Human proteoglycan testican‐1 inhibits the lysosomal cysteine protease cathepsin L

Testican‐1, a secreted proteoglycan enriched in brain, has a single thyropin domain that is highly homologous to domains previously shown to inhibit cysteine proteases. We demonstrate that purified recombinant human testican‐1 is a strong competitive inhibitor of the lysosomal cysteine protease, cathepsin L, with a Ki of 0.7 nm, but it does not inhibit the structurally related lysosomal cysteine protease cathepsin B. Testican‐1 inhibition of cathepsin L is independent of its chondroitin sulfate chains and is effective at both pH 5.5 and 7.2. At neutral pH, testican‐1 also stabilizes cathepsin L, slowing pH‐induced denaturation and allowing the protease to remain active longer, although the rate of proteolysis is reduced. These data indicate that testican‐1 is capable of modulating cathepsin L activity both in intracellular vesicles and in the extracellular milieu.