A-deficient mouse and human plasmas. Collectively, these data indicate that FXIII-A₂B₂ binds fibrinogen residues γ390-396 via the B subunits, and that excess plasma FXIII-B₂ is not free, but rather circulates bound to fibrinogen. These findings provide insight into assembly of the fibrinogen/FXIII-A₂B₂ complex in both physiologic and therapeutic situations.

Covalent structure of single-stranded fibrin oligomers cross-linked by FXIIIa

FXIIIa-mediated isopeptide γ-γ bonds are produced between γ polypeptide chains of adjacent monomeric fibrin. Despite the use of the different methodological approaches there are apparently conflicting ideas regarding the orientation of γ-γ bonds. To identify the orientation of these bonds a novel approach has been applied. It was based on self-assembly of soluble cross-linked fibrin protofibrils ongoing in the urea solution of moderate concentrations followed by dissociation of protofibrils in the conditions of increasing urea concentration. The oligomers were composed of monomeric desA fibrin molecules created by cleavage of the fibrinopeptides A from fibrinogen molecules with thrombin-like enzyme, reptilase. The results of elastic and dynamic light scattering coupled with analytical ultracentrifugation indicated an emergence of the double-stranded rod-like fibrin protofibrils. For the first time, the protofibrils are proved to exhibit an ability to dissociate under increasing urea concentration to yield single-stranded structures. Since no accumulation of α polymers has been found the covalent structure of soluble single-stranded fibrin oligomers is entirely brought about by γ-γ bonds. The results of this study provide an extra evidence to support the model of the longitudinal γ-γ bonds that form between the γ chains end-to-end within the same strand of a protofibril.

Ozone-induced oxidative modification of fibrinogen: role of the D regions

Native fibrinogen is a key blood plasma protein whose main function is to maintain hemostasis by virtue of producing cross-linked fibrin clots under the influence of thrombin and fibrin-stabilizing factor (FXIIIa). The aim of this study was to investigate mechanisms of impairment of both the molecular structure and the spatial organization of fibrinogen under ozone-induced oxidation. FTIR analysis showed that ozone treatment of the whole fibrinogen molecule results in the growth of hydroxyl, carbonyl, and carboxyl group content. A similar analysis of fibrinogen D and E fragments isolated from the oxidized protein also revealed transformation of distinct important functional groups. In particular, a remarkable decay of N-H groups within the peptide backbone was observed along with a lowering of the content of C-H groups belonging to either the aromatic moieties or the aliphatic chain CH2 and CH3 units. The model experiments performed showed that the rather unexpected decay of the aliphatic CH units might be caused by the action of hydroxyl radicals, these being produced in the water solution from ozone. The observed dissimilarities in the shapes of amide I bands of the fibrinogen D and E fragments before and after ozone treatment are interpreted in terms of feasible local conformational changes affecting the secondary structure of the protein. Taken as a whole, the FTIR data suggests that the terminal D fragments of fibrinogen are markedly more susceptible to the ozone-induced oxidation than the central E fragment. The data on elastic and dynamic light scattering provide evidence that, in the presence of FXIIIa, both the unoxidized and the oxidized fibrinogen molecules bind to one another in an "end-to-end" fashion to form the flexible covalently cross-linked fibrinogen homopolymers. The γ and α polypeptide chains of the oxidized fibrinogen proved to be involved in the enzymatic cross-linking more readily than those of unaffected fibrinogen. The experimental data on fibrinogen oxidation acquired in the present study, combined with our earlier findings, make it reasonable to suppose that the spatial structure of fibrinogen could be evolutionarily adapted to some reactive oxygen species actions detrimental to the protein function.

