Cysteine protease. Hydrolyzes casein and synthetic peptide Boc-Val-Leu-Lys-7-amino-4-methylcoumarin (Boc-VLK-AMC) in vitro. Activated by L-cysteine (PubMed:27129273). Inhibited by cysteine protease inhibitor E64 (PubMed:25865353, PubMed:27129273). Inhibited by cysteine/serine protease inhibitor leupeptin. Not inhibited by serine protease inhibitors 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF) and phenylmethanesulfonyl fluoride (PMSF), metallo protease inhibitor bestatin or aspartic protease inhibitor pepstatin A (PubMed:27129273). Optimum pH is between 8 and 9. Homodimer. Expressed in pollen (at protein and mRNA level). Autocatalytic proteolytic cleavage of N-terminal activation peptide. N-glycosylated. Glycosylation is not required for binding to IgE. Causes an allergic reaction in human. Binds to IgE in 54% of the 92 patients, including both Europeans and Americans, tested allergic to ragweed pollen. Purified native protein under non-denaturing conditions binds to IgE in 66% of the same 92 patients tested (PubMed:25865353). Binds to IgE of mice. In mice, the allergicity especially to the proteolytically active form of this protein leads to airway hyperresponsiveness, lung inflammation characterized by eosinophil and type 2 innate lymphoid cell (ILC2) infiltrates in bronchoalveolar lavages (BALs), and responses of the T-helper 2 (Th2) cells (PubMed:27129273). Causes activation of human basophils (PubMed:25865353). Belongs to the peptidase C1 family.