The α-Macroglobulin Bait Region

The Structure of Bovine Complement Component 3 Reveals the Basis for Thioester Function

The third component of complement (C3) is a 190 kDa glycoprotein essential for eliciting the complement response. The protein consists of two polypeptide chains (alpha and beta) held together with a single disulfide bridge. The beta-chain is composed of six MG domains, one of which is shared with the alpha-chain. The disulfide bridge connecting the chains is positioned in the shared MG domain. The alpha-chain consists of the anaphylatoxin domain, three MG domains, a CUB domain, an alpha(6)/alpha(6)-barrel domain and the C-terminal C345c domain. An internal thioester in the alpha-chain of C3 (present in C4 but not in C5) is cleaved during complement activation. This mediates covalent attachment of the activated C3b to immune complexes and invading microorganisms, thereby opsonizing the target. We present the structure of bovine C3 determined at 3 Angstroms resolution. The structure shows that the ester is buried deeply between the thioester domain and the properdin binding domain, in agreement with the human structure. This domain interface is broken upon activation, allowing nucleophile access. The structure of bovine C3 clearly demonstrates that the main chain around the thioester undergoes a helical transition upon activation. This rearrangement is proposed to be the basis for the high level of reactivity of the thioester group. A strictly conserved glutamate residue is suggested to function catalytically in thioester proteins. Structure-based design of inhibitors of C3 activation may target a conserved pocket between the alpha-chain and the beta-chain of C3, which appears essential for conformational changes in C3.

Interaction between human alpha2-macroglobulin and duodenase, a serine proteinase with dual specificity

Interaction between a serine proteinase from bovine duodenum and human serum alpha(2)-macroglobulin (alpha(2)-MG) was studied. alpha(2)-MG is established to be one of the most effective duodenase inhibitors. The enzyme is completely inhibited in less than 30 sec at equimolar ratio of the inhibitor and enzyme (concentration 2 x 10(-8) M). Under identical conditions, the rate of duodenase association with alpha(2)-MG is at least 2.5-fold higher than the rate of chymotrypsin association with this inhibitor. The interaction with duodenase results in proteolysis of the inhibitor subunit in the "bait region". Similarly to other proteases, duodenase in the complex with alpha(2)-MG retains the intact catalytic apparatus and ability to hydrolyze some small substrates. But the duodenase-inhibitor complex is fully inactive to proteins (bovine serum albumin). The stoichiometry of the enzyme interaction with the inhibitor is 2 : 1 (mol/mol). Based on the association rate constant and the termination time of the duodenase and alpha(2)-MG in vivo association, alpha(2)-MG is suggested to be a physiological regulator of the enzyme.

Metal homeostasis in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by neuronal dysfunction and the formation of amyloid plaques in the brain. Although the pathological processes resulting in the onset and progression of AD are not well understood, there is a growing body of evidence to support a central role for biometals in many critical aspects of the illness. Recent reports have described the exciting development of potential therapeutic agents based on the modulation of metal bioavailability. The metal ligand, clioquinol has demonstrated promising results in animal models and small clinical trials and a new generation of metal ligand-based therapeutics are currently under development. However, further research is necessary to fully understand the complex and interdependent pathways of biometal homeostasis and amyloid metabolism in AD. This information will be vital for the development of safe and effective metal-based pharmaceuticals for the treatment of AD and, potentially, other neurodegenerative disorders.

Proteases and protease inhibitors: a balance of activities in host-pathogen interaction

The immune system is the collection of effector molecules and cells of the host that act against invading parasites and their products. Secreted proteases serve important roles in parasitic metabolism and virulence and the several families of protein protease inhibitors of the plasma and blood cells play an important role in immunity by inactivating and clearing the protease virulence factors of parasites. The protease inhibitors are of two classes, the active-site inhibitors and the alpha2-macroglobulins. Inhibitors for the first class bind and inactivate the active site of the target protease. Proteins of the second class bind proteases by a unique molecular trap mechanism and deliver the bound protease to a receptor-mediated endocytic system for degradation in secondary lysosomes. Proteins of the alpha2-macroglobulin family are present in a variety of animal phyla, including the nematodes, arthropods, mollusks, echinoderms, urochordates, and vertebrates. A shared suite of unique functional characteristics have been documented for the alpha2-macroglobulins of vertebrates, arthropods, and mollusks. The alpha2-macroglobulins of nematodes, arthropods, mollusks, and vertebrates show significant sequence identity in key functional domains. Thus, the alpha2-macroglobulins comprise an evolutionarily conserved arm of the innate immune system with similar structure and function in animal phyla separated by 0.6 billion years of evolution.

Purification and characterization of alpha2-macroglobulin from grass carp Ctenopharyngodon idellus: cloning a segment of the corresponding gene

The plasma protein alpha2-macroglobulin (alpha2M) was purified by gel filtration and anion-exchange chromatography from grass carp plasma. The alpha2M consists of two different subunits of molecular weight 95 kDa and 80 kDa, respectively. The characteristics of grass carp alpha2M are similar to mammalian alpha2M, in that grass carp alpha2M exists in two forms: a fast-form and a slow-form. The former is complexed with protease. The sequence of grass carp alpha2M-conserved region and a region containing the bait region was determined by sequence analysis using polymerase chain reaction (PCR). The deduced amino acid sequence of the conserved region is similar to the alpha2M sequence of common carp, however, the bait region amino acid sequence is dramatically distinct from that of common carp. This may partially explain the differential ability of alpha2M of different species to inhibit different proteases. The alpha2M was able to inhibit Aeromonas hydrophila extracellular protease (AhECPase) and thus may play a role in resistance to infection by this pathogen.

Interaction of a plasminogen activator proteinase, LV-PA with human α2-macroglobulin

Lachesis venom plasminogen activator (LV-PA) is a 33-kDa serine proteinase isolated from bushmaster (Lachesis muta muta) snake venom, which activates the fibrinolytic system in vitro. This study has examined the effect of the plasma proteinase inhibitor alpha2-macroglobulin (alpha2-M) towards LV-PA and compares it with the effect on tissue type plasminogen activator (t-PA). The proteolytic activity of LV-PA alone or previously incubated with human plasminogen (Plg) on the large molecular mass protein substrates, dimethylcasein (DMC) and fibrinogen (Fg) was completely inhibited by human alpha2-M. However, the synthetic peptides Tos-Gly-Pro-Lys-pNA and H-D-Pro-Phe-Arg-pNA (S-2302) were hydrolyzed with almost no reduction in rate. At pH 7.4 and 37 degrees C the proteinase (0.15 microM over 15 min) interacted with alpha2-M, and each mole of alpha2-M bound 2 mol of enzyme. Sodium dodecyl sulfate gel electrophoresis of reduced samples showed that the interaction of alpha2-M with either LV-PA or t-PA preincubated with Plg resulted in the formation of approximately 90 kDa fragments and high molecular mass complexes (Mr 180 kDa), generated by the incubation mixture (LV-PA or t-PA) and Plg. The data suggest that LV-PA is a direct-type PA and its fibrinolytic effect can be reduced by alpha2-M in vivo.

A novel protease inhibitor of the alpha2-macroglobulin family expressed in the human epidermis

In the course of a large scale analysis of late-expressed genes in the human epidermis, we identified a new member of the alpha(2)-macroglobulin (alpha2M) protease inhibitor family, A2ML1 (for alpha(2)-macroglobulin-like 1). Like A2M and PZP, A2ML1 is located on chromosome 12p13.31. A2ML1 encodes a protein of 1454 amino acids, which fits the characteristics of alpha2Ms: 1) strong conservation in amino acid sequence including most of cysteine positions with alpha2M; 2) a putative central bait domain; 3) a typical thiol ester sequence. Northern blot and reverse transcriptase-PCR studies revealed a single 5-kb A2ML1 mRNA, mainly in the epidermis granular keratinocytes. A2ML1 is also transcribed in placenta, thymus, and testis. By Western blot analysis, alpha2ML1 is detected as a monomeric, approximately 180-kDa protein in human epidermis. In vitro keratinocyte differentiation is associated with increased expression levels. By immunohistochemistry, alpha2ML1 was detected within keratinosomes in the granular layer of the epidermis, and as a secreted product in the extracellular space between the uppermost granular layer and the cornified layer. Recombinant alpha2ML1 displayed inhibitory activity toward chymotrypsin, papain, thermolysin, subtilisin A, and to a lesser extent, elastase but not trypsin. Incubation with chymotrypsin and the chymotrypsin-like kallikrein 7 protease indicated that alpha2ML1 binds covalently to these proteases, a feature shared with other members of the family. Therefore, alpha2ML1 is the first alpha2M family member detected in the epidermis. It may play an important role during desquamation by inhibiting extracellular proteases.