Modulation of fibrin matrix properties via knob:hole affinity interactions using peptide-PEG conjugates

Fibrin is a widely used biological scaffold in tissue engineering and regenerative medicine. While the polymerization dynamics from its soluble precursor fibrinogen has been studied for decades, few attempts have been made to modulate fibrin network structure through the addition of external agents that actively engage this process. We propose the use of polyethylene glycol (PEG)-based linkers that interact with fibrinogen via knob:hole affinity interactions as a means of controlling thrombin-mediated fibrin polymerization dynamics and resulting network structure. Using bivalent and tetravalent knob-PEG conjugates with sizes ranging from 2 to 20 kDa, we demonstrate that the clotting characteristics of fibrinogen solutions can be altered in a dose-dependent manner, with conjugate size playing a major role in altering fibrin network structure. Interestingly, factor XIIIa-catalyzed fibrin(ogen) crosslinking and plasmin-mediated degradation were not significantly impacted. This work demonstrates the feasibility of modulating fibrin network structure through the addition of knob-PEG conjugates that perturb the polymerization process through non-covalent knob:hole interactions.

Probing interactions between the coagulants thrombin, Factor XIII, and fibrin(ogen)

Thrombin cleaves fibrinopeptides A and B from fibrinogen leading to the formation of a fibrin network that is later covalently crosslinked by Factor XIII (FXIII). Thrombin helps activate FXIII by catalyzing hydrolysis of the FXIII activation peptides (AP). In the current work, the role of exosites in the ternary thrombin-FXIII-fibrin(ogen) complex was further explored. Hydrolysis studies indicate that thrombin predominantly utilizes its active site region to bind extended Factor XIII AP (FXIII AP 33-64 and 28-56) leaving the anion-binding exosites for fibrin(ogen) binding. The presence of fibrin-I leads to improvements in the K(m) for hydrolysis of FXIII AP (28-41), whereas peptides based on the cardioprotective FXIII V34L sequence exhibit less reliance on this cofactor. Surface plasmon resonance measurements reveal that d-Phe-Pro-Arg-chloromethylketone-thrombin binds to fibrinogen faster than to FXIII a(2) and dissociates from fibrinogen more slowly than from FXIII a(2). This system of thrombin exosite interactions with differing affinities promotes efficient clot formation.

Fibrinogen and fibrin structure and functions

Fibrinogen molecules are comprised of two sets of disulfide-bridged Aalpha-, Bbeta-, and gamma-chains. Each molecule contains two outer D domains connected to a central E domain by a coiled-coil segment. Fibrin is formed after thrombin cleavage of fibrinopeptide A (FPA) from fibrinogen Aalpha-chains, thus initiating fibrin polymerization. Double-stranded fibrils form through end-to-middle domain (D:E) associations, and concomitant lateral fibril associations and branching create a clot network. Fibrin assembly facilitates intermolecular antiparallel C-terminal alignment of gamma-chain pairs, which are then covalently 'cross-linked' by factor XIII ('plasma protransglutaminase') or XIIIa to form 'gamma-dimers'. In addition to its primary role of providing scaffolding for the intravascular thrombus and also accounting for important clot viscoelastic properties, fibrin(ogen) participates in other biologic functions involving unique binding sites, some of which become exposed as a consequence of fibrin formation. This review provides details about fibrinogen and fibrin structure, and correlates this information with biological functions that include: (i) suppression of plasma factor XIII-mediated cross-linking activity in blood by binding the factor XIII A2B2 complex. (ii) Non-substrate thrombin binding to fibrin, termed antithrombin I (AT-I), which down-regulates thrombin generation in clotting blood. (iii) Tissue-type plasminogen activator (tPA)-stimulated plasminogen activation by fibrin that results from formation of a ternary tPA-plasminogen-fibrin complex. Binding of inhibitors such as alpha2-antiplasmin, plasminogen activator inhibitor-2, lipoprotein(a), or histidine-rich glycoprotein, impairs plasminogen activation. (iv) Enhanced interactions with the extracellular matrix by binding of fibronectin to fibrin(ogen). (v) Molecular and cellular interactions of fibrin beta15-42. This sequence binds to heparin and mediates platelet and endothelial cell spreading, fibroblast proliferation, and capillary tube formation. Interactions between beta15-42 and vascular endothelial (VE)-cadherin, an endothelial cell receptor, also promote capillary tube formation and angiogenesis. These activities are enhanced by binding of growth factors like fibroblast growth factor-2 (FGF-2) and vascular endothelial growth factor (VEGF), and cytokines like interleukin (IL)-1. (vi) Fibrinogen binding to the platelet alpha(IIb)beta3 receptor, which is important for incorporating platelets into a developing thrombus. (vii) Leukocyte binding to fibrin(ogen) via integrin alpha(M)beta2 (Mac-1), which is a high affinity receptor on stimulated monocytes and neutrophils.

