Structure and ligand binding determinants of the recombinant kringle 5 domain of human plasminogen

Molecular recognition and binding between human plasminogen Kringle 5 and α-chain of human complement component C3b by frontal chromatography and dynamics simulation

The binding and molecular recognition between α-chain of human complement C3b (α-chain of C3b) and human plasminogen Kringle 5 (Kringle 5) were studied and explored by frontal chromatography and dynamics simulation in the combination of bio-specific technologies. The specific interaction between the α-chain of C3b and Kringle 5 was initially confirmed by ligand blot and ELISA (Kd = 4.243×10-6 L/mol). Furthermore, the binding determination conducted via frontal chromatography showed that the presence of a single binding site between them, with the binding constant of 2.98 × 105 L/mol. Then the molecular recognition by dynamics simulation and molecular docking showed that there were 9 and 13 amino acid residues respective in the Kringle 5 and α-chain of C3b directly implicated in the binding and the main stabilizing forces were electrostatic force (-55.99 ± 11.82 kcal/mol) and Van der Waals forces (-42.70 ± 3.45 kcal/mol). Additionally, a loop structure (65-71) in Kringle 5 underwent a conformational change from a random structure to an α-helix and a loop structure (417-425) in α-chain of C3b was closer to the molecular center, both of them were more conducive to the binding between them. Meanwhile, the involvement of the lysine binding site of Kringle 5 played an important role in the binding process. In addition, the erythrocyte-antibody complement rosette assay substantiated that the presence of Kringle 5 hindered the transportation of α-chain of C3b to antigen-antibody complex in a dose-dependent manner. These findings collectively indicated that the α-chain of C3b is very likely a receptor protein for Kringle 5, which provides a methodology for other similar investigations and valuable insights into expansion of the pharmacological effects and potential application of Kringle 5 in immune-related diseases.

Screening, characterization and specific binding mechanism of aptamers against human plasminogen Kringle 5

Plasminogen Kringle 5 is one of the most potent cytokines identified to inhibit the proliferation and migration of vascular endothelial cells. Herein, six aptamer candidates that specifically bind to Kringle 5 were generated by the systematic evolution of ligands by exponential enrichment (SELEX). After 10 rounds of screening against Kringle 5, a highly enriched ssDNA pool was sequenced and the representative aptamers were subjected to binding assays to evaluate their affinity and specificity. The preferred aptamer KG-4, which demonstrated a low dissociation constant (K_d) of ∼ 432 nM and excellent selectivity for Kringle 5. A conserved "motif" of eight bases located at the stem-loop intersection, common to the aptamer, was further confirmed as the recognition element for binding with Kringle 5. The bulge formed by the motif and depression on the lysine binding site of Kringle 5 were both located at the binding interface, and the "induced fit" between their structures played a central role in the recognition process. Kringle 5 interacts KG-4 primarily through enthalpy-driven van der Waals forces and hydrogen bond. The key nucleotides A34 and C35 at motif on KG-4 and the positively charged amino acids in the loop 1 and loop 4 regions on Kringle 5 play a major role in the interaction. Furthermore, KG-4 dose-dependently reduced the proliferation inhibition of vascular endothelial cells by Kringle 5 and had a blocking effect on the function of Kringle 5 in inhibiting migration and promoting apoptosis of vascular endothelial cells in vitro. This study put a new light on protein-aptamer binding mechanism and may provide insight into the treatment of ischemic diseases by target depletion of Kringle 5.

Streptococcus co-opts a conformational lock in human plasminogen to facilitate streptokinase cleavage and bacterial virulence

Virulent strains of Streptococcus pyogenes (gram-positive group A Streptococcus pyogenes [GAS]) recruit host single-chain human plasminogen (hPg) to the cell surface-where in the case of Pattern D strains of GAS, hPg binds directly to the cells through a surface receptor, plasminogen-binding group A streptococcal M-protein (PAM). The coinherited Pattern D GAS-secreted streptokinase (SK2b) then accelerates cleavage of hPg at the R561-V562 peptide bond, resulting in the disulfide-linked two-chain protease, human plasmin (hPm). hPm localizes on the bacterial surface, assisting bacterial dissemination via proteolysis of host defense proteins. Studies using isolated domains from PAM and hPg revealed that the A-domain of PAM binds to the hPg kringle-2 module (K2hPg), but how this relates to the function of the full-length proteins is unclear. Herein, we use intact proteins to show that the lysine-binding site of K2hPg is a major determinant of the activation-resistant T-conformation of hPg. The binding of PAM to the lysine-binding site of K2hPg relaxes the conformation of hPg, leading to a greatly enhanced activation rate of hPg by SK2b. Domain swapping between hPg and mouse Pg emphasizes the importance of the Pg latent heavy chain (residues 1-561) in PAM binding and shows that while SK2b binds to both hPg and mouse Pg, the activation properties of streptokinase are strictly attributed to the serine protease domain (residues 562-791) of hPg. Overall, these data show that native hPg is locked in an activation-resistant conformation that is relaxed upon its direct binding to PAM, allowing hPm to form and provide GAS cells with a proteolytic surface.

Identification and analyses of inhibitors targeting apolipoprotein(a) kringle domains KIV-7, KIV-10, and KV provide insight into kringle domain function

Increased plasma concentrations of lipoprotein(a) (Lp(a)) are associated with an increased risk for cardiovascular disease. Lp(a) is composed of apolipoprotein(a) (apo(a)) covalently bound to apolipoprotein B of low-density lipoprotein (LDL). Many of apo(a)'s potential pathological properties, such as inhibition of plasmin generation, have been attributed to its main structural domains, the kringles, and have been proposed to be mediated by their lysine-binding sites. However, available small-molecule inhibitors, such as lysine analogs, bind unselectively to kringle domains and are therefore unsuitable for functional characterization of specific kringle domains. Here, we discovered small molecules that specifically bind to the apo(a) kringle domains KIV-7, KIV-10, and KV. Chemical synthesis yielded compound AZ-05, which bound to KIV-10 with a K_d of 0.8 μm and exhibited more than 100-fold selectivity for KIV-10, compared with the other kringle domains tested, including plasminogen kringle 1. To better understand and further improve ligand selectivity, we determined the crystal structures of KIV-7, KIV-10, and KV in complex with small-molecule ligands at 1.6-2.1 Å resolutions. Furthermore, we used these small molecules as chemical probes to characterize the roles of the different apo(a) kringle domains in in vitro assays. These assays revealed the assembly of Lp(a) from apo(a) and LDL, as well as potential pathophysiological mechanisms of Lp(a), including (i) binding to fibrin, (ii) stimulation of smooth-muscle cell proliferation, and (iii) stimulation of LDL uptake into differentiated monocytes. Our results indicate that a small-molecule inhibitor targeting the lysine-binding site of KIV-10 can combat the pathophysiological effects of Lp(a).

