INTERACTION OF THE FIBRINOLYTIC RECEPTOR, ANNEXIN-II, WITH THE ENDOTHELIAL-CELL SURFACE - ESSENTIAL ROLE OF ENDONEXIN REPEAT

Endothelial cells express a cell surface co-receptor for plasminogen a nd tissue plasminogen activator (t-PA) which we recently identified as annexin II (Hajjar, K. A., Jacovina, A. T., and Chacko, J. (1994) J. Biol. Chem., 269, 21191-21197). This protein enhances the catalytic ef ficiency of t-PA-dependent plasmin generation by 60-fold (Cesarman, G. M., Guevara, C. A., and Hajjar, K. A, (1994) J. Biol. Chem. 269, 2119 8-21203). Here, we demonstrate that annexin II is constitutively trans located to the endothelial cell surface within 16 h of biosynthesis, a nd that cell surface annexin II comprises 4.3 +/- 1.0% of the total ce llular pool. Exogenous I-125-annexin II bound to EGTA-washed endotheli al cells with high affinity (K-d 49 nM) and in a calcium-dependent (I- 50 = 3 mu M), phospholipid-sensitive manner. Peptides KASMKGLGTDED and YDSMKGKGTRDK, mimicking the calcium-binding ''endonexin'' motif (KGXG T) of annexin II, blocked its interaction with endothelial cells. Reco mbinant annexin II, bearing the calcium-binding site substitution D161 A of core repeat 2, failed to compete with binding of the wild type pr otein to the cell surface, while E246A and D321A mutants, correspondin g to core repeats 3 and 4, behaved as effective competitors. These dat a suggest that translocated annexin II interacts with cell surface pho spholipid via a high affinity calcium-dependent binding site that incl udes residues 118-122 (KGLGT) and the coordinating Asp(161) of core re peat 2. Thus, calcium-regulated expression of annexin II on the endoth elial cell surface may play a central role in control of plasmin-media ted processes.