Deciphering the physiological pathway of clotting of fibrinogen in blood plasma

I have been fortunate to have benefited over the years from the friendship and advice of John Ferry in our research to decipher the physiological reactions and regulatory events involved in the clotting of fibrinogen in blood. The article is a tribute to the memory of this creative scientist and remarkable individual.

Fibrinogen gamma chain functions

This review covers the functional features of the fibrinogen gamma chains including their participation in fibrin polymerization and cross-linking, their role in the initiation of fibrinolysis, their binding and regulation of factor XIII activity, their interactions with platelets and other cells, and their role in mediating thrombin binding to fibrin, a thrombin inhibitory function termed 'antithrombin I'.

Analysis of engineered fibrinogen variants suggests that an additional site mediates platelet aggregation and that "B-b" interactions have a role in protofibril formation

The C-terminal domain of the fibrinogen gamma-chain includes multiple functional sites that have been defined in high-resolution structures and biochemical assays. Calcium binds to this domain through the side chains of gammaD318 and gammaD320 and the backbone carbonyls of gammaF322 and gammaG324. We have examined variant fibrinogens with alanine at position gamma318 and/or gamma320 and found that calcium binding, fibrin polymerization, and fibrinogen-mediated platelet aggregation, but not FXIIIa-catalyzed cross-linking, were abnormal. When measured by turbidity, thrombin-catalyzed polymerization was severely reduced, and batroxobin-catalyzed polymerization was completely obliterated. Moreover, thrombin-catalyzed polymerization was abolished by the peptide GHRP, which binds to the polymerization site in the beta-chain but does not inhibit polymerization of normal fibrinogen. ADP-induced platelet aggregation was also severely impaired. In contrast, as measured by SDS-PAGE, FXIIIa introduced cross-links between gamma-chains for all three variants, as expected if the gamma-chain C-terminal sites were normal. In addition, binding of the monoclonal antibody 4A5, which recognizes the C-terminal residues, was not different from normal. These data suggest two specific conclusions: (1) a site in the gamma-module other than the C-terminus is critical for platelet aggregation and (2) "B-b" interactions have a role in protofibril formation.

Factor XIII: structure, activation, and interactions with fibrinogen and fibrin

Fibrin stabilizing factor (factor XIII or FXIII) plays a critical role in the generation of a viable hemostatic plug. Following exposure to thrombin and calcium, the zymogen is activated to FXIIIa that, in turn, catalyzes the formation of N epsilon(gamma-glutamyl)lysine protein-to-protein side chain bridges within the clot network. Introduction of these covalent crosslinks greatly augments the viscoelastic storage modulus of the structure and its resistance to fibrinolytic enzymes. Analysis of the individual reaction steps and regulatory control mechanisms involved in clot stabilization enabled us to reconstruct the entire physiological process. This also serves as a guide for the differential diagnosis of the variety of molecular defects of fibrin stabilization.