Prostate Cancer-Specific of DD3-driven Oncolytic Virus-harboring mK5 Gene

Prostate cancer (PCa) is the second most diagnosed cancer in Western male population. In this study, we insert mK5 (the mutational kringle5 of human plasminogen) into a DD3-promoted (differential display code 3) oncolytic adenovirus to construct OncoAd.mK5.DD3. E1A.dE1B, briefly, O^Ad.DD3.mK5. DD3 is one of the most prostate cancer specific promoters which can transcriptionally control adenoviral replication. mK5 has been proved to be able to inhibit the tumor angiogenesis and inhibit cell proliferation. Our results suggested that targeting PCa with O^Ad.DD3.mK5 elicited strong antitumor effect.

Voltage-Dependent Anion Channel-1, a Possible Ligand of Plasminogen Kringle 5

Kringle 5, the fifth fragment of plasminogen, is known to be important for inhibiting the proliferation and migration of vascular endothelial cell (VEC), while not having any effects on normal endothelial cells. Therefore, it may be a potential tumor therapy candidate. However, the ligand of the Kringle 5 in VEC has not yet been identified. In this study, the possible ligand of Kringle 5 in vitro was screened and validated using Ph.D.-7 phage display peptide library with molecular docking, along with surface plasma resonance (SPR). After four rounds of panning, the specific clones of Kringle 5 were confirmed using enzyme-linked immunosorbent assay (ELISA). The gene sequence analysis showed that they expressed the common amino sequence IGNSNTL. Then, using a NCBI BLAST, 103 matching sequences were found. Following the molecular docking evaluation and considering the acting function and pathway of the plasminogen Kringle 5 in the human body, the most promising candidate was determined to be voltage-dependent anion channel-1 (VDAC-1), which was able to bind to Kringle 5 at -822.65 J·mol-1 of the binding energy at the residues of Lys12, Thr19, Ser57, Thr188, Arg139, Asn214, Ser240 and Lys274. A strong dose-dependent interaction occurred between the VDAC-1 and Kringle 5 (binding constant 2.43 × 103 L·mol-1) in SPR observation. Therefore, this study proposed that VDAC-1 was a potential ligand of plasminogen Kringle 5, and also demonstrated that the screening and validation of protein ligand using phage display peptide library with the molecular docking, along with SPR, was a practicable application.

Pharmacokinetics of gene recombined angiogenesis inhibitor Kringle 5 in vivo using 131I specific markers and SPECT/CT

The previous pharmacokinetic methods can be only limited to drug analysis in vitro, which provide less information on the distribution and metabolismof drugs, and limit the interpretation and assessment of pharmacokinetics, the determination of metabolic principles, and evaluation of treatment effect. The objective of the study was to investigate the pharmacokinetic characteristics of gene recombination angiogenesis inhibitor Kringle 5 in vivo. The SPECT/CT and specific ¹³¹I-Kringle 5 marked by Iodogen method were both applied to explore the pharmacokinetic characteristics of ¹³¹I-Kringle 5 in vivo, and to investigate the dynamic distributions of ¹³¹I-Kringle 5 in target organs. Labeling recombinant angiogenesis inhibitor Kringle 5 using ¹³¹I with longer half-life and imaging in vivo using SPECT instead of PET, could overcome the limitations of previous methods. When the doses of ¹³¹I-Kringle 5 were 10.0, 7.5 and 5.0 g/kg, respectively, the two-compartment open models can be determined within all the metabolic process in vivo. There were no significant differences in t_1/2α, t_1/2β, apparent volume of distribution and CL between those three levels. The ratio of AUC_(0~∞) among three different groups of 10.0, 7.5 and 5.0 g/kg was 2.56:1.44:1.0, which was close to the ratio (2:1.5:1.0). It could be clear that in the range of 5.0-10.0 g/kg, Kringle 5 was characterized by the first-order pharmacokinetics. Approximately 30 min after ¹³¹I-Kringle 5 was injected, ¹³¹I-Kringle 5 could be observed to concentrate in the heart, kidneys, liver and other organs by means of planar imaging and tomography. After 1 h of being injected, more radionuclide retained in the bladder, but not in intestinal. It could be concluded that ¹³¹I-Kringle 5 is mainly excreted through the kidneys. About 2 h after the injection of ¹³¹I-Kringle 5, the radionuclide in the heart, kidneys, liver and other organs was gradually reduced, while more radionuclide was concentrated in the bladder. The radionuclide was completely metabolized within 24 h, and the distribution of radioactivity in rats was similar to normal levels. In our study, the specific marker ¹³¹I-Kringle 5 and SPECT/CT were successfully used to explore pharmacokinetic characteristics of Kringle 5 in rats. The study could provide a new evaluation platform of the specific, in vivo and real-time functional imaging and pharmacokinetics for the clinical application of ¹³¹I-Kringle 5.

Identifying functionally important cis-peptide containing segments in proteins and their utility in molecular function annotation

Cis-peptide embedded segments are rare in proteins but often highlight their important role in molecular function when they do occur. The high evolutionary conservation of these segments illustrates this observation almost universally, although no attempt has been made to systematically use this information for the purpose of function annotation. In the present study, we demonstrate how geometric clustering and level-specific Gene Ontology molecular-function terms (also known as annotations) can be used in a statistically significant manner to identify cis-embedded segments in a protein linked to its molecular function. The present study identifies novel cis-peptide fragments, which are subsequently used for fragment-based function annotation. Annotation recall benchmarks interpreted using the receiver-operator characteristic plot returned an area-under-curve > 0.9, corroborating the utility of the annotation method. In addition, we identified cis-peptide fragments occurring in conjunction with functionally important trans-peptide fragments, providing additional insights into molecular function. We further illustrate the applicability of our method in function annotation where homology-based annotation transfer is not possible. The findings of the present study add to the repertoire of function annotation approaches and also facilitate engineering, design and allied studies around the cis-peptide neighborhood of proteins.

