Cockroach allergen Bla g 2:: An unusual aspartic proteinase

Background: Enzymatic activity of mite, fungal, and bee venom allergens is thought to potentiate their allergenicity. Bla g 2 is a potent cockroach allergen, but despite sharing sequence homology with aspartic proteinases, it contains critical amino acid substitutions that impair proteolytic activity. The biologic function of Bla g 2 remains unclear. Objective: We sought to investigate the effects of specific amino acid substitutions on enzymatic activity, and the peptide-binding capability of Bla g 2. Methods: Site-directed mutagenesis was used to produce a recombinant Bla g 2 mutant (Mut) with corrected canonical triads and a flap region. Another mutant (MutF(-)) was expressed after additional mutations in the flap region of Mut. Bla g 2 wild-type (Wt), Mut, and MutF(-) were assayed for aspartic proteinase activity, and Bla g 2 Wt was tested for pepstatin binding. Results: Recombinant Bla g 2 Wit and Mut did not show enzymatic activity in a milk-clotting and hemoglobin assay. By using a modified hemoglobin assay, residual activity inhibited by pepstatin was detected for MutF(-) and Wt at 20 mu g/mL, whereas pepsin was active at a 1000-fold lower concentration. Most of Bla g 2 binding to pepstatin-agarose was nonspecific. Conclusion: Residual proteolytic activity was found for Bla g 2 at concentrations of approximately 4 mM. This weak activity suggests that proteolysis is not the primary function of this allergen and that it is unlikely to contribute to the allergenicity of Bla g 2. Bla g 2 has a cleft that might specifically bind ligands other than pepstatin.