The structure and biological features of fibrinogen and fibrin

Fibrinogen and fibrin play important, overlapping roles in blood clotting, fibrinolysis, cellular and matrix interactions, inflammation, wound healing, and neoplasia. These events are regulated to a large extent by fibrin formation itself and by complementary interactions between specific binding sites on fibrin(ogen) and extrinsic molecules including proenzymes, clotting factors, enzyme inhibitors, and cell receptors. Fibrinogen is comprised of two sets of three polypeptide chains termed A alpha, B beta, and gamma, that are joined by disulfide bridging within the N-terminal E domain. The molecules are elongated 45-nm structures consisting of two outer D domains, each connected to a central E domain by a coiled-coil segment. These domains contain constitutive binding sites that participate in fibrinogen conversion to fibrin, fibrin assembly, crosslinking, and platelet interactions (e.g., thrombin substrate, Da, Db, gamma XL, D:D, alpha C, gamma A chain platelet receptor) as well as sites that are available after fibrinopeptide cleavage (e.g., E domain low affinity non-substrate thrombin binding site); or that become exposed as a consequence of the polymerization process (e.g., tPA-dependent plasminogen activation). A constitutive plasma factor XIII binding site and a high affinity non-substrate thrombin binding site are located on variant gamma' chains that comprise a minor proportion of the gamma chain population. Initiation of fibrin assembly by thrombin-mediated cleavage of fibrinopeptide A from A alpha chains exposes two EA polymerization sites, and subsequent fibrinopeptide B cleavage exposes two EB polymerization sites that can also interact with platelets, fibroblasts, and endothelial cells. Fibrin generation leads to end-to-middle intermolecular Da to EA associations, resulting in linear double-stranded fibrils and equilaterally branched trimolecular fibril junctions. Side-to-side fibril convergence results in bilateral network branches and multistranded thick fiber cables. Concomitantly, factor XIII or thrombin-activated factor XIIIa introduce intermolecular covalent epsilon-(gamma glutamyl)lysine bonds into these polymers, first creating gamma dimers between properly aligned C-terminal gamma XL sites, which are positioned transversely between the two strands of each fibrin fibril. Later, crosslinks form mainly between complementary sites on alpha chains (forming alpha-polymers), and even more slowly among gamma dimers to create higher order crosslinked gamma trimers and tetramers, to complete the mature network structure.

Transglutaminase-catalyzed crosslinking of the Aalpha and gamma constituent chains in fibrinogen

Studies on transglutaminases usually focus on the polymerization of protein substrates by intermolecular N(epsilon)(gamma-glutamyl)lysine bridges, without considering the possibility that the monomeric protein units, themselves, could also become crosslinked internally. Both types of crosslinks are produced in the reaction of fibrinogen with red cell transglutaminase. We isolated the transglutaminase-modified, mostly monomeric form (92-96%) of fibrinogen with a N(epsilon)(gamma-glutamyl)lysine content of approximately 1.6 moles/mole of fibrinogen. The preparation was fully clottable by thrombin, but the rates of release of fibrinopeptides and clotting times were delayed compared with control. Hybrid Aalpha.gamma type of crosslinking, the hallmark of the reaction of the transglutaminase with fibrinogen, occurred by bridging the Aalpha(408-421) chain segment of the protein to that of gamma(392-406). Rotary shadowed electron microscope images showed many monomers to be bent, and the crosslinks seemed to bind the otherwise flexible alphaC domain closer to the backbone of fibrinogen.

Autoimmune Antibody in a Hemorrhagic Patient Interacts With Thrombin-Activated Factor XIII in a Unique Manner

Without a prior history of hemorrhagic disease, a 62-year-old man suffered recurrent episodes of bleeding. Solubility of the patient’s clot in 5 mol/L urea indicated a problem with fibrin stabilization. The transamidase activity potential of factor XIII, measured by the incorporation of radioactive putrescine into N,N-dimethylcasein as test substrate, was 62% of control, close to the normal range of values. Examination of the patient’s clot from recalcified plasma by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that essentially none of the  chains and only about two thirds of the γ chains of fibrin became cross-linked under conditions where both were fully cross-linked in the controls. An antibody to factor XIII was isolated which, although recognizing the recombinant rA2subunits, as well as the virgin A2B2 plasma ensemble, showed a 100-fold greater affinity for the thrombin-activated rA2′ and A2′B2 forms of the zymogen, suggesting that the latter would be its main target during coagulation. Furthermore, the patient’s IgG has an ability, never seen before, for inducing an enzymatically active configuration in the thrombin-activated zymogen in the absence of Ca2+.