Rapid binding of plasminogen to streptokinase in a catalytic complex reveals a three-step mechanism

Rapid kinetics demonstrate a three-step pathway of streptokinase (SK) binding to plasminogen (Pg), the zymogen of plasmin (Pm). Formation of a fluorescently silent encounter complex is followed by two conformational tightening steps reported by fluorescence quenches. Forward reactions were defined by time courses of biphasic quenching during complex formation between SK or its COOH-terminal Lys(414) deletion mutant (SKΔK414) and active site-labeled [Lys]Pg ([5-(acetamido)fluorescein]-D-Phe-Phe-Arg-[Lys]Pg ([5F]FFR-[Lys]Pg)) and by the SK dependences of the quench rates. Active site-blocked Pm rapidly displaced [5F]FFR-[Lys]Pg from the complex. The encounter and final SK ·[5F]FFR-[Lys]Pg complexes were weakened similarly by SK Lys(414) deletion and blocking of lysine-binding sites (LBSs) on Pg kringles with 6-aminohexanoic acid or benzamidine. Forward and reverse rates for both tightening steps were unaffected by 6-aminohexanoic acid, whereas benzamidine released constraints on the first conformational tightening. This indicated that binding of SK Lys(414) to Pg kringle 4 plays a role in recognition of Pg by SK. The substantially lower affinity of the final SK · Pg complex compared with SK · Pm is characterized by a ∼ 25-fold weaker encounter complex and ∼ 40-fold faster off-rates for the second conformational step. The results suggest that effective Pg encounter requires SK Lys(414) engagement and significant non-LBS interactions with the protease domain, whereas Pm binding additionally requires contributions of other lysines. This difference may be responsible for the lower affinity of the SK · Pg complex and the expression of a weaker "pro"-exosite for binding of a second Pg in the substrate mode compared with SK · Pm.

Dimerization is not a determining factor for functional high affinity human plasminogen binding by the group A streptococcal virulence factor PAM and is mediated by specific residues within the PAM a1a2 domain

A emm53 subclass of Group A Streptococcus pyogenes (GAS) interacts tightly with human plasma plasminogen (hPg) and plasmin (hPm) via the kringle 2 (K2hPg) domain of hPg/hPm and the N-terminal a1a2 regions of a GAS coiled-coil M-like protein (PAM). Previous studies have shown that a monomeric PAM fragment, VEK30 (residues 97-125 + Tyr), interacted specifically with isolated K2hPg. However, the binding strength of VEK30 (KD = 56 nm) was ∼60-fold weaker than that of full-length dimeric PAM (KD = 1 nm). To assess whether this attenuated binding was due to the inability of VEK30 to dimerize, we defined the minimal length of PAM required to dimerize using a series of peptides with additional PAM residues placed at the NH2 and COOH termini of VEK30. VEK64 (PAM residues 83-145 + Tyr) was found to be the smallest peptide that adopted an α-helical dimer, and was bound to K2hPg with nearly the same affinity as PAM (KD = 1-2 nm). However, addition of two PAM residues (Arg(126)-His(127)) to the COOH terminus of VEK30 (VEK32) maintained a monomeric peptidic structure, but exhibited similar K2hPg binding affinity as full-length dimeric PAM. We identified five residues in a1a2 (Arg(113), His(114), Glu(116), Arg(126), His(127)), mutation of which reduced PAM binding affinity for K2hPg by ∼ 1000-fold. Replacement of these critical residues by Ala in the GAS genome resulted in reduced virulence, similar to the effects of inactivating the PAM gene entirely. We conclude that rather than dimerization of PAM, the five key residues in the binding domain of PAM are essential to mediate the high affinity interaction with hPg, leading to increased GAS virulence.

Pneumococcal phosphoglycerate kinase interacts with plasminogen and its tissue activator

Streptococcus pneumoniae is not only a commensal of the nasopharyngeal epithelium, but may also cause life-threatening diseases. Immune-electron microscopy studies revealed that the bacterial glycolytic enzyme, phosphoglycerate kinase (PGK), is localised on the pneumococcal surface of both capsulated and non-capsulated strains and colocalises with plasminogen. Since pneumococci may concentrate host plasminogen (PLG) together with its activators on the bacterial cell surface to facilitate the formation of plasmin, the involvement of PGK in this process was studied. Specific binding of human or murine PLG to strain-independent PGK was documented, and surface plasmon resonance analyses indicated a high affinity interaction with the kringle domains 1-4 of PLG. Crystal structure determination of pneumococcal PGK together with peptide array analysis revealed localisation of PLG-binding site in the N-terminal region and provided structural motifs for the interaction with PLG. Based on structural analysis data, a potential interaction of PGK with tissue plasminogen activator (tPA) was proposed and experimentally confirmed by binding studies, plasmin activity assays and thrombus degradation analyses.

Bacterial plasminogen receptors utilize host plasminogen system for effective invasion and dissemination

In order for invasive pathogens to migrate beyond the site of infection, host physiological barriers such as the extracellular matrix, the basement membrane, and encapsulating fibrin network must be degraded. To circumvent these impediments, proteolytic enzymes facilitate the dissemination of the microorganism. Recruitment of host proteases to the bacterial surface represents a particularly effective mechanism for enhancing invasiveness. Plasmin is a broad spectrum serine protease that degrades fibrin, extracellular matrices, and connective tissue. A large number of pathogens express plasminogen receptors which immobilize plasmin(ogen) on the bacterial surface. Surface-bound plasminogen is then activated by plasminogen activators to plasmin through limited proteolysis thus triggering the development of a proteolytic surface on the bacteria and eventually assisting the spread of bacteria. The host hemostatic system plays an important role in systemic infection. The interplay between hemostatic processes such as coagulation and fibrinolysis and the inflammatory response constitutes essential components of host defense and bacterial invasion. The goal of this paper is to highlight mechanisms whereby pathogenic bacteria, by engaging surface receptors, utilize and exploit the host plasminogen and fibrinolytic system for the successful dissemination within the host.

Crystal structure of the native plasminogen reveals an activation-resistant compact conformation

BACKGROUND

Plasminogen is the zymogen form of plasmin and the precursor of angiostatin. It has been implicated in a variety of disease states, including thrombosis, bleeding and cancers. The native plasminogen, known as Glu-plasminogen, contains seven domains comprising the N-terminal peptide domain (NTP), five kringle domains (K1-K5) and the C-terminal serine protease domain (SP). Previous studies have established that the lysine binding site (LBS) of the conserved kringle domains plays a crucial role in mediating the regulation of plasminogen function. However, details of the related conformational mechanism are unknown.

