Factor XII and kininogen asymmetric assembly with gC1qR/C1QBP/P32 is governed by allostery

Bubacarr Kaira, Alexandre Slater, Keith McCrae, Ingrid Dreveny, Sumya Ummay, Nicola Mutch, Mark Searle*, Jonas Emsley*

*Corresponding author for this work

Research output: Contribution to journalArticle


The contact system is composed of Factor XII (FXII), prekallikrein (PK) and co-factor kininogen (HK). The globular C1q receptor (gC1qR) has been shown to interact with FXII and HK. We reveal the FXII fibronectin type II domain (FnII) binds gC1qR in a Zn 2+dependent fashionand determined the complex crystal structure. FXIIFnII binds the gC1qR trimer in an asymmetric fashion with residues Arg36 and Arg65 forming contacts with two distinct negatively charged pockets. gC1qR residues Asp185 and His187 coordinate a Zn 2+adjacent to the FXII binding site and a comparison with the ligand free gC1qR crystal structure reveals the anionic G1-loop becomes ordered upon FXIIFnII binding. Additional
conformational changes in the region of the Zn 2+binding site reveal an allosteric basis for Zn 2+modulation of FXII binding. Mutagenesis coupled
with SPR demonstrate the gC1qR Zn 2+site contributes to FXII binding and plasma based assays reveal gC1qR stimulates coagulation in a FXIIdependent manner. Analysis of the binding of HK domain 5 (HKD5) to gC1qR shows only one high affinity binding site per trimer. Mutagenesis studies identify a critical G3-loop located at the center of the gC1qR trimer suggesting steric occlusion as the mechanism for HKD5 asymmetric binding. Gel filtration experiments reveal that gC1qR clusters FXII and HK into a higher order 500kDa ternary complex. These results support the conclusion that extracellular gC1qR can act as a chaperone to cluster contact factors which may be a prelude for initiating the cascades which drive bradykinin generation and the intrinsic pathway of coagulation.
Original languageEnglish
Early online date19 Jun 2020
Publication statusE-pub ahead of print - 19 Jun 2020


  • Plasma kallikrein
  • Factor XII
  • Globular complement C1q receptor
  • Kininogen

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