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 an d, upon activation, initiate tissue remodeling by proteolytic degradat ion of collagens and proteoglycans. Activation of the secreted proenzy mes and interaction with their specific inhibitors determine the net e nzymatic activity in the extracellular space. We have previously demon strated that 72T4Cl can be activated by a plasma membrane-dependent me chanism specific for this enzyme. Here, we report purification of the membrane activator of 72T4Cl, which is a new metalloprotease identical to a recently cloned membrane-type matrix metalloprotease (MT-MMP). W e demonstrate that activated MT-MMP acts as a cell surface tissue inhi bitor of metalloprotease 2 (TIMP-2) receptor with K-d = 2.54 x 10(-9) M. The activator . TLMP-2 complex in turn acts as a receptor for 72T4C l (K-d = 0.56 x 10(-9) M), binding to the carboxyl-end domain of the e nzyme. Activation of 72T4Cl on the cell membrane provides a basic mech anism for spatially regulated extracellular proteolysis and presents a new target for prognosis and treatment of metastatic disease. The act ivator, purified as a tri-moIecular complex of MT-MMP . TIMPS . carbox yl-end domain of 72T4Cl, is itself an activated form of MT-MMP, posing the following question: what is the mechanism of the activator's acti vation?