Stabilizing a Flexible Interdomain Hinge Region Harboring the SMB Binding Site Drives uPAR into Its Closed Conformation

The urokinase-type plasminogen activator receptor (uPAR) is a multidomain glycolipid-anchored membrane protein, which facilitates extracellular matrix remodeling by focalizing plasminogen activation to cell surfaces via its high-affinity interaction with uPA. The modular assembly of its three LU domains is inherently flexible and binding of uPA drives uPAR into its closed conformation, which presents the higher affinity state for vitronectin thus providing an allosteric regulatory mechanism. Using a new class of epitope mapped monoclonal anti-uPAR antibodies, we now demonstrate that the reciprocal stabilization is indeed also possible. By surface plasmon resonance studies we show that these mAbs and vitronectin have overlapping binding sites on uPAR and that they share Arg(91) as hotspot residue in their binding interfaces. The crystal structure solved for one of these uPAR•mAb complexes at 3.0Å clearly shows that this mAb preselects the closed uPAR conformation with an empty, but correctly assembled large hydrophobic binding cavity for uPA. Accordingly, these mAbs inhibit the uPAR-dependent lamellipodia formation and migration on vitronectin coated matrices irrespective of the conformational status of uPAR and its occupancy with uPA. This is to the best of our knowledge the first study, showing that the dynamic assembly of the three LU domains in uPAR(wt) can be driven towards the closed form by an external ligand, which is not engaging the hydrophobic uPA-binding cavity. As this binding interface is also exploited by the SMB domain of vitronectin this relationship should therefore be taken into consideration when exploring uPAR-dependent cell adhesion and migration in vitronectin-rich environments.