Protease nexins: Cell‐secreted proteins that mediate the binding, internalization, and degradation of regulatory serine proteases

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Abstract
The protease nexins (PN‐I, Mr ∼︁38,000; PN‐II, Mr ∼︁95,000; and PN‐III, Mr ∼︁31,000) are recently described cell‐secreted proteins that selectively link to regulatory serins proteases in the extracellular environment and mediate their cellular binding, internalization, and degradation. In the present studies we compared the protease nexins with respect to protease specificity, heparin sensitivity, and general mode of action. By competitive binding assays using [125l]‐thrombin, [125l]‐nerve growth factor‐γ(125l‐NGF‐γ), and [125l]‐epidermal growth factor binding protein (125l‐EGF‐binding protein), we characterized the nexins in terms of protease specificity and determined that PN‐I links to and mediates the cellular binding of thrombin or urokinase, whereas PN‐II and PN‐III preferentially link to and mediate the cellular binding of the EGF binding protein and NGF‐γ, respectively. In addition, whereas the ability of PN‐I to link to thrombin is strongly modulated by heparin, PN‐II and PN‐III are essentially unaffected by heparin. The linkage of each of the nexins to their respective proteases requires the catalytic site serine of the protease, judged by the inability of diisopropylphospho (DIP) derivatives of the proteases tested to link to their respective nexins. Subsequent to linkage, the nexin:protease complexes are bound to cells, rapidly internalized, and ultimately degraded via a monensin‐sensitive apparently lysosomal pathway, although each nexin:protease complex is degraded at its own characteristic rate. Importantly, the protease nexins provide the major pathway through which human fibroblasts interact with each of the serine proteases studied. Taken together, these data suggest that the nexins are a unique class of cell‐secreted proteins that enable cells to monitor and selectively regulate specific serine proteases in their environment.