Autoimmune Antibody in a Hemorrhagic Patient Interacts With Thrombin-Activated Factor XIII in a Unique Manner

Without a prior history of hemorrhagic disease, a 62-year-old man suffered recurrent episodes of bleeding. Solubility of the patient’s clot in 5 mol/L urea indicated a problem with fibrin stabilization. The transamidase activity potential of factor XIII, measured by the incorporation of radioactive putrescine into N,N-dimethylcasein as test substrate, was 62% of control, close to the normal range of values. Examination of the patient’s clot from recalcified plasma by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that essentially none of the  chains and only about two thirds of the γ chains of fibrin became cross-linked under conditions where both were fully cross-linked in the controls. An antibody to factor XIII was isolated which, although recognizing the recombinant rA2subunits, as well as the virgin A2B2 plasma ensemble, showed a 100-fold greater affinity for the thrombin-activated rA2′ and A2′B2 forms of the zymogen, suggesting that the latter would be its main target during coagulation. Furthermore, the patient’s IgG has an ability, never seen before, for inducing an enzymatically active configuration in the thrombin-activated zymogen in the absence of Ca2+.

Three-dimensional structural studies on fragments of fibrinogen and fibrin

Fibrinogen is a 340 kDa glycoprotein found in the blood plasma of all vertebrates. It is transformed into a fibrin clot by the action of thrombin. Recent X-ray structures of core fragments of both fibrinogen and fibrin have revealed many details about this polymerization event. These include structures of a 30 kDa recombinant gammaC domain, an 86 kDa fragment D from human fibrinogen and a cross-linked double-D fragment from fibrin.

The location of the carboxy-terminal region of gamma chains in fibrinogen and fibrin D domains

Elongated fibrinogen molecules are comprised of two outer "D" domains, each connected through a "coiled-coil" region to the central "E" domain. Fibrin forms following thrombin cleavage in the E domain and then undergoes intermolecular end-to-middle D:E domain associations that result in double-stranded fibrils. Factor XIIIa mediates crosslinking of the C-terminal regions of gamma chains in each D domain (the gammaXL site) by incorporating intermolecular epsilon-(gamma-glutamyl)lysine bonds between amine donor gamma406 lysine of one gamma chain and a glutamine acceptor at gamma398 or gamma399 of another. Several lines of evidence show that crosslinked gamma chains extend "transversely" between the strands of each fibril, but other data suggest instead that crosslinked gamma chains can only traverse end-to-end-aligned D domains within each strand. To examine this issue and determine the location of the gammaXL site in fibrinogen and assembled fibrin fibrils, we incorporated an amine donor, thioacetyl cadaverine, into glutamine acceptor sites in fibrinogen in the presence of XIIIa, and then labeled the thiol with a relatively small (0.8 nm diameter) electron dense gold cluster compound, undecagold monoaminopropyl maleimide (Au11). Fibrinogen was examined by scanning transmission electron microscopy to locate Au11-cadaverine-labeled gamma398/399 D domain sites. Seventy-nine percent of D domain Au11 clusters were situated in middle to proximal positions relative to the end of the molecule, with the remaining Au11 clusters in a distal position. In fibrin fibrils, D domain Au11 clusters were located in middle to proximal positions. These findings show that most C-terminal gamma chains in fibrinogen or fibrin are oriented toward the central domain and indicate that gammaXL sites in fibrils are situated predominantly between strands, suitably aligned for transverse crosslinking.

