The matrix metalloproteinases (MMPs), 1 a multidomain family of zinc-dependent endopeptidases, degrade all structural components of the extracellular matrix (ECM) and many bioactive molecules, thereby playing essential roles in many physiological and pathological processes (1-4). Based on structural organization and subcellular localization, the MMP family is divided into secreted and membrane-anchored enzymes (1, 5). The membrane type-MMPs (MT-MMPs) comprise six members of plasma membrane-tethered MMPs, which include four type I transmembrane enzymes: MT1-, MT2-, MT3-, and MT5-MMP, and two glycosylphosphatidylinositol-anchored enzymes: MT4-and MT6-MMP (4, 6 -8). Although there is some tissue-specific expression of MT-MMPs, there is also significant overlap both in expression and in function raising the question of how cells command the repertoire of MT-MMPs at their disposition during proteolytic events. A major mechanism for controlling MT-MMP activity in the pericellular space is mediated by the action of tissue inhibitors of metalloproteinases (TIMPs). However, as opposed to soluble MMPs, which are almost equally inhibited by all TIMPs (9, 10), the MT-MMPs are highly selective when it comes to TIMP interactions. For example, the type I transmembrane MT-MMPs are poorly inhibited by TIMP-1 but are relatively well inhibited by TIMP-2, TIMP-3, and TIMP-4. In contrast, the glycosylphosphatidylinositol-anchored MT-MMPs are inhibited by both TIMP-1 and TIMP-2 (11). The differential sensitivity to TIMP inhibition among members of the MT-MMP family may facilitate the control of MT-MMP activity in the pericellular space.In addition to being potent ECM-degrading enzymes, the type I transmembrane MT-MMPs can also initiate a cascade of zymogen activation on the cell surface. MT-MMPs activate the zymogenic form of MMP-2 (pro-MMP-2 or pro-gelatinase A) (6, 8), which in turn can activate pro-MMP-9 (pro-gelatinase B) (12). MT1-MMP also promotes the activation of pro-collagenase 3 (MMP-13) (13), a potent collagenolytic protease. Because the gelatinases are efficient gelatinolytic enzymes, the MT-MMP/ MMP-13/gelatinase axis represents a well adapted proteolytic system designed to promote coordinated and complete collagen degradation in the pericellular space. Although the type I