IgA proteases are metal chelator-sensitive neutral proteolytic enzymes elaborated into the extracellular environment by bacteria that cause human infections. Enzymepositive bacteria that have been identified are Neisseria gonorrhoeae (1), Neisseria meningitidis (1), Hemophilus influenzae (2-4), Streptococcus pneumoniae (2-4), and Streptococcus sanguis (5,6). The IgA proteases each cleave a single peptide bond in the unusual duplicated octapeptide Thr-Pro-Pro-Thr-Pro-Ser-Pro-Ser that is found in the human IgA1 heavy-chain hinge region (1, 7); proteins of the IgA2 subclass, having a hingeregion deletion that includes a large portion of the duplicated segment (8), are resistant to IgA protease cleavage (9), as are all other proteins yet examined. The peptide bonds cleaved by the proteases of these bacteria are shown diagramatically in Fig. 1, where it can be seen that a protine residue contributes the carboxyl group in each susceptible bond. Each enzyme catalyzes the hydrolysis of a single specific peptide linkage in the alpha chain, despite the abundance of pro-R linkages (R denoting threonyl or seryl residues) throughout the hinge-region segment; because cleavage is at a single peptide bond, the products of hydrolysis are intact Faba and Fca fragments.While studying the IgA proteases of N. meningitidis, it was noted that the IgA1 cleavage products differed in size and eleetrophoretic migration among N. meningitidis isolates. We have now shown that this species releases two types of enzymes, each bacterial isolate elaborating one or the other enzyme but not both. Limited aminoterminal sequence analysis of the Fca digestion products identified the peptide linkages cleaved in the alpha chain by each enzyme, and showed that the susceptible bonds are separated by only two amino acid residues.
Materials and MethodsN. meningitidis strains were clinical isolates of human origin grown in pure culture on chocolate agar plates (Baltimore Biological Laboratories, Cockeysville, Md.) at 37°C under 5% CO2. Two selected isolates were grown overnight at 37°C in brain-heart infusion (Difco Laboratories, Detroit, Mich.) that contained 1% Isovitalex (Baltimore Biological Laboratories), and IgA protease was purified from the spent culture medium by sequential salt precipitation and column chromatography, as previously described (1, 5).
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