Plasminogen bound to fibrin is cleaved and activated by tissue plasminogen activator also bound to the fibrin. The association of both plasminogen and tissue plasminogen activator with a fibrin clot juxtaposes the two molecules, facilitating their interaction (Hoylaerts et al. 1982). Early studies suggested that tissue plasminogen activator itself might require activation (conversion to its two-chain form) before it could catalyze this reaction (e.g., Higgins and Vehar 1987). More recent work (Boose et al. 1989) indicates that the single-chain form of the molecule is catalytically active, although cleavage increases its activity and may thus serve to accelerate the later stages of fibrinolysis.
Annexin A2 (ANXA2) is a multicompartmental, multifunctional protein that forms a heterotetramer with its endothelial cell-surface binding partner protein S100-A10 (S100A10) (Rety et al. 1999). The tetramer is able to positively modulate tissue plasminogen activator-dependent activation of the fibrinolytic protease, plasmin from its plasminogen precursor (Luo et al. 2013, Hedhli et al. 2012).