Activated mast cells trigger diverse processes in innate and adaptive immunity due to their capability to secrete a wide array of newly-synthesized as well as preformed, granule-stored mediators. Among the preformed mediators, tryptases are most prominent, accounting for up to 25% of the cell’s protein. In man, this group of serine proteases comprises at least four distinct proteins, i.e. tryptase α, β, γ and δ. Within the group, tryptases α, β and δ share considerable sequence similarity (> 80% amino acid identity) whereas tryptase γ is more distantly related (~ 50% identity) and also differs in an additional C-terminal hydrophobic domain.
To foster structural and functional comparisons we have expressed the tryptases α, β, δ and soluble forms of γ as well as their zymogens in Pichia pastoris. Among them, tryptases β and γ are enzymatically active with trypsin-like substrate specificity whereas α and δ as well as the zymogens have minute if any activity. The active tryptases differ, however, in their pH optimum, heparin affinity and quaternary structure. In respect to the interaction with substrates and inhibitors, tryptases β and γ have different but overlapping specificities; analysis of substrate specificity revealed distinct P2 subsite preferences whereas P3 and P4 preferences are comparable. Similarly, some inhibitors discriminate between the tryptases whereas others inhibit both proteases. These results demonstrate that the tryptases β and γ share properties including trypsin-like enzymatic activity, cleavage of prototypical ‘tryptase’ substrates and resistance against inhibition by various plasma and proteinaceous inhibitors, and thus can contribute to the mediator actions ascribed to ‘tryptase’.
Supported by DFG Priority Program 1394 “Mast Cells – Promoters of health and modulators of disease”