OBJECTIVES

We aim to understand in more detail the conformational mechanism of plasminogen activation involving the kringles.

METHODS

We crystallized the native plasminogen under physiologically relevant conditions and determined the structure at 3.5 Å resolution. We performed structural analyses and related these to the literature data to gain critical understanding of the plasminogen activation.

RESULTS AND CONCLUSIONS

The structure reveals the precise architecture of the quaternary complex. It shows that the Glu-plasminogen renders its compact form as an activation-resistant conformation for the proteolytic activation. The LBSs of all kringles, except K1, are engaged in intra-molecular interactions while only K1-LBS is readily available for ligand binding or receptor anchorage. The structure also provides insights into the interactions between plasminogen and α2-antiplasmin, the primary physiological inhibitor of plasmin. Furthermore, the data presented explain why a conformational transition to the open form is necessary for plasminogen activation as well as angiostatin generation, and provide a rationale for the functional hierarchy of the different kringles.

The plasmin-antiplasmin system: structural and functional aspects

The plasmin-antiplasmin system plays a key role in blood coagulation and fibrinolysis. Plasmin and α(2)-antiplasmin are primarily responsible for a controlled and regulated dissolution of the fibrin polymers into soluble fragments. However, besides plasmin(ogen) and α(2)-antiplasmin the system contains a series of specific activators and inhibitors. The main physiological activators of plasminogen are tissue-type plasminogen activator, which is mainly involved in the dissolution of the fibrin polymers by plasmin, and urokinase-type plasminogen activator, which is primarily responsible for the generation of plasmin activity in the intercellular space. Both activators are multidomain serine proteases. Besides the main physiological inhibitor α(2)-antiplasmin, the plasmin-antiplasmin system is also regulated by the general protease inhibitor α(2)-macroglobulin, a member of the protease inhibitor I39 family. The activity of the plasminogen activators is primarily regulated by the plasminogen activator inhibitors 1 and 2, members of the serine protease inhibitor superfamily.

Small-molecule modulators of zymogen activation in the fibrinolytic and coagulation systems

The coagulation and fibrinolytic systems are central to the hemostatic mechanism, which works promptly on vascular injury and tissue damage. The rapid response is generated by specific molecular interactions between components in these systems. Thus, the regulation mechanism of the systems is programmed in each component, as exemplified by the elegant processes in zymogen activation. This review describes recently identified small molecules that modulate the activation of zymogens in the fibrinolytic and coagulation systems.

NMR backbone dynamics of VEK-30 bound to the human plasminogen kringle 2 domain

To gain insights into the mechanisms for the tight and highly specific interaction of the kringle 2 domain of human plasminogen (K2(Pg)) with a 30-residue internal peptide (VEK-30) from a group A streptococcal M-like protein, the dynamic properties of free and bound K2(Pg) and VEK-30 were investigated using backbone amide (15)N-NMR relaxation measurements. Dynamic parameters, namely the generalized order parameter, S(2), the local correlation time, tau(e), and the conformational exchange contribution, R(ex), were obtained for this complex by Lipari-Szabo model-free analysis. The results show that VEK-30 displays distinctly different dynamic behavior as a consequence of binding to K2(Pg), manifest by decreased backbone flexibility, particularly at the binding region of the peptide. In contrast, the backbone dynamics parameters of K2(Pg) displayed similar patterns in the free and bound forms, but, nonetheless, showed interesting differences. Based on our previous structure-function studies of this interaction, we also made comparisons of the VEK-30/K2(Pg) dynamics results from different kringle modules complexed with small lysine analogs. The differences in dynamics observed for kringles with different ligands provide what we believe to be new insights into the interactions responsible for protein-ligand recognition and a better understanding of the differences in binding affinity and binding specificity of kringle domains with various ligands.

Skizzle is a novel plasminogen- and plasmin-binding protein from Streptococcus agalactiae that targets proteins of human fibrinolysis to promote plasmin generation

Skizzle (SkzL), secreted by Streptococcus agalactiae, has moderate sequence identity to streptokinase and staphylokinase, bacterial activators of human plasminogen (Pg). SkzL binds [Glu]Pg with low affinity (K(D) 3-16 mum) and [Lys]Pg and plasmin (Pm) with indistinguishable high affinity (K(D) 80 and 50 nm, respectively). Binding of SkzL to Pg and Pm is completely lysine-binding site-dependent, as shown by the effect of the lysine analog, 6-aminohexanoic acid. Deletion of the COOH-terminal SkzL Lys(415) residue reduces affinity for [Lys]Pg and active site-blocked Pm 30-fold, implicating Lys(415) in a lysine-binding site interaction with a Pg/Pm kringle. SkzL binding to active site fluorescein-labeled Pg/Pm analogs demonstrates distinct high and low affinity interactions. High affinity binding is mediated by Lys(415), whereas the source of low affinity binding is unknown. SkzL enhances the activation of [Glu]Pg by urokinase (uPA) approximately 20-fold, to a maximum rate indistinguishable from that for [Lys]Pg and [Glu]Pg activation in the presence of 6-aminohexanoic acid. SkzL binds preferentially to the partially extended beta-conformation of [Glu]Pg, which is in unfavorable equilibrium with the compact alpha-conformation, thereby converting [Glu]Pg to the fully extended gamma-conformation and accelerating the rate of its activation by uPA. SkzL enhances [Lys]Pg and [Glu]Pg activation by single-chain tissue-type Pg activator, approximately 42- and approximately 650-fold, respectively. SkzL increases the rate of plasma clot lysis by uPA and single-chain tissue-type Pg activator approximately 2-fold, confirming its cofactor activity in a physiological model system. The results suggest a role for SkzL in S. agalactiae pathogenesis through fibrinolytic enhancement.

