Mechanism of cell surface activation of 72-kDa type IV collagenase. Isolation of the activated form of the membrane metalloprotease

Matrix metalloproteases are secreted by mammalian cells as zymogens and, upon activation, initiate tissue remodeling by proteolytic degradation of collagens and proteoglycans. Activation of the secreted proenzymes and interaction with their specific inhibition determine the net enzymatic activity in the extracellular space. We have previously demonstrated that 72T4C1 can be activated by a plasma membrane-dependent mechanism specific for this enzyme. Here, we report purification of the membrane activator of 72T4C1, which is a new metalloprotease identical to a recently cloned membrane-type matrix metalloprotease (MT-MMP). We demonstrate that activated MT-MMP acts as a cell surface tissue inhibitor of metalloprotease 2 (TIMP-2) receptor with K(d) = 2.54 x 10^-9 M. The activator·TIMP-2 complex in turn acts as a receptor for 72T4C1 (K(d) = 0.56 x 10^-9 M), binding to the carboxyl-end domain of the enzyme. Activation of 72T4Cl on the cell membrane provides a basic mechanism for spatially regulated extracellular proteolysis and presents a new target for prognosis and treatment of metastatic disease. The activator, purified as a tri-molecular complex of MT- MMP·TIMP2·carboxyl-end domain of 72T4Cl, is itself an activated form of MT- MMP, posing the following question: what is the mechanism of the activator's activation?