Identification and characterization of syntenin binding protein in the black tiger shrimp Penaeus monodon

Shrimp exhibit a diverse response to viral infection that is manifested in drastic up- and down-regulations of a variety of genes. In our previous work, we identified syntenin of the shrimp Penaeus monodon (Pm) as a dynamic responder to white spot syndrome virus (WSSV) infection, its message being greatly upregulated in the acute phase of the infection. In order to further explore the link between Pm-syntenin and viral infection, we performed a yeast two-hybrid screening of a P. monodon cDNA library, using Pm-syntenin as bait. One of the molecules that specifically interacted with Pm-syntenin was the receptor-binding domain of alpha-2-macroglobulin (alpha2M). A GST pull-down assay showed that GST-alpha2M, but not GST alone, was capable of co-precipitating syntenin. Another GST pull-down assay showed that GST-syntenin, but not GST alone, was capable of co-precipitating alpha2M. In addition, mutant analyses showed that the N-terminal 131 amino acids of syntenin were both necessary and sufficient to bind the C-terminus receptor-binding domain of alpha2M. Furthermore, WSSV-infected Pm showed a significant upregulation of the alpha2M message, suggesting that both syntenin and its protein partner alpha2M are upregulated in the acute phase of a WSSV infection. Taken together with a previous report showing the co-localization of alpha2M and syntenin in the exosome of a dendritic cell line, it is likely that syntenin, through its interaction with alpha2M, plays an important role in the immune defense mechanisms of viral infections of shrimps.

Structures of complement component C3 provide insights into the function and evolution of immunity

The mammalian complement system is a phylogenetically ancient cascade system that has a major role in innate and adaptive immunity. Activation of component C3 (1,641 residues) is central to the three complement pathways and results in inflammation and elimination of self and non-self targets. Here we present crystal structures of native C3 and its final major proteolytic fragment C3c. The structures reveal thirteen domains, nine of which were unpredicted, and suggest that the proteins of the alpha2-macroglobulin family evolved from a core of eight homologous domains. A double mechanism prevents hydrolysis of the thioester group, essential for covalent attachment of activated C3 to target surfaces. Marked conformational changes in the alpha-chain, including movement of a critical interaction site through a ring formed by the domains of the beta-chain, indicate an unprecedented, conformation-dependent mechanism of activation, regulation and biological function of C3.

Brain, aging and neurodegeneration: role of zinc ion availability

Actual fields of research in neurobiology are not only aimed at understanding the different aspects of brain aging but also at developing strategies useful to preserve brain compensatory capacity and to prevent the onset of neurodegenerative diseases. Consistent with this trend much attention has been addressed to zinc metabolism. In fact, zinc acts as a neuromodulator at excitatory synapses and has a considerable role in the stress response and in the functionality of zinc-dependent enzymes contributing to maintaining brain compensatory capacity. In particular, the mechanisms that modulate the free zinc pool are pivotal for safeguarding brain health and performance. Alterations in zinc homeostasis have been reported in Parkinson's and Alzheimer's disease as well as in transient forebrain ischemia, seizures and traumatic brain injury, but little is known regarding aged brain. There is much evidence that that age-related changes, frequently associated to a decline in brain functions and impaired cognitive performances, could be related to dysfunctions affecting the intracellular zinc ion availability. A general agreement emerges from studies of humans' and rodents' old brains about an increased expression of metallothionein (MT) isoforms I and II, but dyshomogenous results are reported for MT-III, and it is still uncertain whether these proteins maintain in aging the protective role, as it occurs in adult/young age. At the same time, there is considerable evidence that amyloid-beta deposition in Alzheimer's disease is induced by zinc, but the pathological significance and the causes of this phenomenon are still an open question. The scientific debate on the role of zinc and of some zinc-binding proteins in aging and neurodegenerative disorders, as well as on the beneficial effect of zinc supplementation in aged brain and neurodegeneration, is extensively discussed in this review.

Biochemical and enzymatic characterization of human kallikrein 5 (hK5), a novel serine protease potentially involved in cancer progression

Human kallikrein 5 (KLK5) is a member of the human kallikrein gene family of serine proteases. Preliminary results indicate that the protein, hK5, may be a potential serological marker for breast and ovarian cancer. Other studies implicate hK5 with skin desquamation and skin diseases. To gain further insights on hK5 physiological functions, we studied its substrate specificity, the regulation of its activity by various inhibitors, and identified candidate physiological substrates. After producing and purifying recombinant hK5 in yeast, we determined the k(cat)/K(m) ratio of the fluorogenic substrates Gly-Pro-Arg-AMC and Gly-Pro-Lys-AMC, and showed that it has trypsin-like activity with strong preference for Arg over Lys in the P1 position. The serpins alpha(2)-antiplasmin and antithrombin were able to inhibit hK5 with an inhibition constant (k(+2)/K(i)) of 1.0 x 10(-) (2)and 4.2 x 10(-4) m(-1) min(-1), respectively. No inhibition was observed with the serpins alpha(1)-antitrypsin and alpha(1)-antichymotrypsin, although alpha(2)-macroglobulin partially inhibited hK5 at high concentrations. We also demonstrated that hK5 can efficiently digest the extracellular matrix components, collagens type I, II, III, and IV, fibronectin, and laminin. Furthermore, our results suggest that hK5 can potentially release (a) angiostatin 4.5 from plasminogen, (b) "cystatin-like domain 3" from low molecular weight kininogen, and (c) fibrinopeptide B and peptide beta15-42 from the Bbeta chain of fibrinogen. hK5 could also play a role in the regulation of the binding of plasminogen activator inhibitor 1 to vitronectin. Our findings suggest that hK5 may be implicated in tumor progression, particularly in invasion and angiogenesis, and may represent a novel therapeutic target.

Biochemical properties of a bushmaster snake venom serine proteinase (LV-Ka), and its kinin releasing activity evaluated in rat mesenteric arterial rings

A serine proteinase with kallikrein-like activity (LV-Ka) has been purified to homogeneity from bushmaster snake (Lachesis muta muta) venom. Physicochemical studies indicated that LV-Ka is a single chain glycoprotein with a molecular mass (Mr) of 33 kDa under reducing conditions which was reduced to 28 kDa after treatment with N-Glycosidase F (PNGase F). LV-Ka can be bounded and neutralized by serum alpha2-macroglobulin (alpha2-M), a prevalent mammalian protease inhibitor that is capable of forming a macromolecular complex with LV-Ka (Mr >180 kDa). Cleavage of alpha2-M by the enzyme resulted in the formation of 90-kDa fragments. The proteolytic activity of LV-Ka against dimethylcasein could be inhibited by alpha2-M, and the binding ratio of the inhibitor:enzyme complex was found to be 1:1. The Michaelis constant, Km, and catalytic rate constant, kcat, of LV-Ka on four selective chromogenic substrates were obtained from Lineweaver-Burk plots. LV-Ka exhibits substrate specificities not only for the glandular kallikrein H-D-Val-Leu-Arg-pNA (S-2266) but also for the plasmin substrates S-2251 and Tos-Gly-Pro-Lys-pNA. Bovine kininogen incubated with LV-Ka generated a polypeptide that dose dependently contracted mesenteric arterial rings from spontaneously hypertensive rats (SHR) in a similar way as bradykinin (BK) does. As it happens with BK, LV-Ka generated polypeptide was inhibited by HOE-140, a bradykinin B2-receptor antagonist and by indomethacin, a cyclo-oxygenase inhibitor. These results strongly suggest that the polypeptide generated by LV-Ka by cleavage of bovine kininogen is bradykinin. In addition, our studies may help to understand the mechanism of action involved in hypotension produced by envenomation of bushmaster snake.

alpha2-Macroglobulin-proteinase complexes protect Streptococcus pyogenes from killing by the antimicrobial peptide LL-37

The significant human bacterial pathogen Streptococcus pyogenes expresses GRAB, a surface protein that binds alpha(2)-macroglobulin (alpha(2)M), a major proteinase inhibitor of human plasma. alpha(2)M inhibits proteolysis by trapping the proteinase, which, however, still remains proteolytically active against smaller peptides that can penetrate the alpha(2)M-proteinase complex. Here we report that SpeB, a cysteine proteinase secreted by S. pyogenes, is trapped by alpha(2)M bound to protein GRAB. As a consequence, SpeB is retained at the bacterial surface and protects S. pyogenes against killing by the antibacterial peptide LL-37.

