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. (+info)
Heme and acute inflammation role in vivo of heme in the hepatic expression of positive acute-phase reactants in rats.
Acute-phase protein synthesis in the liver during inflammation is regulated via cytokines and glucocorticoids. Using quantitative reverse transcription (RT)-PCR analysis and immunoassay, we explored, in the rat, the response of the acute-phase protein, alpha-2 macroglobulin (A2M), after systemic inflammation induced by lipopolysaccharide (LPS) or localized inflammation induced by turpentine oil (TO). The results indicate that synthesis of A2M is higher following TO-induced inflammation than LPS-induced inflammation and is not correlated with interleukin (IL)-6 or glucocorticoid levels. We studied the putative role of heme in this differential A2M expression following localized vs. systemic inflammation; addition of heme during LPS-induced inflammation can boost the expression of A2M, whereas blocking heme synthesis (by succinyl acetone) or enhancing its consumption in parallel biosynthetic pathways (cytochrome P450 induction by phenobarbital) decreases A2M expression. This decrease was abolished by exogenous heme supplementation. Finally, we demonstrate that heme supplementation is also able to increase the A2M response in female rats to a level similar to that in male rats providing a new insight into the puzzling sexual dimorphism observed previously during localized inflammation. We propose that heme should be considered a new regulatory element in controlling liver A2M expression during inflammation. (+info)
Pig kidney legumain: an asparaginyl endopeptidase with restricted specificity.
Legumain was recently discovered as a lysosomal endopeptidase in mammals [Chen, Dando, Rawlings, Brown, Young, Stevens, Hewitt, Watts and Barrett (1997) J. Biol. Chem. 272, 8090-8098], having been known previously only from plants and invertebrates. It has been shown to play a key role in processing of the C fragment of tetanus toxin for presentation by the MHC class-II system [Manoury, Hewitt, Morrice, Dando, Barrett and Watts (1998) Nature (London) 396, 695-699]. We examine here the specificity of the enzyme from pig kidney by use of protein, oligopeptide and synthetic arylamide substrates, all determinations being made at pH 5.8. In proteins, only about one in ten of the asparaginyl bonds were hydrolysed, and these were mostly predicted to be located at turns on the protein surface. Bonds that were not cleaved in tetanus toxin were cleaved when presented in oligopeptides, sometimes faster than an equivalent oligopeptide based on a bond that was cleaved in the protein. Legumain cleaved the bait region of rat alpha1-macroglobulin and was 'trapped' by the macroglobulin, as most other endopeptidases are, but did not interact with human alpha2-macroglobulin, which contains no asparagine residue in its bait region. Glycosylation of asparagine totally prevented hydrolysis by legumain. Specificity for arylamide substrates was evaluated with reference to benzyloxycarbonyl-Ala-Ala-Asn-aminomethylcoumarin, and the preference for the P3-position amino acid was Ala>Tyr(tertiary butyl)>Val>Pro>Phe=Tyr>Leu=Gly. There was no hydrolysis of substrate analogues containing mono- or di-N-methylasparagines, l-2-amino-3-ureidopropionic acid or citrulline in the P1 position. We conclude that mammalian legumain appears to be totally restricted to the hydrolysis of asparaginyl bonds in substrates of all kinds. There seem to be no strong preferences for particular amino acids in other subsites, and yet there are still unidentified factors that prevent hydrolysis of many asparaginyl bonds in proteins. (+info)
Alpha2-macroglobulin and eosinophil cationic protein in the allergic airway mucosa in seasonal allergic rhinitis.
As previously demonstrated in seasonal allergic rhinitis, increased microvascular permeability and eosinophil activation are key features of allergic airway inflammation. In the present study, the hypothesis that exudation of alpha2-macroglobulin may cause the appearance of eosinophil cationic protein (ECP) in the airway lumen was explored. Nasal lavages were carried out using the nasal pool device before and during the pollen season both at baseline and after histamine challenge in 10 children with allergic rhinitis. Nasal lavage fluid levels of alpha2-macroglobulin and ECP were determined. All patients experienced nasal symptoms of allergic rhinitis during the pollen season (p<0.01-0.05). Baseline nasal lavage fluid levels of alpha2-macroglobulin and ECP were increased during the season (p<0.01-0.05) and were found to be well correlated (p<0.0001). Histamine produced concentration-dependent plasma exudation before and during the pollen season, but it was only during the pollen season that this caused an increase in the lavage fluid levels of ECP (p<0.05). These data suggest that exudation of plasma and increased tissue levels and output of eosinophil cationic protein characterize nasal mucosal inflammation in children with seasonal allergic rhinitis. The plasma exudation process in part may account for lumenal entry of eosinophil cationic protein molecules that have been released in mucosal tissue compartments. A combination of induced exudation and nasal lavage may improve the yield of important markers of inflammation in studies of nasal diseases. (+info)
NMR solution structure of complement-like repeat CR8 from the low density lipoprotein receptor-related protein.
The low density lipoprotein receptor-related protein is a member of the low density lipoprotein receptor family and contains clusters of cysteine-rich complement-like repeats of about 42 residues that are present in all members of this family of receptors. These clusters are thought to be the principal binding sites for protein ligands. We have expressed one complement-like repeat, CR8, from the cluster in lipoprotein receptor-related protein that binds certain proteinase inhibitor-proteinase complexes and used three-dimensional NMR on the 13C/15N-labeled protein to determine the structure in solution of the calcium-bound form. The structure is very similar in overall fold to repeat 5 from the low density lipoprotein receptor (LB5), with backbone root mean square deviation of 1.5 A. The calcium-binding site also appears to be homologous, with four carboxyl and two backbone carbonyl ligands. However, differences in primary structure are such that equivalent surfaces that might represent the binding interfaces are very different from one another, indicating that different domains will have very different ligand specificities. (+info)
Abnormality in urinary protein excretion in Japanese men with impaired glucose tolerance.
