Molecular interactions of MMP-13 C-terminal domain with chondrocyte proteins.

Matrix metalloproteinases (MMP)-13 activity is necessary for normal skeletal development and plays a central role in cartilage degeneration associated with osteoarthritis (OA). The studies we described here examine the interactions of the hemopexin domain of MMP-13 with proteins secreted by human chondrocytes in culture. The hemopexin domain of the MMPs and many other proteins in which this structure is found mediates protein function by forming the primary site of interaction with other proteins. We have modified a tandem affinity expression tag (hTAP) to enable efficient expression of the tagged bait protein. In this case the MMP-13 C-terminal domain (CTD) comprises hinge and hemopexin domain, and we immobilized the fusion construct on a column of agarose bound immunoglobin G. The MMP-13 CTD affinity column so generated enabled the efficient and gentle isolation of interacting proteins from the culture medium of human articular chondrocytes. TIMP1 and alpha2-macroglobulin previously shown to interact with MMP-13 as well as several proteins, fibronectin, type VI collagen and xylosyltransferase 1 and several proteoglycans, decorin, syndecan 4 and serglycin not previously recognized as interacting with MMP-13 were identified by mass spectrometry. The interaction between isolated proteins and MMP-13 CTD was verified by yeast two hybrid analysis. We also demonstrated serglycin expression by chondrocytes for the first time and its co localization with MMP-13 in a cytoplasmic granular morphology. The consequence of these interactions remains to be demonstrated, however; binding to MMP-13 suggests a role in the regulation of cartilage degradation.