Molecular cloning and expression analysis of alpha 2-macroglobulin in the kuruma shrimp, Marsupenaeus japonicus

The cDNA encoding the kuruma shrimp, Marsupenaeus japonicus alpha(2)-macroglobulin (alpha(2)M) was obtained by screening a haemocyte cDNA library and 5' RACE PCR amplification. The full length cDNA of 4748 bp contains an open reading frame of 4518 nucleotides that translates into a 1505-amino acid putative peptide, with a 5'untranslated region (UTR) of 59 bp and a 3'UTR of 171 bp. The open reading frame encodes an N-terminal signal sequence of 17 residues and a mature protein of 1488 residues. The entire amino acid sequence is similar to the alpha(2)M sequences of arthropods (30-31% identity), mammals (26-27% identity) and fish (25-28% identity). The M. japonicus alpha(2)M sequence contains putative functional domains including a bait region, an internal thiol ester site, and a receptor-binding domain, which are present in mammalian alpha(2)Ms. In a healthy shrimp, the mRNA of alpha(2)M was mainly expressed in haemocytes. In addition, the expression level of alpha(2)M mRNA was dramatically increased by through time upon oral administration of peptidoglycan (PG), which is an immune stimulant. The highest expression of alpha(2)M mRNA was observed 7 days after feeding with PG. These results suggest that the shrimp alpha(2)M is an important molecule in immune system.

α2-Macroglobulin Is a Novel Substrate for ADAMTS-4 and ADAMTS-5 and Represents an Endogenous Inhibitor of These Enzymes

Osteoarthritis is characterized by the loss of aggrecan and collagen from the cartilage extracellular matrix. The proteinases responsible for the breakdown of cartilage aggrecan include ADAMTS-4 (aggrecanase 1) and ADAMTS-5 (aggrecanase 2). Post-translational inhibition of ADAMTS-4/-5 activity may be important for maintaining normal homeostasis of aggrecan metabolism, and thus, any disruption to this inhibition could lead to accelerated aggrecan breakdown. To date TIMP-3 (tissue inhibitor of matrix metalloproteinases-3) is the only endogenous inhibitor of ADAMTS-4/-5 that has been identified. In the present studies we identify alpha(2)-macroglobulin (alpha(2)M) as an additional endogenous inhibitor of ADAMTS-4 and ADAMTS-5. alpha(2)M inhibited the activity of both ADAMTS-4 and ADAMTS-5 in a concentration-dependent manner, demonstrating 1:1 stoichiometry with second-order rate constants on the order of 10(6) and 10(5) m(-1) s(-1), respectively. Inhibition of the aggrecanases was mediated by proteolysis of the bait region within alpha(2)M, resulting in physical entrapment of these proteinases. Both ADAMTS-4 and ADAMTS-5 cleaved alpha(2)M at Met(690)/Gly(691), representing a novel proteinase cleavage site within alpha(2)M and a novel site of cleavage for ADAMTS-4 and ADAMTS-5. Finally, the use of the anti-neoepitope antibodies to detect aggrecanase-generated alpha(2)M-fragments in synovial fluid was investigated and found to be uninformative.

Expression and characterization of the C345C/NTR domains of complement components C3 and C5

Complement components C3, C4, and C5 are members of the thioester-containing alpha-macroglobulin protein superfamily. Within this superfamily, a unique feature of the complement proteins is a 150-residue-long C-terminal extension of their alpha-subunits that harbors three internal disulfide bonds. Previous reports have suggested that this is an independent structural module, homologous to modules found in other proteins, including netrins and tissue inhibitors of metalloproteinases. Because of its distribution, this putative module has been named both C345C and NTR. To assess the structures of these segments of the complement proteins, their relationships with other domains, and activities as independent structures, we expressed C345C from C3 and C5 in a bacterial strain that permits cytoplasmic disulfide bond formation. Affinity purification directly from cell lysates yielded recombinant C3- and C5-C345C with properties consistent with multiple intramolecular disulfide bonds and high beta-sheet contents. rC5-, but not rC3-C345C inhibited complement hemolytic activity, and surface plasmon resonance studies revealed that rC5-C345C binds to complement components C6 and C7 with dissociation constants of 10 and 3 nM, respectively. Our results provide strong evidence that this binding corresponds to the previously described reversible binding of C5 to C6 and C7, and taken together with earlier work, indicate that the C5-C345C module interacts directly with the factor I modules in C6 and C7. The high binding affinities suggest that complexes composed of C5 bound to C6 or C7 exist in plasma before activation and may facilitate assembly of the complement membrane attack complex.

The human kallikrein protein 5 (hK5) is enzymatically active, glycosylated and forms complexes with two protease inhibitors in ovarian cancer fluids

The kallikrein family is a group of 15 serine protease genes clustered on chromosome 19q13.4. Binding of kallikreins to protease inhibitors is an important mechanism for regulating their enzymatic activity and may have potential clinical applications. Human kallikrein gene 5 (KLK5) is a member of this family and encodes for a secreted serine protease (hK5). This kallikrein was shown to be differentially expressed at the mRNA and protein levels in diverse malignancies. Our objective was to study the enzymatic activity and the interaction of recombinant hK5 protein with protease inhibitors. Recombinant hK5 protein was produced in yeast and mammalian expression systems and purified by chromatography. HPLC fractionation, followed by ELISA-type assays, immunoblotting and radiolabeling experiments were performed to detect the possible interactions between hK5 and proteinase inhibitors in serum. Enzymatic deglycosylation was performed to examine the glycosylation pattern of the protein. The enzymatic activity of hK5 was tested using trypsin and chymotrypsin-specific synthetic fluorogenic substrates. In serum and ascites fluid, in addition to the free ( approximately 40 kDa) form, hK5 forms complexes with alpha(1)-antitrypsin and alpha(2)-macroglobulin. These complexes were detected by hybrid ELISA-type assays using hK5-specific coating antibodies and inhibitor detection antibodies. The ability of hK5 to bind to these inhibitors was further verified in vitro. Spiking of serum samples with 125I-labeled hK5 results in the distribution of the protein in two higher molecular mass (bound) forms, in addition to the unbound form. The hK5 mature enzyme is active and shows trypsin, but not chymotrypsin-like, activity. The pro-form of hK5 is not active. Recombinant hK5 shows a higher than predicted molecular mass due to glycosylation. hK5 is partially complexed with alpha(1)-antitrypsin and alpha(2)-macroglobulin in serum and ascites fluid of ovarian cancer patients. The recombinant protein is glycosylated and its mature form shows trypsin-like activity.

Purification and characterization of alpha 2-macroglobulin from the white shrimp (Penaeus vannamei)

alpha(2)-Macroglobulin (alpha(2)M) is a broad-spectrum protease-binding protein abundant in plasma from vertebrates and several invertebrate phyla. This protein was purified from cell-free hemolymph of the white shrimp, Penaeus vannamei, using Blue-Sepharose and Phenyl-Sepharose chromatography. The shrimp alpha(2)M is a 380 kDa protein, a homodimer of two apparently identical subunits of approximately 180 kDa linked by disulphide bridges. The amino acid sequence of the N-terminus is similar to the Limulus alpha(2)M counterpart. The shrimp alpha(2)M has a wide inhibition spectrum against different proteinase types including trypsin, leucine amino peptidase, chymotrypsin, elastase and papain. The secondary structure of shrimp alpha(2)M is mainly beta-sheet (36%), with a characteristic minimum elipticity at 217 nm. Evidence for a thiolester-mediated inhibition mechanism of proteases by alpha(2)M was provided by inactivation with methylamine.