Solution structure of the complex of VEK-30 and plasminogen kringle 2

The solution structure of the complex containing the isolated kringle 2 domain of human plasminogen (K2(Pg)) and VEK-30, a 30-amino acid residue internal peptide from a streptococcal M-like plasminogen (Pg) binding protein (PAM), has been determined by multinuclear high-resolution NMR. Complete backbone and side-chain assignments were obtained from triple-resonance experiments, after which structure calculations were performed and ultimately refined by restrained molecular simulation in water. We find that, in contrast with the dimer of complexes observed in the asymmetric unit of the crystal, global correlation times and buoyant molecular weight determinations of the complex and its individual components showed the monomeric nature of all species in solution. The NMR-derived structure of K2(Pg) in complex with VEK-30 presents a folding pattern typical of other kringle domains, while bound VEK-30 forms an end-to-end alpha-helix (residues 6-27) in the complex. Most of the VEK-30/K2(Pg) interactions in solution occur between a single face of the alpha-helix of VEK-30 and the lysine binding site (LBS) of K2(Pg). The canonical LBS of K2(Pg), consisting of Asp54, Asp56, Trp60, Arg69, and Trp70 (kringle numbering), interacts with an internal pseudo-lysine of VEK-30, comprising side-chains of Arg17, His18, and Glu20. Site-specific mutagenesis analysis confirmed that the electrostatic field formed by the N-terminal anionic residues of the VEK-30 alpha-helix, viz., Asp7, and the non-conserved cationic residues of K2(Pg), viz., Lys43 and Arg55, play additional important roles in the docking of VEK-30 to K2(Pg). Structural analysis and kringle sequence alignments revealed several important features related to exosite binding that provide a structural rationale for the high specificity and affinity of VEK-30 for K2(Pg).

Recombinant kringle 5 from plasminogen antagonises hepatocyte growth factor-mediated signalling

The blood protein plasminogen is proteolytically cleaved to produce angiostatin and kringle 5 (K5), both of which are known angiogenesis inhibitors. A common structural element between K5, angiostatin and other endogenous angiogenesis inhibitors is the presence of the kringle protein-interacting domain. Another kringle domain-containing protein, hepatocyte growth factor (HGF), promotes angiogenesis by binding to and stimulating the tyrosine kinase receptor Met. HGF binding to Met is dependent on the kringle domains of HGF. Because both K5 and HGF contain kringle motifs and because these proteins have opposite effects on angiogenesis, we hypothesised that K5 can antagonise HGF-mediated signalling in a Met-dependent manner. We determined that K5 binding to H1299 cells is competed by HGF suggesting that these two proteins bind to the same protein. Purified K5 immunoprecipitates with Met and this interaction is abolished by increasing doses of HGF. Using proliferation, phosphorylation of Met and Akt as markers of HGF activity, we determined that K5 inhibits HGF-mediated signalling. Taken together, these data support a model by which K5 binds to Met and functions as a competitive antagonist of HGF signalling and presents a novel mechanism of action of K5.

Solution structure and functional characterization of human plasminogen kringle 5

The ligand binding properties of the kringle 5 (K5) domain of human plasminogen have been investigated via intrinsic tryptophan fluorescence. The oleic acid (OA) affinity for K5 was quantified, yielding an association constant K(a) approximately 2.08 x 10(4) mM(-1). Simultaneously, it was determined that OA and trans-4-(aminomethyl)cyclohexanecarboxylic acid (AMCHA) (K(a) approximately 50 mM(-1)) compete for binding to K5. The solution structure of K5 in the presence of 11 mM AMCHA was solved via NMR spectroscopy (protein heavy atom RMSD approximately 0.93 +/- 0.12 A). The AMCHA binding site was localized via (1)H/(15)N chemical shift perturbation mapping assisted by in silico docking. We have found that AMCHA binds at the canonical kringle lysine binding site (LBS), structured by the Pro54-Gly60 segment plus the neighboring Phe36, Thr37, Trp62, Leu71, and Tyr72 residues. The segment 30-42, encompassing LBS residues, appears to be endowed with a higher degree of structural flexibility as suggested by the relatively lower value of S(2), the generalized order parameter, consistent with a higher backbone heavy atom RMSD of approximately 1.22 A (vs 0.84 A overall) between the two monomeric units in the crystal unit cell, of potential significance for ligand binding. OA was found to perturb the same area of the protein, namely, the LBS, as well as Tyr74. Combined with previous studies, the observation of OA binding expands the range of ligands that interact with kringle 5 while it widens the scope of potential biological functions for kringle domains.

Plasminogen substrate recognition by the streptokinase-plasminogen catalytic complex is facilitated by Arg253, Lys256, and Lys257 in the streptokinase beta-domain and kringle 5 of the substrate

Streptokinase (SK) conformationally activates the central zymogen of the fibrinolytic system, plasminogen (Pg). The SK.Pg* catalytic complex binds Pg as a specific substrate and cleaves it into plasmin (Pm), which binds SK to form the SK.Pm complex that propagates Pm generation. Catalytic complex formation is dependent on lysine-binding site (LBS) interactions between a Pg/Pm kringle and the SK COOH-terminal Lys(414). Pg substrate recognition is also LBS-dependent, but the kringle and SK structural element(s) responsible have not been identified. SK mutants lacking Lys(414) with Ala substitutions of charged residues in the SK beta-domain 250-loop were evaluated in kinetic studies that resolved conformational and proteolytic Pg activation. Activation of [Lys]Pg and mini-Pg (containing only kringle 5 of Pg) by SK with Ala substitutions of Arg(253), Lys(256), and Lys(257) showed decreases in the bimolecular rate constant for Pm generation, with nearly total inhibition for the SK Lys(256)/Lys(257) double mutant. Binding of bovine Pg (BPg) to the SK.Pm complex containing fluorescently labeled Pm demonstrated LBS-dependent assembly of a SK.labeled Pm.BPg ternary complex, whereas BPg did not bind to the complex containing the SK Lys(256)/Lys(257) mutant. BPg was activated by SK.Pm with a K(m) indistinguishable from the K(D) for BPg binding to form the ternary complex, whereas the SK Lys(256)/Lys(257) mutant did not support BPg activation. We conclude that SK residues Arg(253), Lys(256), and Lys(257) mediate Pg substrate recognition through kringle 5 of the [Lys]Pg and mini-Pg substrates. A molecular model of the SK.kringle 5 complex identifies the putative interactions involved in LBS-dependent Pg substrate recognition.

Reduction of canine plasminogen leads to an expanded molecule which precipitates

Canine plasminogen is made up of seven domains. In each domain there are several cysteines that are linked by disulfide bonds. Reduction of a limited number of the cystines destabilizes the protein such that it precipitates. The bond or bonds that are broken provide about 14 kcal of stabilization energy. Circular dichroism and dynamic light scattering indicate that there is probably an intermediate that is formed prior to precipitation and that the intermediate is somewhat larger than the compact form of plasminogen.

