Inhibition of an active zymogen protease: the zymogen form of matriptase is regulated by HAI-1 and HAI-2

The membrane-bound serine protease matriptase belongs to a rare subset of serine proteases that display significant activity in the zymogen form. Matriptase is critically involved in epithelial differentiation and homeostasis, and insufficient regulation of its proteolytic activity directly causes onset and development of malignant cancer. There is strong evidence that the zymogen activity of matriptase is sufficient for its biological function(s). Activated matriptase is inhibited by the two Kunitz-type inhibitor domain-containing hepatocyte growth factor activator inhibitors 1 (HAI-1) and HAI-2, however, it remains unknown whether the activity of the matriptase zymogen is regulated. Using both purified proteins and a cell-based assay, we show that the catalytic activity of the matriptase zymogen towards a peptide-based substrate as well as the natural protein substrates, pro-HGF and pro-prostasin, can be inhibited by HAI-1 and HAI-2. Inhibition of zymogen matriptase by HAI-1 and HAI-2 appears similar to inhibition of activated matriptase and occurs at comparable inhibitor concentrations. This indicates that HAI-1 and HAI-2 interact with the active sites of zymogen and activated matriptase in a similar manner. Our results suggest that HAI-1 and HAI-2 regulate matriptase zymogen activity and thus may act as regulators of matriptase trans(auto)-activation. Due to the main localisation of HAI-2 in the ER and HAI-1 in the secretory pathway and on the cell surface, this regulation likely occurs both in the secretory pathway and on the plasma membrane. Regulation of an active zymogen form of a protease is a novel finding.