The intermediate filament protein, vimentin, in the lens is a target for cross-linking by transglutaminase

Mere addition of Ca2+ to a lens cortical homogenate (bovine) generates a series of products composed of a variety of high molecular weight vimentin species. The Ca2+-induced cross-linking of this cytoskeletal element seems to be mediated by the intrinsic transglutaminase of lens, because the reaction could be blocked at the monomeric state of vimentin by the inclusion of small synthetic substrates of the enzyme dansylcadaverine or dansyl-epsilon-aminocaproyl-Gln-Gln-Ile-Val. These compounds are known to compete against the Gln or Lys functionalities of proteins that would participate in forming the Nepsilon(gamma-glutamyl)lysine protein-to-protein cross-links. The cytosolic transglutaminase-catalyzed reactions could be reproduced with purified bovine lens vimentin and also with recombinant human vimentin preparations. Employing the latter system, we have titrated the transglutaminase-reactive sites of vimentin and, by sequencing the dansyl-tracer-labeled segments of the protein, we have shown that residues Gln453 and Gln460 served as acceptor functionalities and Lys97, Lys104, Lys294, and Lys439 as electron donor functionalities in vimentin. The transglutaminase-dependent reaction of this intermediate filament protein might influence the shape and plasticity of the fiber cells, and the enzyme-catalyzed cross-linking of vimentin, in conjunction with other lens constituents, may contribute to the process of cataract formation.

The complementary aggregation sites of fibrin investigated through examination of polymers of fibrinogen with fragment E

Fibrin polymerizes through the interaction of sites exposed by the thrombin-mediated cleavage of fibrinopeptides in the central E region of the protein and complementary sites near the ends of the molecules, open in the D regions of both fibrinogen and fibrin. A preparation of fragment E, containing the central domain and part of the coiled-coil regions of fibrin, was used in mixtures with fibrinogen in this electron microscopy study to investigate the formation of fibrillar structures. At short times, linearly ordered oligomers of fibrinogen were observed with an additional mass of E fragments at the end-to-end junctions. At later times, long flexible polymers made up of 30 or more fibrinogen and fragment E units, with a tendency for lateral aggregation and tangle formation, were seen. These single-stranded assemblies could be readily dissociated in dilute acetic acid into their fibrinogen and fragment E components. However, if the aggregates were treated with factor XIIIa so that all gamma chains became ligated by Nepsilon(gamma-glutamyl)lysine linkages, the polymers could no longer be taken apart. Because the only gamma chains in the preparation are present in the fibrinogen molecules interacting end-to-end, the findings show that the factor XIIIa-induced cross-linking of gamma chains in the clotting of fibrinogen or fibrin must occur between molecules that are longitudinal (or end-to-end) rather than transverse (or half-staggered).

A double-headed Gly-Pro-Arg-Pro ligand mimics the functions of the E domain of fibrin for promoting the end-to-end crosslinking of gamma chains by factor XIIIa

The E domain of fibrinogen represents the central region of the protein that, after the removal of fibrinopeptides from the N-termini of its alpha chains by thrombin, orders the noncovalent assembly of fibrin units into a half-staggered array. This structural organization is accomplished purely through noncovalent binding between the E domain of one molecule and the distal D domains of two others. The process of assembly has a physiologically important up-regulatory effect on the next enzymatic phase of blood coagulation, which is the factor XIIIa-catalyzed end-to-end ligation of the gamma chains at the D domains of the protein. Fibrin assembly, as well as the acceleration of the factor XIIIa reaction, could be prevented by Gly-Pro-Arg-Pro, a homologue of the natural sequence of amino acids at the N termini of alpha chains in the E domain. We have now succeeded with a simple double-headed ligand, bis(Gly-Pro-Arg-Pro-amido)polyethylene glycol, in fully replacing the regulatory functions of the large E domains of the native protein.