Localization of carbohydrate attachment sites and disulfide bridges in limulus alpha 2-macroglobulin. Evidence for two forms differing primarily in their bait region sequences

The primary structure determination of the dimeric invertebrate alpha(2)-macroglobulin (alpha(2)M) from Limulus polyphemus has been completed by determining its sites of glycosylation and disulfide bridge pattern. Of seven potential glycosylation sites for N-linked glycosylation, six (Asn(275), Asn(307), Asn(866), Asn(896), Asn(1089), and Asn(1145)) carry common glucosamine-based carbohydrates groups, whereas one (Asn(80)) carries a carbohydrate chain containing both glucosamine and galactosamine. Nine disulfide bridges, which are homologues with bridges in human alpha(2)M, have been identified (Cys(228)-Cys(269), Cys(456)-Cys(580), Cys(612)-Cys(799), Cys(657)-Cys(707), Cys(849)-Cys(876), Cys(874)-Cys(910), Cys(946)-Cys(1328), Cys(1104)-Cys(1155), and Cys(1362)-Cys(1475)). In addition to these bridges, Limulus alpha(2)M contains three unique bridges that connect Cys(361) and Cys(382), Cys(1370) and Cys(1374), respectively, and Cys(719) in one subunit with the same residue in the other subunit of the dimer. The latter bridge forms the only interchain disulfide bridge in Limulus alpha(2)M. The location of this bridge within the bait region is discussed and compared with other alpha-macroglobulins. Several peptides identified in the course of determining the disulfide bridge pattern provided evidence for the existence of two forms of Limulus alpha(2)M. The two forms have a high degree of sequence identity, but they differ extensively in large parts of their bait regions suggesting that they have different inhibitory spectra. The two forms (Limulus alpha(2)M-1 and -2) are most likely present in an approximately 2:1 ratio in the hemolymph of each animal, and they can be partially separated on a Mono Q column at pH 7.4 by applying a shallow gradient of NaCl.

Linkage relationships of genes coding for alpha2-macroglobulin, C3 and C4 in the zebrafish: implications for the evolution of the complement and Mhc systems

The alpha2-macroglobulin (A2M) and the complement components C3 and C4 are related proteins derived from a common ancestor. Theoretically, this derivation could have occurred either by tandem duplications of their encoding genes or by polyploidization involving chromosomal segments, a chromosome or the whole genome. In tetrapods the A2M-, C3- and C4-encoding genes are generally each located on a different chromosome. This observation has been interpreted as supporting their origin by polyploidization. We identified and mapped (with the help of a radiation hybrid panel of cell lines) the A2M, C3 and C4 loci in the zebrafish, Danio rerio. Each of the three types of loci is present in the zebrafish in multiple copies, but all of the identified copies of a given type map to the same region in linkage groups 1 (C3) and 15 (A2M, C4). The A2M and C4 loci are mapped in the same region not linked to any of the class I or class II major histocompatibility complex (Mhc) loci. These observations are interpreted as supporting the origin of the A2M family of genes by tandem duplications, followed by the dispersal of the copies to different chromosomes. It is also argued that the association of C4 with the class I/II loci in tetrapods is accidental and without functional significance.

The three-dimensional structure of the human alpha 2-macroglobulin dimer reveals its structural organization in the tetrameric native and chymotrypsin alpha 2-macroglobulin complexes

Three-dimensional electron microscopy reconstructions of the human alpha(2)-macroglobulin (alpha(2)M) dimer and chymotrypsin-transformed alpha(2)M reveal the structural arrangement of the two dimers that comprise native and proteinase-transformed molecules. They consist of two side-by-side extended strands that have a clockwise and counterclockwise twist about their major axes in the native and transformed structures, respectively. This and other studies show that there are major contacts between the two strands at both ends of the molecule that evidently sequester the receptor binding domains. Upon proteinase cleavage of the bait domains and subsequent thiol ester cleavages, which occur near the central region of the molecule, the two strands separate by 40 A at both ends of the structure to expose the receptor binding domains and form the arm-like extensions of the transformed alpha(2)M. During the transformation of the structure, the strands untwist to expose the alpha(2)M central cavity to the proteinase. This extraordinary change in the architecture of alpha(2)M functions to completely engulf two molecules of chymotrypsin within its central cavity and to irreversibly encapsulate them.

New member of the trefoil factor family of proteins is an alpha-macroglobulin protease inhibitor

The amino acid sequence of the monomeric alpha-macroglobulin (alphaM) from the American bullfrog, Rana catesbiana, was determined. The mature protein consisted of 1469 amino acid residues and shared sequence identity with other members of the alphaM family of protein. The central portion of the frog monomeric alphaM contained Cys residues positioned analogously to the Cys residues in human alpha(2)-macroglobulin (alpha(2)M), known to be involved in disulfide bridges. Additionally, the frog monomeric alphaM contained six Cys residues in a approximately 60 residue COOH-terminal extension not present in previously characterized alphaMs. The spacing of the Cys residues and the overall sequence identity of this COOH-terminal extension were consistent with a trefoil motif. This is the first time a member of the trefoil factor family has been identified in the circulatory system. The "bait region" was located between Arg(675)-Lys(685) and contained mainly basic amino acid residues. The COOH-terminal receptor-binding domain was not exposed prior to proteolysis of this highly susceptible region. The proximity of the receptor-binding and trefoil domains implied that the trefoil domain is similarly concealed before bait region cleavage.

Distinct binding sites in the structure of alpha 2-macroglobulin mediate the interaction with beta-amyloid peptide and growth factors

Alpha(2)-macroglobulin (alpha(2)M) and its receptor, low density lipoprotein receptor-related protein (LRP), function together to facilitate the cellular uptake and degradation of beta-amyloid peptide (Abeta). In this study, we demonstrate that Abeta binds selectively to alpha(2)M that has been induced to undergo conformational change by reaction with methylamine. Denatured alpha(2)M subunits, which were immobilized on polyvinylidene difluoride membranes, bound Abeta, suggesting that alpha(2)M tertiary and quaternary structure are not necessary. To determine whether a specific sequence in alpha(2)M is responsible for Abeta binding, we prepared and analyzed defined alpha(2)M fragments and glutathione S-transferase-alpha(2)M peptide fusion proteins. A single sequence, centered at amino acids (aa) 1314-1365, was identified as the only major Abeta-binding site. Importantly, Abeta did not bind to the previously characterized growth factor-binding site (aa 718-734). Although the Abeta binding sequence is adjacent to the binding site for LRP, the results of experiments with mutated fusion proteins indicate that the two sites are distinct. Furthermore, a saturating concentration of Abeta did not inhibit LRP-mediated clearance of alpha(2)M-MA in mice. Using various methods, we determined that the K(D) for the interaction of Abeta with its binding site in the individual alpha(2)M subunit is 0.7-2.4 microm. The capacity of alpha(2)M to bind Abeta and deliver it to LRP may be greater than that predicted by the K(D), because each alpha(2)M subunit may bind Abeta and the bound Abeta may multimerize. These studies suggest a model in which alpha(2)M has three protein interaction sites with distinct specificities, mediating the interaction with Abeta, growth factors, and LRP.

Comparative binding of biotinylated neurotrophins to alpha(2)-macroglobulin family of proteins: relationship between cytokine-binding and neuro-modulatory activities of the macroglobulins

Human alpha(2)-macroglobulin (alpha(2)M), pregnancy zone protein (PZP), rat alpha(1)M and acute-phase rat alpha(2)M belong to the alpha(2)M gene family of proteins, which can react covalently with nucleophilic monoamines to yield monoamine-activated (MA) macroglobulins. The MA forms of human alpha(2)M, PZP and rat alpha(2)M have been demonstrated previously to inhibit various neurotrophin-promoted neuronal activities, whereas MA-alpha(1)M is neurostimulatory and all native macroglobulins are generally inactive. The mechanism of neuromodulation is unknown, but it has been postulated that MA macroglobulins might inhibit neurons via their binding and sequestration of neurotrophins. This study employed a novel biotinylation-Western blot technique to compare the neurotrophin-binding properties of the four macroglobulins, and to correlate their binding activities with their known neuro-modulatory activities. In comparison with their respective native counterparts, human and rat MA-alpha(2)M bound slightly more NGF, but significantly less BDNF or NT-3. Native human alpha(2)M and PZP in general have no neuro-modulatory activity, but native PZP bound significantly more NGF, BDNF or NT-3 than either native alpha(2)M or MA-alpha(2)M, which is neuro-inhibitory. It is known that MA-PZP is neuro-inhibitory, but it fails to bind more NGF, BDNF, or NT-3 than native PZP. MA-alpha(1)M is the only macroglobulin known to stimulate NGF-promoted neurite outgrowth, but it bound NGF with similar affinities as native alpha(1)M and rat alpha(2)M; in addition, it bound significantly less BDNF or NT-3 than native alpha(1)M. All the bindings were non-covalent and appeared specific. In conclusion, PZP and rat macroglobulins are versatile carriers of neurotrophins with diverse binding capacities, and the neurotrophin-binding property does not appear to mediate the neuro-modulatory activity of these human and rat macroglobulins.

