The integrin I-domain anchors proMMP-9 at the cell surface. (A) ¹²⁵I-labeled MMP-9 domains bind to OCI-AML-3 cells in a dose-dependent manner. (B) ¹²⁵I-ΔproMMP-9 binding was competed with peptides (200 μmol/L), α_L and α_M I domains (20 μg/mL), or the antibodies (20 μg). (C) Binding of ¹²⁵I-PEX domain was studied as in panel B. (D) ¹²⁵I-ΔproMMP-9 binding to siRNA-treated or untreated cells. (E) Immunoblots of siRNA-treated cells with α_M antibodies and control α-actinin antibodies. (F) Binding of siRNA-treated cells to immobilized proMMP-9, intercellular adhesion molecule-1, or BSA. Adherent cells were quantitated by phosphatase assay. Data are means ± SD from triplicates. *P < .001 by t test.