A novel heparin-dependent processing pathway for human tryptase: Autocatalysis followed by activation with dipeptidyl peptidase I

Abstract

Tryptase is the major protein constituent of human mast cells, where it is stored within the secretory granules as a fully active tetramer. Two tryptase genes (α and β) are expressed by human mast cells at the level of mRNA and protein, each with a 30 amino acid leader sequence. Recombinant precursor forms of human α- and β-tryptase were produced in a baculovirus system, purified, and used to study their processing. Monomeric β-protryptase first is shown to be intermolecularly autoprocessed to monomeric β-pro'tryptase at acid pH in the presence of heparin by cleavage between Arg-3 and Val-2 in the leader peptide. The precursor of α-tryptase has an Arg-3 to Gln 3 mutation that precludes autoprocessing. This may explain why α-tryptase is not stored in secretory granules, but instead is constitutively secreted by mast cells and is the predominant form of tryptase found in blood in both healthy subjects and those with systemic mastocytosis under nonacute conditions. Second, the NH2-terminal activation dipeptide on β-pro'tryptase is removed by dipeptidyl peptidase I at acid pH in the absence of heparin to yield an inactive monomeric form of tryptase. Conversion of the catalytic portion of β-tryptase to the active homotetramer at acid pH requires heparin. Thus, β-tryptase homotetramers probably account for active enzyme detected in vivo. Also, processing of tryptase to an active form should occur optimally only in cells that coexpress heparin proteoglycan, restricting this pathway to a mast cell lineage.