Inhibition of mutalysin II, a metalloproteinase from bushmaster snake venom by human alpha2-macroglobulin and rabbit immunoglobulin

Mutalysin II is a 22.5-kDa zinc endopeptidase isolated from Lachesis muta muta snake venom. In order to determine whether the inhibitors human alpha2-macroglobulin (alpha2-M) and rabbit antibody to mutalysin II share a common mechanism, we have investigated the inhibition of mutalysin II by these two different glycoproteins. The proteolytic activity of mutalysin II with dimethylcasein as substrate was completely inhibited by human alpha2-M and by a purified rabbit antibody to mutalysin II. The protection of fibrin(ogen) digestion by alpha2-M was slightly better than the protection offered by the antibody. In addition, the purified antibody reacted only with the metalloproteinase in bushmaster venom, as demonstrated by immunodiffusion. SDS-PAGE analysis of reduced samples showed that the interaction of mutalysin II with alpha2-M resulted in the formation of high molecular complex ( approximately 180000) and M(r) 90000 fragments generated by the venom enzyme. Also, fragments at 85 and 23 kDa were detected under non-reducing conditions after incubation of rabbit immunoglobulin with enzyme. Proteolysis of dimethylcasein as substrate revealed that the stoichiometry of inhibition was 1.0 mol of human alpha2-M and 1.5 mol of rabbit IgG antimutalysin II per mole of enzyme. Furthermore, dimethylcasein hydrolysis indicated that several viperid snake venoms, including Bothrops atrox, B. alternatus and Trimeresurus flavoviridis cross-reacted with the specific rabbit antibody to varying degrees.

The contribution of proteinase inhibitors to immune defense

The colonization of a potential host by a parasite requires an ability to cross the integuments and then to escape from the host immune defenses. Proteinases are important virulence factors that assist these processes. Host proteinase inhibitors potentially contribute to immunity by inactivating the proteinase virulence factors of pathogens.

A 16-amino acid peptide from human alpha2-macroglobulin binds transforming growth factor-beta and platelet-derived growth factor-BB

Alpha2-macroglobulin (alpha2M) is a major carrier of transforming growth factor-beta (TGF-beta) in vitro and in vivo. By screening glutathione S-transferase (GST) fusion proteins with overlapping sequences, we localized the TGFbeta-binding site to aa 700-738 of the mature human alpha2M subunit. In separate experiments, we screened overlapping synthetic peptides corresponding to aa 696-777 of alpha2M and identified a single 16-mer (718-733) that binds TGF-beta1. Platelet-derived growth factor-BB (PDGF-BB) bound to the same peptide, even though TGF-beta and PDGF-BB share almost no sequence identity. The sequence of the growth factor-binding peptide, WDLVVVNSAGVAEVGV, included a high proportion of hydrophobic amino acids. The analogous peptide from murinoglobulin, a human alpha2M homologue that does not bind growth factors, contained only three nonconservative amino acid substitutions; however, the MUG peptide failed to bind TGF-beta1 and PDGF-BB. These results demonstrate that a distinct and highly-restricted site in alpha2M, positioned near the C-terminal flank of the bait region, mediates growth factor binding. At least part of the growth factor-binding site is encoded by exon 18 of the alpha2M gene, which is notable for a 5' splice site polymorphism that has been implicated in Alzheimer's Disease.

Determination of prostate-specific antigen complexed to alpha(2)-macroglobulin in serum increases the specificity of free to total PSA for prostate cancer

OBJECTIVES

To determine whether prostate-specific antigen (PSA) complexed to alpha(2)-macroglobulin (A2M) increases the specificity of free PSA (fPSA) and total PSA (tPSA) for the diagnosis of prostate cancer (PCa).

METHODS

In a series of 73 patients with PCa and 58 with benign prostatic hyperplasia (BPH), fPSA, tPSA, and PSA complexed with A2M (PSA-A2M) in serum were determined by specific immunoassays. The assay for PSA-A2M was based on the immunoadsorption of immunoreactive PSA in serum and the measurement of the PSA immunoreactivity released by denaturation of PSA-A2M at pH 11.4.

RESULTS

The median proportion of PSA-A2M [ %PSA-A2M=PSA-A2M/(tPSA+PSA-A2M)] and that of fPSA ( %fPSA=fPSA/tPSA) were significantly lower in patients with PCa (8.2% and 12.4%, respectively) than in patients with BPH (11.6% and 22.5%, P = 0.0014 and P <0.0001, respectively). The median sum of %PSA-A2M and %fPSA was 22.4% in PCa and 38.2% in BPH (P <0.0001). When the sum of %PSA-A2M and %fPSA was used as a diagnostic test for PCa, 57% of patients with "falsely" elevated PSA concentrations (4 to 10 ng/mL) caused by BPH could be correctly identified without missing patients with PCa compared with 18% of the patients with BPH but not PCa using %fPSA alone.

CONCLUSIONS

Measurement of the sum of %PSA-A2M and %fPSA in serum significantly improves the cancer specificity of the PSA test compared with the use of tPSA and %fPSA.

The structure of the C949S mutant human alpha(2)-macroglobulin demonstrates the critical role of the internal thiol esters in its proteinase-entrapping structural transformation

A three-dimensional reconstruction of a protein-engineered mutant alpha(2)-macroglobulin (alpha(2)M) in which a serine residue was substituted for the cysteine 949 (C949S), making it unable to form internal thiol ester moieties, was compared with native and methylamine-transformed alpha(2)Ms. The native alpha(2)M structure consists of two oppositely oriented Z-shaped strands. Thiol ester cleavage following an encounter with a proteinase or a nucleophilic attack by methylamine causes a structural transformation in which the strands assume an opposite handedness and a significant portion of the protein density migrates from the distal ends of the molecule toward the center. The C949S mutant showed a protein density distribution very similar to that of transformed alpha(2)M, with a compact central region of protein density connected to two receptor-binding arms on each end of the molecule. Since no particle shapes characteristic of native or half-transformed alpha(2)Ms were seen in electron micrographs and the C949S mutant and alpha(2)M-methylamine structures are highly similar, we conclude that the intact thiol esters maintain native alpha(2)M in a quasi-stable state. In their absence, alpha(2)M folds into the more stable transformed structure, which displays the functionally important receptor-binding domains and contains the proteinase-entrapping internal cavity.

Aberrant forms of alpha(2)-macroglobulin purified from patients with multiple sclerosis

The biochemical properties of alpha(2)-macroglobulin were investigated in four patients with multiple sclerosis and compared to alpha(2)-macroglobulin from healthy controls. An impaired stability of alpha(2)-macroglobulin from the multiple sclerosis patients was demonstrated as a spontaneous conversion to an electrophoretic"fast" form of alpha(2)-macroglobulin upon purification and storage, with a concomitant decrease in functional capacity to inhibit proteinases. The ability to form complexes with proteinases was significantly reduced in alpha(2)-macroglobulin purified from the multiple sclerosis patients. The aberrant molecular arrangements of the protein were not due to proteinase cleavages in the bait regions of alpha(2)-macroglobulin, as demonstrated by gel electrophoresis and protein sequencing. The number of functional thiol esters, however, was reduced in alpha(2)-macroglobulin purified from the multiple sclerosis patients, an observation compatible with the impaired proteinase binding property. Furthermore, differences in isoelectric points were observed between alpha(2)-macroglobulin from the multiple sclerosis patients and alpha(2)-macroglobulin from healthy controls. The results suggest that aberrant forms of alpha(2)-macroglobulin may be present in patients with multiple sclerosis.

