Summary
The physiologic activation of the plasma kallikrein-kinin system requires the assembly
of its constituents on a cell membrane. High-molecular-weight kininogen (HK) and cleaved
HK (HKa) both interact with at least three endothelial cell binding proteins: urokinase
plasminogen activator receptor (uPAR), globular C1q receptor (gC1qR,) and cytokeratin
1 (CK1). The affinity of HK and HKa for endothelial cells are KD=7–52 nM. The contribution
of each protein is unknown. We examined the direct binding of HK and HKa to the soluble
extracellular form of uPAR (suPAR), gC1qR and CK1 using surface plasmon resonance.
Each binding protein linked to a CM-5 chip and the association, dissociation and KD
(equilibrium constant) were measured. The interaction of HK and HKa with each binding
protein was zinc-dependent. The affinity for HK and HKa was gC1qR>CK1>suPAR, indicating
that gC1qR is dominant for binding. The affinity for HKa compared to HK was the same
for gC1qR, 2.6-fold tighter for CK1 but 53-fold tighter for suPAR. Complex between
binding proteins was only observed between gC1qR and CK1 indicating that a binary
CK1-gC1qR complex can form independently of kininogen. Although suPAR has the weakest
affinity of the three binding proteins, it is the only one that distinguished between
HK and HKa. This finding indicates that uPAR may be a key membrane binding protein
for differential binding and signalling between the cleaved and uncleaved forms of
kininogen. The role of CK1 and gC1qR may be to initially bind HK to the membrane surface
before productive cleavage to HKa.
Keywords Kininogen - surface plasmon resonance - suPAR - gC1qR - cytokeratin 1