OBJECTIVE: To examine whether subjects with impaired glucose tolerance (IGT) for more than 2 years have any abnormality in the kidney. RESEARCH DESIGN AND METHODS: We measured urinary excretion rate and clearance of various plasma proteins with different molecular radii and different isoelectric points in 22 Japanese men with IGT (IGT group) and 37 age-matched healthy control subjects (control group). RESULTS: Clearance of ceruloplasmin (molecular radius approximately 45 A; isoelectric point 4.4), IgG4 (molecular radius 55 A; isoelectric point 5.4), and IgG (molecular radius 55 A; isoelectric point 7.4) was significantly higher in the IGT group than in the control group, whereas there were no significant differences in urinary excretion rate of albumin (molecular radius 36 A; isoelectric points 4.8-5.2) and clearance of alpha 2-macroglobulin (molecular radius 88 A; isoelectric point 5.4) between the two groups. CONCLUSIONS: In the present study, we found that clearance of neutral-charged IgG, negatively charged IgG4, and ceruloplasmin with molecular radii of approximately 45-55 A was selectively increased in IGT subjects. This finding does not seem to be explained by impairment of charge and pore-size selectivity in the glomerulus. Therefore, considering the present result together with our recent finding that enhanced glomerular filtration rate (GFR) after acute protein loading in healthy subjects induced a selective increase in clearance of IgG, IgG4, and ceruloplasmin, we suggest that increased intraglomerular hydraulic pressure, although enhanced GFR was not demonstrated, may be at work in these mildly hyperglycemic subjects. (+info)
Protein GRAB of streptococcus pyogenes regulates proteolysis at the bacterial surface by binding alpha2-macroglobulin.
In the molecular interplay between pathogenic microorganisms and their host, proteolytic mechanisms are believed to play a crucial role. Here we find that the important human pathogen Streptococcus pyogenes (group A Streptococcus) expresses a surface protein with high affinity (Ka = 2.0 x 10(8) M-1) for alpha2-macroglobulin (alpha2M), the dominating proteinase inhibitor of human plasma. The immunoglobulin-binding protein G of group C and G streptococci also contains an alpha2M-binding domain and a gene encoding protein GRAB (protein G-related alpha2M-binding protein) was identified in the S. pyogenes Genome Sequencing data base. The grab gene is present in most S. pyogenes strains and is well conserved. Protein GRAB has typical features of a surface-attached protein of Gram-positive bacteria. It also contains a region homologous to parts of the alpha2M-binding domain of protein G and a variable number of a unique 28-amino acid-long repeat. Using Escherichia coli-produced protein GRAB and synthetic GRAB peptides, the alpha2M-binding region was mapped to the NH2-terminal part of protein GRAB, which is the region with homology to protein G. An isogenic S. pyogenes mutant lacking surface-associated protein GRAB showed no alpha2M binding activity and was attenuated in virulence when injected intraperitoneally in mice. Finally, alpha2M bound to the bacterial surface via protein GRAB was found to entrap and inhibit the activity of both S. pyogenes and host proteinases, thereby protecting important virulence determinants from proteolytic degradation. This regulation of proteolytic activity at the bacterial surface should affect the host-microbe relation during S. pyogenes infections. (+info)
Destruction of articular cartilage by alpha 2 macroglobulin elastase complexes: role in rheumatoid arthritis.
OBJECTIVE: Neutrophil elastase accounts for the ability of some fresh rheumatoid synovial fluids to degrade cartilage matrix in vitro. The aim of this study was to determine if enzyme activity could result from depletion of synovial fluid inhibitors or protection of the enzyme from inhibition. METHODS: The ability of synovial fluids to inhibit porcine pancreatic elastase was investigated together with chemical pretreatments capable of inactivating alpha 1 protease inhibitor (alpha 1PI) or preventing formation of alpha 2 macroglobulin (alpha 2M) elastase complexes. Subsequently, complexes of human neutrophil elastase with alpha 2M were prepared and applied to frozen sections of cartilage. Proteoglycan loss was quantified by alcian blue staining and scanning and integrating microdensitometry. Parallel studies were carried out using a low molecular weight chromogenic elastase substrate. The effects of alpha 1PI and SF on these systems were investigated. Finally, synovial fluids were subjected to gel filtration and the fractions assayed for elastase activity. High molecular weight fractions were pooled, concentrated, and tested for their ability to degrade cartilage sections. RESULTS: All synovial fluids reduced the activity of porcine pancreatic elastase, the inhibition mainly being attributable to alpha 1PI, whereas remaining activity resulted from complexes of elastase with alpha 2M. Complexes of human neutrophil elastase with alpha 2M were shown to cause proteoglycan degradation in frozen sections of human articular cartilage. Alpha 1PI prevented alpha 2M elastase complexes from degrading cartilage but not the chromogenic substrate. The data suggested that alpha 1PI does not inhibit elastase bound to alpha 2M but sterically hinders the complex. However, only one of five synovial fluids was able to completely block the actions of alpha 2M elastase complexes against cartilage. Gel filtration of rheumatoid synovial fluids showed elastase and cartilage degrading activity to be associated with fractions that contained alpha 2M, and not with fractions expected to contain free enzyme. CONCLUSIONS: The data suggest that synovial fluid alpha 2M elastase complexes can degrade cartilage matrix in rheumatoid arthritis. (+info)