Identical or overlapping sequences in the primary structure of human alpha(2)-macroglobulin are responsible for the binding of nerve growth factor-beta, platelet-derived growth factor-BB, and transforming growth factor-beta

alpha(2)-Macroglobulin (alpha(2)M) functions as a proteinase inhibitor and as a carrier of diverse growth factors. In this study, we localized binding sites for platelet-derived growth factor-BB (PDGF-BB) and nerve growth factor-beta (NGF-beta) to a linear sequence in the 180-kDa human alpha(2)M subunit which includes amino acids 591-774. A glutathione S-transferase fusion protein containing amino acids 591-774 (FP3) bound PDGF-BB and NGF-beta in ligand blotting assays whereas five other fusion proteins, which collectively include amino acids 99-590 and 775-1451 did not. The K(D) values for PDGF-BB and NGF-beta binding to immobilized FP3 were 300 +/- 40 and 180 +/- 30 nM, respectively; these values were comparable with those determined using methylamine-modified alpha(2)M, suggesting that higher-order alpha(2)M structure is not necessary for PDGF-BB and NGF-beta binding. PDGF-BB and NGF-beta blocked the binding of transforming growth factor-beta1 (TGF-beta1) to FP3. Furthermore, murinoglobulin, which is the only known member of the alpha-macroglobulin family that does not bind TGF-beta, also failed to bind PDGF-BB and NGF-beta. These results support the hypothesis that either a single linear sequence in human alpha(2)M or overlapping sequences are responsible for the binding of TGF-beta, PDGF-BB, and NGF-beta, even though there is minimal sequence identity between these three growth factors. FP3 blocked the binding of PDGF-BB to a purified chimeric protein, in which the extracellular domain of the PDGF beta receptor was fused to the IgG(1) Fc domain, and to PDGF receptors on NIH 3T3 cells. Thus, FP3 may inhibit the activity of PDGF-BB.

A high affinity binding site for the polypyrimidine tract binding protein (PTB) is located in the 5'-untranslated region of the rat proteinase alpha1-inhibitor 3 variant I gene

As a first step towards understanding the mechanism underlying the differential gene expression of the two variants of the rat proteinase-inhibitor alpha1-inhibitor 3 (alpha1-I3) corresponding genomic clones were isolated. The 100% similarity between the sequence of one genomic clone and that of the alpha1-13 variant I cDNA strongly suggested that its 5'-sequence represented the upstream region of the corresponding gene. Several putative cis-regulatory elements were identified as well as a polypyrimidine tract located between the transcription start site of the alpha1-I3 variant I mRNA and the AUG codon. The polypyrimidine tract functions as a positive cis-element in a heterologous promoter. By electrophoretic mobility shift assays (EMSA) we have shown that a GST (glutathione S-transferase) fusion of the rat polypyrimidine tract binding protein (PTB) has a high affinity for the pyrimidine-rich sense strand but not for the complementary sequence of the 5'-untranslated region of the alpha1-I3 variant I gene.

Lebetase, an alpha(beta)-fibrin(ogen)olytic metalloproteinase of Vipera lebetina snake venom, is inhibited by human alpha-macroglobulins

The effects of the plasma proteinase inhibitors alpha(2)-macroglobulin (alpha(2)M) and the alpha(2)M-related pregnancy zone protein (PZP) were evaluated towards the metalloproteinase lebetase, isolated from Vipera lebetina venom. We demonstrate that lebetase interacts with both inhibitors. Cleavage of alpha(2)M by lebetase resulted in the formation of 90-kDa fragments, and covalent complexes of alpha(2)M with lebetase were observed. The proteolytic activity of lebetase against fibrinogen and azocasein could be inhibited by alpha(2)M. Cleavage of PZP also resulted in the formation of 90-kDa fragments, and complexes of both dimer and tetramer forms of PZP with lebetase were detected. The amino acid sequence identification of the sites of specific proteolysis of alpha(2)M and PZP demonstrate that the cleavage sites are within the bait regions of both proteins. Lebetase I cleaves between Arg(696)-Leu(697), which is one of the most common cleavage sites in alpha(2)M by proteinases. The other two cleavage sites in alpha(2)M by lebetase are Gly(679)-Leu(680) and His(694)-Ala(695). The cleavage between Pro(689)-Gln(690) is the only cleavage site identified in PZP. In that lebetase is an anticoagulation agent in vivo, we propose that the interaction of lebetase with alpha(2)M may suggest a reduced fibrin(ogen)olytic activity of lebetase in human.

Highly efficient inhibition of human chymase by alpha(2)-macroglobulin

The inhibition of human chymase by the protease inhibitor alpha(2)-macroglobulin (alpha2M) was investigated. Titration of chymase hydrolytic activity with purified alpha2M showed that approximately 1 mol of alpha2M tetramer inhibits 1 mol of chymase. Inhibition was associated with cleavage of the alpha2M bait region and formation of a 200-kDa covalent complex. NH(2)-terminal sequencing of chymase-treated alpha2M revealed cleavage at bonds Phe684-Tyr685 and Tyr685-Glu686 of the bait region. alpha2M pretreated with methylamine, an inactivator of alpha2M, did not inhibit chymase. The apparent second-order rate constant for inhibition (k(ass)) was 5 x 10(6) M(-1) s(-1), making alpha2M the most efficient natural protein protease inhibitor of chymase so far described. The k(ass) value for inhibition was decreased approximately 10-fold by addition of heparin, a glycosaminoglycan produced by mast cells that binds to chymase. Heparin did not change significantly the stoichiometry of inhibition or block covalent complex formation. These results indicate that alpha2M is an important inhibitor to consider in the regulation of human chymase.

Ca(2+) and Zn(2+) binding properties of peptide substrates of vertebrate collagenase, MMP-1

To understand the role of Ca(2+) in vertebrate in the structure and action of collagenase, we have examined peptides that interact with recombinant human fibroblast collagenase for their affinities towards Ca(2+) and Zn(2+) in a non-polar solvent. Two of the peptides, GPQGIAGQ and GNVGLAGA, had sequences in collagen which are, respectively, cleaved and not cleaved by collagenase. A third peptide, PSYFLNAG, had a collagenase-cleaved sequence in ovostatin, a globular protein substrate. Peptides TVGCEECTV and CLPREPGL were derived from TIMP-1; the former competitively inhibits collagenase while the latter does not. The relative rates of hydrolysis of the peptides by collagenase had the order GPQGIAGQ>PSYFLNAG>GNVGLAGA. Circular dichroism spectral data in trifluoroethanol showed that while the TIMP control peptide, CLPREPGL, bound only Zn(2+), the other four peptides bound both Ca(2+) and Zn(2+) with definite stoichiometries. Ca(2+) could displace Zn(2+) in the substrate peptides while Zn(2+) displaced Ca(2+) in the TIMP peptide. GPQGIAGQ, PSYFLNAG and TVGCEECTV formed peptide:Ca(2+):Zn(2+) ternary complexes. Our results suggest that both collagen and globular protein substrates of collagenase may bind Ca(2+) and Zn(2+) in the enzyme's active site. This, in turn, may account for the known importance of the non-catalytic Ca(2+) and Zn(2+) in collagenase activity.

ADAMTS-1 is an active metalloproteinase associated with the extracellular matrix

Cellular disintegrin and metalloproteinases (ADAMs) are a family of genes with a sequence similar to the snake venom metalloproteinases and disintegrins. ADAMTS-1 is a unique ADAM family protein with respect to the presence of thrombospondin type I motifs and the capacity to bind to the extracellular matrix. Because ADAMTS-1 has a potential zinc-binding motif in the metalloproteinase domain, we examined in this study whether ADAMTS-1 is an active metalloproteinase by means of the proteinase trapping mechanism of alpha2-macroglobulin. We found that the soluble type of ADAMTS-1 protein is able to form a covalent-binding complex with alpha2-macroglobulin. Furthermore, the point mutation within the zinc-binding motif of ADAMTS-1 protein eliminates its capacity to bind to alpha2-macroglobulin. These data demonstrate that the metalloproteinase domain of ADAMTS-1 is catalytically active. In addition, we showed that the removal of the pro-domain from the ADAMTS-1 precursor is impaired in the furin-deficient cell line, LoVo, and that the processing ability of the cells is restored by the co-expression of the furin cDNA. These data provide evidence that the ADAMTS-1 precursor is processed in vivo by furin endopeptidase in the secretory pathway. Consequently, ADAMTS-1 is an active metalloprotease that is associated with the extracellular matrix. This study strongly suggests that ADAMTS-1 may play a role in the inflammatory process through its protease activity.

