Synthetic peptide substrates for the immunoglobulin A1 protease from Neisseria gonorrhoeae (type 2)

Active-site gating regulates substrate selectivity in a chymotrypsin-like serine protease the structure of haemophilus influenzae immunoglobulin A1 protease

We report here the first structure of a member of the immunoglobulin A protease (IgAP) family at 1.75-A resolution. This protease is a founding member of the type V (autotransporter) secretion system and is considered a virulence determinant among the bacteria expressing the enzyme. The structure of the enzyme fits that of a classic autotransporter in which several unique domains necessary for protein function are appended to a central, 100-A-long beta-helical domain. The N-terminal domain of the IgAP is found to possess a chymotrypsin-like fold. However, this catalytic domain contains a unique loop D that extends over the active site acting as a lid, gating substrate access. The data presented provide a structural basis for the known ability of IgAPs to cleave only the proline/serine/threonine-rich hinge peptide unique to IgA1 (isotype 1) in the context of the intact fold of the immunoglobulin. Based upon the structural data, as well as molecular modeling, a model suggesting that the unique extended loop D in this IgAP sterically occludes the active-site binding cleft in the absence of immunoglobulin binding is presented. Only in the context of binding of the IgA1-Fc domain in a valley formed between the N-terminal protease domain and another domain appended to the beta-helix spine (domain 2) is the lid stabilized in an open conformation. The stabilization of this open conformation through Fc association subsequently allows access of the hinge peptide to the active site, resulting in recognition and cleavage of the substrate.

Sites in the CH3 Domain of Human IgA1 That Influence Sensitivity to Bacterial IgA1 Proteases

The influence of regions, other than the hinge, on the susceptibility of human IgA1 to cleavage by diverse bacterial IgA1 proteases, was examined using IgA1 mutants bearing amino acid deletions, substitutions, and domain swaps. IgA1 lacking the tailpiece retained its susceptibility to cleavage by all of the IgA1 proteases. The domain swap molecule alpha1alpha2gamma3, in which the CH3 domain of IgA1 was exchanged for that of human IgG1, was resistant to cleavage with the type 1 and 2 serine IgA1 proteases of Neisseria meningitidis, Neisseria gonorrhoeae, and Haemophilus influenzae, but remained sensitive to cleavage with the metallo-IgA1 proteases of Streptococcus pneumoniae, Streptococcus oralis, Streptococcus sanguis, and Streptococcus mitis. Substitution of the IgA1 Calpha3 domain motif Pro440 -Phe443 into the corresponding position in the Cgamma3 domain of alpha1alpha2gamma3 resulted now in sensitivity to the type 2 IgA1 protease of N. meningitidis, indicating the possible requirement of these amino acids for sensitivity to this protease. For the H. influenzae type 2 protease, resistance of an IgA1 mutant in which the CH3 domain residues 399-409 were exchanged with those from IgG1, but sensitivity of mutant HuBovalpha3 in which the Calpha3 domain of bovine IgA replaces that of human IgA1, suggests that CH3 domain residues Glu403, Gln406, and Thr409 influence sensitivity to this enzyme. Hence, unlike the situation with the metallo-IgA1 proteases of Streptococcus spp., the sensitivity of human IgA1 to cleavage with the serine IgA1 proteases of Neisseria and Haemophilus involves their binding to different sites specifically in the CH3 domain.

Effect of mutations in the human immunoglobulin A1 (IgA1) hinge on its susceptibility to cleavage by diverse bacterial IgA1 proteases

Components of the human immunoglobulin A1 (IgA1) hinge governing sensitivity to cleavage by bacterial IgA1 proteases were investigated. Recombinant antibodies with distinct hinge mutations were constructed from a hybrid comprised of human IgA2 bearing half of the human IgA1 hinge region. This hybrid antibody and all the mutant antibodies derived from it were resistant to cleavage by the IgA1 proteases from Streptococcus oralis and Streptococcus mitis biovar 1 strains but were cleaved to various degrees by those of Streptococcus pneumoniae, some Streptococcus sanguis strains, and the type 1 and 2 IgA1 proteases of Haemophilus influenzae, Neisseria meningitidis, and Neisseria gonorrhoeae. Remarkably, those proteases that cleave a Pro-Ser peptide bond in the wild-type IgA1 hinge were able to cleave mutant antibodies lacking a Pro-Ser peptide bond in the hinge, and those that cleave a Pro-Thr peptide bond in the wild-type IgA1 hinge were able to cleave mutant antibodies devoid of a Pro-Thr peptide bond in the hinge. Thus, the enzymes can cleave alternatives to their preferred postproline peptide bond when such a bond is unavailable. Peptide sequence analysis of a representative antibody digestion product confirmed this conclusion. The presence of a cleavable peptide bond near the CH2 end of the hinge appeared to result in greater cleavage than if the scissile bond was at the CH1 end of the hinge. Proline-to-serine substitution at residue 230 in a hinge containing potentially cleavable Pro-Ser and Pro-Thr peptide bonds increased the resistance of the antibody to cleavage by many IgA1 proteases.

