The difference in the intracellular Arg/Lys-rich and EHLVY motifs contributes to distinct subcellular distribution of HAI-1 versus HAI-2.

The integral membrane, Kunitz-type, serine protease inhibitors, HAI-1 and HAI-2, closely resemble one another structurally and with regard to their specificity and potency against proteases. Structural complementarity between the Kunitz domains and serine protease domains renders the membrane-associated serine proteases, matriptase and prostasin, the primary target proteases of the HAIs. The shared biochemical enzyme-inhibitor relationships are, however, at odds with their behavior at the cellular level, where HAI-1 appears to be the default inhibitor of these proteases and HAI-2 a cell-type-selective inhibitor, even though they are widely co-expressed. The limited motility of these proteins caused by their membrane anchorages may require their co-localization within a certain distance to allow the establishment of a cellular level functional relationship between the proteases and the inhibitors. The differences in their subcellular localization with HAI-1 both inside the cell and on the cell surface, compared to HAI-2 predominately in intracellular granules has, therefore, been implicated in the differential manner of their control of matriptase and prostasin proteolysis. The targeting signals present in the intracellular domains of the HAIs are systematically investigated herein. Studies involving domain swap and point mutation, in combination with immunocytochemistry and cell surface biotinylation/avidin depletion, reveal that the different subcellular localization between the HAIs can largely be attributed to differences in the intracellular Arg/Lys-rich and EHLVY motifs. These intrinsic differences in the targeting signal render the HAIs as two independent rather than redundant proteolysis regulators.