Systemic and Periocular Deliveries of Plasminogen Kringle 5 Reduce Vascular Leakage in Rat Models of Oxygen-Induced Retinopathy and Diabetes

PURPOSE

Increased retinal vascular permeability is a common complication of diabetes and a major cause of vision loss in diabetic patients. The current study is to determine the effect of plasminogen kringle 5 (K5) on vascular leakage via systemic and periocular deliveries.

METHODS

Oxygen-induced retinopathy (OIR) was generated by exposing newborn rats to 75% oxygen. Diabetes was induced in adult rats by injection of streptozotocin (STZ). Retinal vascular permeability was measured by the Evans blue-albumin leakage method.

RESULTS

Subcutaneous, intraperitoneal, subconjunctival, and retrobulbar injections and topical eyedrop application of K5 significantly reduced retinal vascular permeability in both the OIR and STZ-diabetic rat models. Compared with the periocular deliveries, systemic administration requires higher doses of K5. K5 deliveries downregulated VEGF expression in the retina.

CONCLUSIONS

K5 can reduce retinal vascular permeability through systemic and periocular deliveries. These delivery routes of K5 have therapeutic potential in the treatment of vascular leakage.

Increased suppression of oncolytic adenovirus carrying mutant k5 on colorectal tumor

Angiogenesis plays a key role in the development of a wide variety of malignant tumors. The approach of targeting antiangiogenesis has become an important field of cancer gene therapy. In this study, the antiangiogenesis protein K5 (the kringle 5 of human plasminogen) has been mutated by changing leucine71 to arginine to form mK5. Then the ZD55-mK5, which is an oncolytic adenovirus expressing mK5, was constructed. It showed stronger inhibition on proliferation of human umbilical vein endothelial cell. Moreover, in tube formation and embryonic chorioallantoic membrane assay, ZD55-mK5 exhibited more effective antiangiogenesis than ZD55-K5. In addition, ZD55-mK5 generated obvious suppression on the growth of colorectal tumor xenografts and prolonged the life span of nude mice. These results indicate that ZD55-mK5 is a potent agent for inhibiting the tumor angiogenesis and tumor growth.

Plasminogen Kringle 5 blocks tumor progression by antiangiogenic and proinflammatory pathways

Proteolytic processing of human plasminogen generates potent antiangiogenic peptides such as angiostatin. The plasminogen kringle 5 (K5) domain, which is distinct from angiostatin, possesses potent antiangiogenic properties on its own, which can be exploited in cancer therapy. It has been recently observed that antiangiogenic agents promote leukocyte-vessel wall interaction as part of their antitumor effect. Although we have previously shown that K5 suppresses cancer growth in tumor xenograft models, its modulation of inflammation in experimental mice with intact immune systems is unknown. To determine whether K5 possesses immune proinflammatory properties, we investigated the effects of K5 in an immune competent model of breast cancer and observed that tumor rejection is substantially reduced in nonobese diabetic/severe combined immunodeficient and BALB/c nude when compared with wild-type BALB/c mice, suggesting an important role for T-lymphoid cells in the antitumor effect of K5. Tumor explant analysis shows that K5 enhances tumor recruitment of CD3(+) lymphoid cells, in particular, the NKT phenotype. We also observed a significant decrease in tumor-associated microvessel length and density consistent with antiangiogenic activity. Histologic analysis of K5 tumors also revealed a robust neutrophilic infiltrate, which may be explained by the neutrophil chemotactic activity of K5 as well as its ability to promote CD64 up-regulation within the CD11b(+) adhesive neutrophil population. In sum, our findings confirm that the K5 protein acts as a potent angiostatic agent and possesses a novel proinflammatory role via its ability to recruit tumor-associated neutrophils and NKT lymphocytes, leading to a potent antitumor response.

Canine Plasminogen: Spectral Responses to Changes in 6-Aminohexanoate and Temperature

We studied the near UV absorption spectrum of canine plasminogen. There are 19 tryptophans, 19 phenylalanines and 34 tyrosines in the protein. 4th derivative spectra optimized for either tryptophan or tyrosine give a measure of the polarity of the environments of these two aromatic amino acids. Plasminogen at temperatures between 0°C and 37°C exists as a mixture of four conformations: closed-relaxed, open-relaxed, closed-compact, and open-compact. The closed to open transition is driven by addition of ligand to a site on the protein. The relaxed to compact transition is driven by increasing temperature from 0°C to above 15-20°C.

When the conformation of plasminogen is mainly closed-relaxed, the 4th derivative spectra suggest that the average tryptophan environment is similar to a solution of 20% methanol at the same temperature. Under the same conditions, 4th derivative spectra suggest that the average tyrosine environment is similar to water. These apparent polarities change as the plasminogen is forced to assume the other conformations. We try to rationalize the information based on the known portions of the plasminogen structure.

Plasminogen kringle 5-engineered glioma cells block migration of tumor-associated macrophages and suppress tumor vascularization and progression

Angiostatin, a well-characterized angiostatic agent, is a proteolytic cleavage product of human plasminogen encompassing the first four kringle structures. The fifth kringle domain (K5) of human plasminogen is distinct from angiostatin and has been shown, on its own, to act as a potent endothelial cell inhibitor. We propose that tumor-targeted K5 cDNA expression may act as an effective therapeutic intervention as part of a cancer gene therapy strategy. In this study, we provide evidence that eukaryotically expressed His-tagged human K5 cDNA (hK5His) is exported extracellularly and maintains predicted disulfide bridging conformation in solution. Functionally, hK5His protein produced by retrovirally engineered human U87MG glioma cells suppresses in vitro migration of both human umbilical vein endothelial cells and human macrophages. Subcutaneous implantation of Matrigel-embedded hK5His-producing glioma cells in nonobese diabetic/severe combined immunodeficient mice reveals that hK5His induces a marked reduction in blood vessel formation and significantly suppresses the recruitment of tumor-infiltrating CD45+ Mac3+ Gr1- macrophages. Therapeutically, we show in a nude mouse orthotopic brain cancer model that tumor-targeted K5 expression is capable of effectively suppressing glioma growth and promotes significant long-term survival (>120 days) of test animals. These data suggest that plasminogen K5 acts as a novel two-pronged anticancer agent, mediating its inhibitory effect via its action on host-derived endothelial cells and tumor-associated macrophages, resulting in a potent, clinically relevant antitumor effect.

