Spatial Regulation and Affinity Modulation of Plasmin by High-Affinity Binding to the G Domain of the 3 Subunit of Laminin-5.

Cells in complex tissues contact extracellular matrix that interacts with integrin receptors to influence gene expression,proliferation,apoptosis,adhesion,and motility. During development,tissue remodeling,and tumorigenesis,matrix components are modified by enzymatic digestion with subsequent effects on integrin binding and signaling. We are interested in understanding the mechanisms by which broad spectrum proteinases such as plasmin are targeted to their extracellular matrix protein substrates. We have utilized plasmin-mediated cleavage of the epithelial basement membrane glycoprotein laminin-5 as a model to evaluate molecular events that direct plasmin activity to specific structural domains. We report that plasminogen and tissue plasminogen activator (tPA) exhibit high affinity,specific binding to the G(1) subdomain of the N terminus of the laminin-5 alpha(3) subunit,with equilibrium dissociation constants of 50 nm for plasminogen and 80 nm for tPA. No high affinity binding to the G(2),G(3),and G(4) subdomains was observed. As a result of binding to the G(1) subdomain,the catalytic efficiency of tPA-catalyzed plasminogen activation is enhanced 32-fold,leading to increased matrix-associated plasmin that is positioned favorably for cleavage within the G(4) subdomain as we have reported previously (Goldfinger,L. E.,Stack,M. S.,and Jones,J. C. R. (1998) J. Cell Biol. 141,255-265). Thus,physical constraints dictated by interaction of proteinase and matrix macromolecule control not only enzymatic activity but may regulate substrate targeting of proteinases.