Alpha2-macroglobulin: an evolutionarily conserved arm of the innate immune system

All animals and plants have immune systems that protect them from the diversity of pathogens that would otherwise threaten their survival. The different components of the immune system may inactivate the pathogens themselves or promote the inactivation and clearance of toxic products produced by the pathogens. An important category of virulence factors of bacterial and prokaryotic pathogens are the proteases, which act to facilitate the invasion of the pathogens and to promote their destructive growth in the host organism. The present review concentrates on the comparative biology of an evolutionarily conserved arm of the immune system, the protein, alpha2-macroglobulin. alpha2-Macroglobulin is an abundant protein of the plasma of vertebrates and members of several invertebrate phyla and functions as a broad-spectrum protease-binding protein. Protease-conjugated alpha2-macroglobulin is selectively bound by cells contacting the body fluids and alpha2-macroglobulin and its protease cargo are then internalized and degraded in secondary lysosomes of those cells. In addition to this function as an agent for protease clearance, alpha2-macroglobulin binds a variety of other ligands, including several peptide growth factors and modulates the activity of a lectin-dependent cytolytic pathway in arthropods.

Structural details of proteinase entrapment by human alpha2-macroglobulin emerge from three-dimensional reconstructions of Fab labeled native, half-transformed, and transformed molecules

Three-dimensional electron microscopy reconstructions of native, half-transformed, and transformed alpha2-macroglobulins (alpha2Ms) labeled with a monoclonal Fab Fab offer new insight into the mechanism of its proteinase entrapment. Each alpha2M binds four Fabs, two at either end of its dimeric protomers approximately 145 A apart. In the native structure, the epitopes are near the base of its two chisel-like features, laterally separated by 120 A, whereas in the methylamine-transformed alpha2M, the epitopes are at the base of its four arms, laterally separated by 160 A. Upon thiol ester cleavage, the chisels on the native alpha2M appear to split with a separation and rotation to give the four arm-like extensions on transformed alpha2M. Thus, the receptor binding domains previously enclosed within the chisels are exposed. The labeled structures further indicate that the two protomeric strands that constitute the native and transformed molecules are related and reside one on each side of the major axes of these structures. The half-transformed structure shows that the two Fabs at one end of the molecule have an arrangement similar to those on the native alpha2M, whereas on its transformed end, they have rotated. The rotation is associated with a partial untwisting of the strands and an enlargement of the openings to the cavity. We propose that the enlarged openings permit the entrance of the proteinase. Then cleavage of the remaining bait domains by a second proteinase occurs with its entrance into the cavity. This is followed by a retwisting of the strands to encapsulate the proteinases and expose the receptor binding domains associated with the transformed alpha2M.

Inhibition of a snake venom hemorrhagic metalloproteinase by human and rat alpha-macroglobulins

Jararafibrase I is a hemorrhagic metalloproteinase purified from Bothrops jararaca venom, which induces local hemorrhage by degrading the basement membrane components. The present study was undertaken to investigate the inhibition of jararafibrase I by human and rat serum proteinase inhibitors. The proteolytic activity of jararafibrase I was completely inhibited by human and rat sera. In particular, rat serum displayed a greater inhibitory capacity. The inhibitory capacities of both sera were dependent on alpha-macroglobulins. SDS-PAGE analysis revealed that jararafibrase I formed complexes with alpha-macroglobulins that were present in normal sera. The proteolytic activity of jararafibrase I was completely inhibited by alpha1-macroglobulin and murinoglobulin in rat serum, and by human alpha2-macroglobulin. The inhibition molar ratios of alpha-macroglobulin/jararafibrase I were 1.5 for rat alpha1-macroglobulin and human alpha2-macroglobulin, and 2.4 for rat murinoglobulin. SDS-PAGE under reducing conditions demonstrated that the bait region of human alpha2-macroglobulin and rat murinoglobulin was cleaved by jararafibrase I. The bait region cleavage sites were identified as being situated at the 696Arg-697Leu peptide bond in human alpha2-macroglobulin, and at the 686Ala-687Val peptide bond in rat murinoglobulin.

Localization of the binding site for transforming growth factor-beta in human alpha2-macroglobulin to a 20-kDa peptide that also contains the bait region

alpha2-Macroglobulin (alpha2M) functions as a major carrier of transforming growth factor-beta (TGF-beta) in vivo. The goal of this investigation was to characterize the TGF-beta-binding site in alpha2M. Human alpha2M, which was reduced and denatured to generate 180-kDa subunits, bound TGF-beta1, TGF-beta2, and NGF-beta in ligand blotting experiments. Cytokine binding was not detected with bovine serum albumin that had been reduced and alkylated, and only minimal binding was detected with purified murinoglobulin. To localize the TGF-beta-binding site in alpha2M, five cDNA fragments, collectively encoding amino acids 122-1302, were expressed as glutathione S-transferase (GST) fusion proteins. In ligand blotting experiments, TGF-beta2 bound only to the fusion protein (FP3) that includes amino acids 614-797. FP3 bound 125I-TGF-beta1 and 125I-TGF-beta2 in solution, preventing the binding of these growth factors to immobilized alpha2M-methylamine (alpha2M-MA). The IC50 values were 33 +/- 5 and 26 +/- 6 nM for TGF-beta1 and TGF-beta2, respectively; these values were comparable with or lower than those determined with native alpha2M or alpha2M-MA. A GST fusion protein that includes amino acids 798-1082 of alpha2M (FP4) and purified GST did not inhibit the binding of TGF-beta to immobilized alpha2M-MA. FP3 (0.2 microM) neutralized the activity of TGF-beta1 and TGF-beta2 in fetal bovine heart endothelial (FBHE) cell proliferation assays; FP4 was inactive in this assay. FP3 also increased NO synthesis by RAW 264.7 cells, mimicking an alpha2M activity that has been attributed to the neutralization of endogenously synthesized TGF-beta. Thus, we have isolated a peptide corresponding to 13% of the alpha2M sequence that binds TGF-beta and neutralizes the activity of TGF-beta in two separate biological assays.

Crystal structure of the receptor-binding domain of alpha 2-macroglobulin

BACKGROUND

The large plasma proteinase inhibitors of the alpha 2-macroglobulin superfamily inhibit proteinases by capturing them within a central cavity of the inhibitor molecule. After reaction with the proteinase, the alpha-macroglobulin-proteinase complex binds to the alpha-macroglobulin receptor, present in the liver and other tissues, and becomes endocytosed and rapidly removed from the circulation. The complex binds to the receptor via recognition sites located on a separate domain of approximately 138 residues positioned at the C terminus of the alpha-macroglobulin subunit.

RESULTS

The crystal structure of the receptor-binding domain of bovine alpha 2-macroglobulin (bRBD) has been determined at a resolution of 1.9 A. The domain primarily comprises a nine-strand beta structure with a jelly-roll topology, but also contains two small alpha helices.

CONCLUSIONS

The surface patch responsible for receptor recognition is thought to involve residues located on one of the two alpha helices of the bRBD as well as residues in two of the beta strands. Located on this alpha helix are two lysine residues that are important for receptor binding. The structure of bRBD is very similar to the approximately 100-residue C-terminal domain of factor XIII, a transglutaminase from the blood coagulation system.

Bait region involvement in the dimer-dimer interface of human alpha 2-macroglobulin and in mediating gross conformational change. Evidence from cysteine variants that form interdimer disulfides

We have characterized four human alpha 2-macroglobulin (alpha 2M) bait region variants (G679C, M690C, V700C, and T705C) to test the hypothesis that the bait regions are involved in the interface between noncovalently associated dimers. All four variants folded correctly as judged by many normal properties. However, the presence of a cysteine resulted in disulfide formation between otherwise noncovalently associated dimers in all four variants. The extent of disulfide cross-linking varied with the location of the cysteine and gave a mixture of species that probably contained two, one, or zero interdimer disulfides in the tetramer. This was reflected in heterogeneity of conformational change upon thiol ester cleavage by methylamine, with the presence of crosslinks correlating with blockage of conformational change. The stoichiometry of trypsin inhibition was less in all cases than for wild-type alpha 2M. The M690C variant also showed evidence of some species with an intramolecular disulfide between bait regions of monomers within the same dimer. Taken together, the results are consistent with a location of the four bait regions in contact with, or in very close proximity to, one another. This suggests that they form all or part of the "cavity body" seen in the low resolution x-ray structure of transformed alpha 2M.