Kringle 5 of human plasminogen induces apoptosis of endothelial and tumor cells through surface-expressed glucose-regulated protein 78

Kringle 5 (K5) of human plasminogen has been shown to inhibit angiogenesis by inducing the apoptosis of proliferating endothelial cells. Peptide regions around the lysine-binding pocket of K5 largely mediate these effects, particularly the peptide PRKLYDY, which we show to compete with K5 for the binding to endothelial cells. The cell surface binding site for K5 that mediates these effects has not been defined previously. Here, we report that glucose-regulated protein 78, exposed on cell surfaces of proliferating endothelial cells as well as on stressed tumor cells, plays a key role in the antiangiogenic and antitumor activity of K5. We also report that recombinant K5-induced apoptosis of stressed HT1080 fibrosarcoma cells involves enhanced activity of caspase-7, consistent with the disruption of glucose-regulated protein 78-procaspase-7 complexes. These results establish recombinant K5 as an inhibitor of a stress response pathway, which leads to both endothelial and tumor cell apoptosis.

New Class of Blue Animal Pigments Based on Frizzled and Kringle Protein Domains

The nature of coloration in many marine animals remains poorly investigated. Here we studied the blue pigment of a scyfoid jellyfish Rhizostoma pulmo and determined it to be a soluble extracellular 30-kDa chromoprotein with a complex absorption spectrum peaking at 420, 588, and 624 nm. Furthermore, we cloned the corresponding cDNA and confirmed its identity by immunoblotting and mass spectrometry experiments. The chromoprotein, named rpulFKz1, consists of two domains, a Frizzled cysteine-rich domain and a Kringle domain, inserted into one another. Generally, Frizzleds are members of a basic Wnt signal transduction pathway investigated intensely with regard to development and cancerogenesis. Kringles are autonomous structural domains found throughout the blood clotting and fibrinolytic proteins. Neither Frizzled and Kringle domains association with any type of coloration nor Kringle intrusion into Frizzled sequence was ever observed. Thus, rpulFKz1 represents a new class of animal pigments, whose chromogenic group remains undetermined. The striking homology between a chromoprotein and members of the signal transduction pathway provides a novel node in the evolution track of growth factor-mediated morphogenesis compounds.

Kringle 5 peptide–albumin conjugates with anti-migratory activity

Three peptide fragments of the kringle 5 region of plasminogen and their respective N- and C-terminus maleimido derivatives conjugated to Cys34 of human serum albumin were evaluated in vitro using a human umbilical vein endothelial cell (HUVEC) migration assay and a human plasma stability assay. The N-terminus maleimido derivative of the 64 to 74 segment of kringle 5 conjugated to human serum albumin possessed remarkable anti-migratory activity.

What the structure of angiostatin may tell us about its mechanism of action

Originally discovered in 1994 by Folkman and coworkers, angiostatin was identified through its antitumor effects in mice and later shown to be a potent inhibitor of angiogenesis. An internal fragment of plasminogen, angiostatin consists of kringle domains that are known to be lysine-binding. The crystal structure of angiostatin was the first multikringle domain-containing structure to be published. This review will focus on what is known about the structure of angiostatin and its implications in function from the current literature.

A fast-acting, modular-structured staphylokinase fusion with Kringle-1 from human plasminogen as the fibrin-targeting domain offers improved clot lysis efficacy

To develop a fast-acting clot dissolving agent, a clot-targeting domain derived from the Kringle-1 domain in human plasminogen was fused to the C-terminal end of staphylokinase with a linker sequence in between. Production of this fusion protein in Bacillus subtilis and Pichia pastoris was examined. The Kringle domain in the fusion protein produced from B. subtilis was improperly folded because of its complicated disulfide-bond profile, whereas the staphylokinase domain produced from P. pastoris was only partially active because of an N-linked glycosylation. A change of the glycosylation residue, Thr-30, to alanine resulted in a non-glycosylated biologically active fusion. The resulting mutein, designated SAKM3-L-K1, was overproduced in P. pastoris. Each domain in SAKM3-L-K1 was functional, and this fusion showed fibrin binding ability by binding directly to plasmin-digested clots. In vitro fibrin clot lysis in a static environment and plasma clot lysis in a flow-cell system demonstrated that the engineered fusion outperformed the non-fused staphylokinase. The time required for 50% clot lysis was reduced by 20 to 500% under different conditions. Faster clot lysis can potentially reduce the degree of damage to occluded heart tissues.

Disulfide bonds as switches for protein function

The prevailing view is that disulfide bonds have been added during evolution to enhance the stability of proteins that function in a fluctuating cellular environment. However, recent evidence indicates that disulfide bonds can be more than inert structural motifs. The function of some secreted soluble proteins and cell-surface receptors is controlled by cleavage of one or more of their disulfide bonds; this cleavage is mediated by catalysts or facilitators that are specific for their substrate.

Biological regulation through protein disulfide bond cleavage

It is thought that disulfide bonds in secreted proteins are inert because of the oxidizing nature of the extracellular milieu. We have suggested that this is not necessarily the case and that certain secreted proteins contain one or more disulfide bonds that can be cleaved and that this cleavage is central to the protein's function. This review discusses disulfide bond cleavage in the secreted soluble protein, plasmin. Cleavage of plasmin disulfide bond(s) triggers peptide bond cleavage and formation of the tumour angiogenesis inhibitor, angiostatin. Tumour cells secrete phosphoglycerate kinase which facilitates cleavage of the plasmin disulfide bond(s). Phosphoglycerate kinase is not a conventional disulfide bond reductase. We propose that phosphoglycerate kinase facilitates cleavage of a particular plasmin disulfide bond by hydroxide ion, which results in formation of a sulfenic acid and a free thiol. The free thiol is then available to exchange with another nearby disulfide bond resulting in formation of a new disulfide and a new free thiol. The reduced plasmin is then susceptible to discreet proteolysis which results in release of angiostatin.

