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Annexin II tetramer (AIIt) is an important endothelial cell surface protein receptor for plasminogen and t-PA. AIIt, a heterotetramer, is composed of two p36 subunits (called annexin II) and two p11 subunits. In this report, we have compared the ability of the isolated p36 and p11 subunits to stimulate t-PA-dependent [Glu]plasminogen activation. The fluid-phase recombinant p11 subunit stimulated the rate of t-PA-dependent activation of [Glu]plasminogen about 46-fold compared to an approximate stimulation of 2-fold by the recombinant p36 subunit and 77-fold by recombinant AIIt. The stimulation of t-PA-dependent activation of [Glu]plasminogen by the p11 subunit was Ca²⁺-independent and inhibited by ε-aminocaproic acid. [Glu]Plasminogen bound to a p11 subunit affinity column and could be eluted with ε-aminocaproic acid. Both AIIt and the p11 subunit protected t-PA and plasmin from inactivation by PAI-1 and α₂ …