Interaction of matrix metalloproteinases-2 and -9 with pregnancy zone protein and alpha2-macroglobulin

The binding of matrix metalloproteinases-2 and -9 to pregnancy zone protein and alpha2-macroglobulin was studied. The binding was demonstrated by formation of dimeric as well as tetrameric complexes of pregnancy zone protein and by the formation of alpha2-macroglobulin complexes with fast and intermediate mobility in native gel electrophoresis. The complex formation was confirmed by the use of 125I-labeled matrix metalloproteinase-2. The cleavage sites in the "bait" regions following formation of high-molecular-weight complexes of matrix metalloproteinases with the alpha-macroglobulins were determined by protein sequence analysis. Pregnancy zone protein was cleaved at Thr693-Tyr694 and alpha2-macroglobulin at Gly679-Leu680 and Arg696-Leu697 by matrix metalloproteinase-2. Matrix metalloproteinase-9 cleaved alpha2-macroglobulin at the same site as matrix metalloproteinase-2, but cleavage of pregnancy zone protein was at Leu753-Ser754. The sequences of the bands, visualized in the SDS gel, of approximately 90 and 165 kDa or higher molecular weight complexes were the same. This indicates that the matrix metalloproteinases cleaved the inhibitors with or without binding to them. The present results suggest that matrix metalloproteinases-2 and -9 may interact with pregnancy zone protein and alpha2-macroglobulin in vivo.

The potential role of alpha 2-macroglobulin in the control of cysteine proteinases (gingipains) from Porphyromonas gingivalis

Porphyromonas gingivalis is closely associated with the development of some forms of periodontitis. The major cysteine proteinases released by this bacterium hydrolyze peptide bonds only after arginyl (gingipain R) or lysyl residues (gingipain K). No target protein inhibitors have been identified for either enzyme, leading us to investigate their inhibition by human plasma alpha 2-macroglobulin (alpha 2M). Both 50- and 95 kDa gingipain R were efficiently inhibited by alpha 2M, whereas the catalytic activity of gingipain K could not be eliminated. All 3 enzymes were, however, inhibited by a homologous macroglobulin from rat plasma, alpha 1-inhibitor-3 (alpha 1I3). alpha-Macroglobulins must be cleaved in the so-called "bait region" in order to inhibit proteinases by a mechanism involving physical entrapment of the enzyme. A comparison of the amino acid sequences of the 2 macroglobulins indicates that the lack of lysyl residues within the bait region of alpha 2M protects Lys-specific proteinases from being trapped. On this basis, other highly specific proteinases might also not be inhibited by alpha 2M, possibly explaining the inability of the inhibitor to control proteolytic activity in some bacterially induced inflammatory states, despite its abundance (2-5 mg/ml) in vascular fluids.

Molecular cloning of Limulus alpha 2-macroglobulin

The American horseshoe crab Limulus polyphemus contains alpha 2-macroglobulin (alpha 2M) in the hemolymph plasma and hemocytes. alpha 2M from Limulus shows many of the typical characteristics of mammalian alpha 2M, including the presence of an internal thiol-ester, reactivity with a diversity of endopeptidases, a unique proteinase-trapping mechanism, and reactivity with the mammalian alpha 2M receptor. Additionally, Limulus alpha 2M has the unique property that it regulates the limulin-based hemolytic system of the plasma. A cDNA encoding Limulus alpha 2M has been obtained from a hemocyte cDNA library. The open reading frame encodes an N-terminal signal sequence of 25 amino acid residues and a mature protein of 1482 residues. The entire amino acid sequence is similar to those of the mammalian alpha 2Ms (28-29% identity) and contains common features found in mammalian alpha 2Ms. a bait region, an internal thiol-ester site, and a receptor-binding domain. However, the N-terminal portion (positions 24-105) has no sequence similarity with those of mammalian alpha 2Ms, and it is structurally related to that of the human complement factor C8 chain, consistent with a role for Limulus alpha 2M in host defense. The component sugar analysis of Limulus alpha 2M showed the existence of a complex type of oligosaccharide chain similar to those of mammalian alpha 2M. However, unlike mammalian alpha 2M, no sialic acid was detected in Limulus alpha 2M and it contained approximately 3 mol/mol N-acetylgalactosamine, suggesting the presence of O-linked sugar chains, which have not been found in mammalian alpha 2M. Expression of alpha 2M was detected in hemocytes, but not in hepatopancreas, heart, stomach, intestine, coxal gland, brain and skeletal muscle. Furthermore, immunoblotting of large and small granules of the hemocytes with antiserum against alpha 2M indicated the presence of the alpha 2M in large granules. Trypsin-treated Limulus alpha 2M, but not the native alpha 2M, displaced methylamine-treated human 125I-alpha 2M from the human alpha 2M receptor with a Kd of 30 nM, suggesting conservation of the proteinase-clearance mechanisms between mammalian and arthropod evolutionary lineages.

Accelerated evolution in inhibitor domains of porcine elafin family members

Through the analysis of the porcine gene encoding the elastase inhibitor elafin, we demonstrated that there are at least three closely related members of the elafin family, and their genes have arisen by accelerated evolution. A porcine genomic DNA library was screened with a previously cloned human elafin cDNA probe, and several positive clones were obtained that can be distinguished by a combination of restriction enzymes. Sequence analysis of these clones revealed the presence of three homologous members whose genes, all consisting of three exons and two introns, are almost identical except the exon 2 sequences encoding the inhibitor domain called "WAP motif"; the intron sequences are related to each other with sequence similarities of 93-98%, whereas the exon 2 sequences exhibited only 60-77% similarities among the three members. The extreme divergence in the exon 2 sequences compared to the highly conserved intron sequences may be generated by accelerated mutations confined in a short stretch of the genes following recent duplication events of a single ancestral gene. An RNase protection assay indicated that the messages of the elafin family members are abundantly expressed in the trachea and intestine, suggesting that the most likely selective forces for the accelerated evolution are extrinsic proteinases produced by invasive microorganisms.

Metalloproteinase inhibitors and the prevention of connective tissue breakdown

The primary agents responsible for cartilage and bone destruction in joint diseases are active proteinases degrading collagen and proteoglycan. All four main classes of proteolytic enzymes are involved in either the normal turnover of connective tissue or its pathological destruction. These proteinases are made by different cells found within the joints. Both extracellular and intracellular pathways exist, and individual enzymes can be inhibited by specific proteinaceous inhibitors that block their activity. Recent research has implicated the matrix metalloproteinases in many of the processes involved in joint diseases. Conventional treatments do little to affect the underlying disease processes, and recently, the use of proteinase inhibitors has been suggested as a new therapeutic approach. A large variety of different synthetic approaches have been used and highly effective metalloproteinase inhibitors have been designed, synthesised and tested. These metalloproteinase inhibitors can prevent the destruction of animal cartilage in model systems and slow the progression of human tumours. Future patient trials will test the effectiveness of these compounds in vivo for the treatment of joint diseases.

Low resolution X-ray structure of human methylamine-treated alpha 2-macroglobulin

The structure of methylamine-treated human alpha 2-macroglobulin (alpha 2M-Ma), a 720-kDa tetrameric inactivated proteinase inhibitor from plasma, has been determined to a resolution of 10 A. Data were collected with synchrotron radiation at 120 K, and phases were calculated by multiple isomorphous replacement and solvent flattening. A novel feature of the structure of alpha 2-M is present in its proteinase-binding cavity, dividing it into two compartments. The potential sites for proteinase entrapment in these compartments are sterically restricted. The positions of the thiol groups appearing from the functional important thiol esters upon their cleavage have been determined. They are found at the walls of the compartments at the center of the structure. The overall structure of alpha 2M-MA is much more sphere-like than previously inferred from electron microscopy studies. However, several aspects of the structure are well described by recent three-dimensional reconstructions. Possible models for the monomer, the disulfide bridged dimer, and native alpha 2M are discussed.