Cleavage of the human immunoglobulin A1 (IgA1) hinge region by IgA1 proteases requires structures in the Fc region of IgA

Secretory immunoglobulin A (IgA) protects the mucosal surfaces against inhaled and ingested pathogens. Many pathogenic bacteria produce IgA1 proteases that cleave in the hinge of IgA1, thus separating the Fab region from the Fc region and making IgA ineffective. Here, we show that Haemophilus influenzae type 1 and Neisseria gonorrhoeae type 2 IgA1 proteases cleave the IgA1 hinge in the context of the constant region of IgA1 or IgA2m(1) but not in the context of IgG2. Both C(alpha)2 and C(alpha)3 but not C(alpha)1 are required for the cleavage of the IgA1 hinge by H. influenzae and N. gonorrhoeae proteases. While there was no difference in the cleavage kinetics between wild-type IgA1 and IgA1 containing only the first GalNAc residue of the O-linked glycans, the absence of N-linked glycans in the Fc increased the ability of the N. gonorrhoeae protease to cleave the IgA1 hinge. Taken together, these results suggest that, in addition to the IgA1 hinge, structures in the Fc region of IgA are required for the recognition and cleavage of IgA1 by the H. influenzae and N. gonorrhoeae proteases.

SepA, the 110 kDa protein secreted by Shigella flexneri: two-domain structure and proteolytic activity

Shigellosis is characterized by a strong inflammatory response which is induced by bacteria invading the colonic mucosa. Characterization of a sepA mutant indicated that SepA, the major protein secreted by Shigella flexneri growing in laboratory media, might be involved in invasion and destruction of the host intestinal epithelium. The sequence of the first 500 residues of mature SepA (110 kDa) is homologous to that of the N-terminal region of IgA1 proteases. To investigate the potential proteolytic activity of SepA, the activity of the purified protein on a wide range of synthetic peptides was tested. SepA hydrolysed several of these substrates and the activity was inhibited by PMSF. Several peptides which were hydrolysed by SepA have been described as specific substrates for cathepsin G, a serine protease produced by polymorphonuclear leukocytes that was proposed to play a role in inflammation. However, unlike cathepsin G, SepA degraded neither fibronectin nor angiotensin I and had no effect on aggregation of human platelets. In addition, analysis of SepA hydrolysis by proteinase K suggested that the protein is composed of two domains of about 450 residues separated by a hinge region of 100 residues. The 47 kDa N-terminal domain was stable and endowed with proteolytic activity.

IgA protease from Neisseria gonorrhoeae inhibits exocytosis in bovine chromaffin cells like tetanus toxin

When tetanus toxin from Clostridium tetani or IgA protease from Neisseria gonorrhoeae is translocated artificially into the cytosol of chromaffin cells, both enzymes inhibit calcium-induced exocytosis, which can be measured by changes in membrane capacitance. The block of exocytosis caused by both proteases cannot be reversed by enforced stimulation with increased calcium concentration. This effect differs from the botulinum A neurotoxin-induced block of exocytosis that can be overcome by elevation of the intracellular calcium concentration. Tetanus toxin is about 50-fold more potent than IgA protease in cells stimulated by carbachol. In this case, the release of [3H]noradrenaline was determined. Trypsin and endoprotease Glu-C are hardly effective and only at concentrations that disturb the integrity of the cells. Like tetanus toxin, IgA protease also splits synaptobrevin II, though at a different site of the molecule. However, unlike tetanus toxin, it does not cleave cellubrevin. It is concluded that the membranes of chromaffin vesicles contain synaptobrevin II, which, as in neurons, appears to play a crucial part in exocytosis.

Sequence-specific cleavage of protein fusions using a recombinant Neisseria type 2 IgA protease

Sequence-specific enzymatic cleavage of protein fusions is an important application in recombinant protein technology. We have used the Neisseria type 2 IgA protease (EC 3.4.24.13), produced and secreted by Escherichia coli host cells, for efficiently processing polypeptides at authentic or engineered target sites. In different substrates, the microbial protease specifically cleaves the peptide bond distal to the second Pro residue of the sequence Yaa-Pro-/-Xaa-Pro, where Yaa stands for Pro (or rarely for Pro in combination with Ala, Gly or Thr) and Xaa stands for Thr, Ser or Ala. Highly specific proteolysis has been obtained not only with soluble and purified protein fusions but also with insoluble aggregates derived from cytoplasmic inclusion bodies. The sequence-specificity and simple production of the recombinant IgA protease make it a versatile tool for the in vitro processing of recombinant proteins.