Function of the 90-loop (Thr90-Glu100) region of staphylokinase in plasminogen activation probed through site-directed mutagenesis and loop deletion

Staphylokinsae (SAK) forms a bimolecular complex with human plasmin(ogen) and changes its substrate specificity by exposing new exosites that enhances accession of substrate plasminogen (PG) to the plasmin (Pm) active site. Protein modelling studies indicated the crucial role of a loop in SAK (SAK 90-loop; Thr(90)-Glu(100)) for the docking of the substrate PG to the SAK-Pm complex. Function of SAK 90-loop was studied by site-directed mutagenesis and loop deletion. Deletion of nine amino acid residues (Tyr(92)-Glu(100)) from the SAK 90-loop, resulted in approximately 60% reduction in the PG activation, but it retained the ability to generate an active site within the complex of loop mutant of SAK (SAKDelta90) and Pm. The preformed activator complex of SAKDelta90 with Pm, however, displayed a 50-60% reduction in substrate PG activation that remained unaffected in the presence of kringle domains (K1+K2+K3+K4) of PG, whereas PG activation by SAK-Pm complex displayed approximately 50% reduction in the presence of kringles, suggesting the involvement of the kringle domains in modulating the PG activation by native SAK but not by SAKDelta90. Lysine residues (Lys(94), Lys(96), Lys(97) and Lys(98)) of the SAK 90-loop were individually mutated into alanine and, among these four SAK loop mutants, SAK(K97A) and SAK(K98A) exhibited specific activities about one-third and one-quarter respectively of the native SAK. The kinetic parameters of PG activation of their 1:1 complex with Pm indicated that the K(m) values of PG towards the activator complex of these two SAK mutants were 4-6-fold higher, suggesting the decreased accessibility of the substrate PG to the activator complex formed by these SAK mutants. These results demonstrated the involvement of the Lys(97) and Lys(98) residues of the SAK 90-loop in assisting the interaction with substrate PG. These interactions of SAK-Pm activator complex via the SAK 90-loop may provide additional anchorage site(s) to the substrate PG that, in turn, may promote the overall process of SAK-mediated PG activation.

The X-ray crystallographic structure of the angiogenesis inhibitor angiostatin

Angiogenesis inhibitors have gained much public attention recently as anti-cancer agents and several are currently in clinical trials, including angiostatin (Phase I, Thomas Jefferson University Hospital, Philadelphia, PA). We report here the bowl-shaped structure of angiostatin kringles 1-3, the first multi-kringle structure to be determined. All three kringle lysine-binding sites contain a bound bicine molecule of crystallization while the former of kringle 2 and kringle 3 are cofacial. Moreover, the separation of the kringle 2 and kringle 3 lysiner binding sites is sufficient to accommodate the alpha-helix of the 30 residue peptide VEK-30 found in the kringle 2/VEK-30 complex. Together the three kringles produce a central cavity suggestive of a unique domain where they may function in concert.

Plasmin reduction by phosphoglycerate kinase is a thiol-independent process

Phosphoglycerate kinase (PGK) is secreted by tumor cells and facilitates reduction of disulfide bond(s) in plasmin (Lay, A. J., Jiang, X.-M., Kisker, O., Flynn, E., Underwood, A., Condron, R., and Hogg, P. J. (2000) Nature 408, 869-873). The angiogenesis inhibitor, angiostatin, is cleaved from the reduced plasmin by a combination of serine- and metalloproteinases. The chemistry of protein reductants is typically mediated by a pair of closely spaced Cys residues. There are seven Cys in human PGK, and mutation of all seven to Ala did not appreciably affect plasmin reductase activity, although some of the mutations perturbed the tertiary structure of the protein. Cys-379 and Cys-380 are close to the hinge that links the N- and C-terminal domains of PGK. Alkylation/oxidation of Cys-379 and -380 by four different thiol-reactive compounds reduced plasmin reductase activity to 7--35% of control. Binding of 3-phosphoglycerate and/or MgATP to the N- and C-terminal domains of PGK, respectively, triggers a hinge bending conformational change in the enzyme. Incubation of PGK with 3-phosphoglycerate and/or MgATP ablated plasmin reductase activity, with half-maximal inhibitory effects at approximately 1 mm concentration. In summary, reduction of plasmin by PGK is a thiol-independent process, although either alkylation/oxidation of the fast-reacting Cys near the hinge or hinge bending conformational change in PGK perturbs plasmin reduction by PGK, perhaps by obstructing the interaction of plasmin with PGK or perturbing conformational changes in PGK required for plasmin reduction.

Proteomic analysis of differential protein expression in human nasopharyngeal carcinoma cells induced by NAG7 transfection

Nasopharyngeal carcinoma (NPC) is a commonly occurring tumor in southern China and south east Asia. A genetic factor has now been recognized to be associated with this cancer. A new gene, named NAG7, was cloned from the common minimal deletion region in 3p25.3-26.3. In order to investigate the function of NAG7 gene, proteomic methods were used to find and identify the differential proteins and expected to elucidate the mechanism of NAG7. The NAG7 eukaryotic expression vector was constructed and transfected into NPC cell line HNE1 with liposome. Twenty-two differential protein spots in transfected cells were found significant and reproducible using high-resolution two-dimensional electrophoresis. Nine proteins that were up-regulated and seven proteins that were down-regulated were identified by matrix assisted laser desorption/ionization time of flight mass spectrometry and database analysis. These proteins included growth arrest specific protein, DNA binding protein, caspase 6, pinch protein and ras-related protein rab-36, which are involved in cell cycling, transcription regulation, signaling pathways and apoptosis. NAG7 may exert its functions by mediating differential expression of these proteins.

The col-1 module of human matrix metalloproteinase-2 (MMP-2): structural/functional relatedness between gelatin-binding fibronectin type II modules and lysine-binding kringle domains

Human matrix metalloproteinase-2 (MMP-2) contains three in-tandem fibronectin type II (FII) repeats that bind gelatin. Here, we report the NMR solution structure of the first FII module of MMP-2 (col-1). The latter is described as a characteristic, globular FII fold containing two beta-sheets, a stretch of 3(1)-helix, a turn of alpha-helix, and an exposed hydrophobic surface lined with aromatic residues. We show that col-1 binds (Pro-Pro-Gly)6, a mimic of gelatin, with a Ka of approx. 0.42 mm(-1), and that its binding site involves a number of aromatic residues as well as Arg34, as previously found for the second and third homologous repeats. Moreover, the affinity of the in-tandem col-1+2 construct (col-12) toward the longer ligand (Pro-Pro-Gly)12 is twice that for (Pro-Pro-Gly)6, as expected from mass action. A detailed structural comparison between FII and kringle domains indicates that four main conformational features are shared: two antiparallel beta-sheets, a central 3(1)-helix, and the quasiperpendicular orientation of the two proximal Cys-Cys bonds. Structure superposition by optimizing overlap of cystine bridge areas results in close juxtaposition of their main beta-sheets and 31-helices, and reveals that the gelatin binding site of FII modules falls at similar locations and exhibits almost identical topological features to those of the lysine binding site of kringle domains. Thus, despite the minor (<15%) consensus sequence relating FII modules to kringles, there is a strong folding and binding site structural homology between the two domains, enforced by key common conformational determinants.