Abstract Number: PB0251
Meeting: ISTH 2020 Congress
Background: A key receptor for appropriate assembly and activation of the contact system (consisting of factor XII (FXII), prekallikrein (PK) and kininogen (HK)), is trimeric gC1qR/C1QBP/P32.
Aims: To understand:
(1) the structure of the complex formed between FXII, HK and gC1qR,
(2) how Zn2+ ions regulate complex formation.
Methods: The key residues involved in the FXII interaction with gC1qR were analysed by protein crystallography coupled with ITC and SPR. The functional consequences of the gC1qR-FXII interaction were studied using thrombin generation and clotting assays.
Results: The FXIIFnII-gC1qR-Zn2+ complex crystal structure provides the first insight into a gC1qR ligand interaction and shows a novel asymmetric stoichiometry of one FXIIFnII bound to the gC1qR trimer. The structure reveals key gC1qR residues coordinate a Zn2+ ion adjacent to the FXII binding site and a comparison with the Zn2+ free gC1qR crystal structure reveals the crucial conformational change underlying the Zn2+ dependent ligand binding. We also studied HKD5 binding and discovered that this is also asymmetric driven by the central location of a critical negatively charged loop in the gC1qR trimer and thus steric occlusion. We tested the effects of gC1qR on plasma coagulation to establish whether contact activation can occur under conditions where both gC1qR and Zn2+ are added. In the absence of additional stimuli, a shortened clotting time with pooled normal plasma was observed in a dose and FXII-dependent manner. Gel filtration experiments reveal that gC1qR clusters FXII and HK into a higher order 500kDa ternary complex a schemtic of which is should below in Figure 1.
Conclusions: The crystal structure of FXII in complex with gC1qR reveals an asymmetric interaction and bound Zn2+ ions. gC1qR undergoes a major conformational change upon FXII binding and the gC1qR-FXII-HK complex forms a higher order 500kDa complex.
To cite this abstract in AMA style:Emsley J, Kaira B, Slater A, Mccrae K, Ummay S, Mutch N, Searle M. Factor XII and Kininogen Complex with gC1qR/C1QBP/P32 Is Governed by Zinc Ions [abstract]. Res Pract Thromb Haemost. 2020; 4 (Suppl 1). https://abstracts.isth.org/abstract/factor-xii-and-kininogen-complex-with-gc1qr-c1qbp-p32-is-governed-by-zinc-ions/. Accessed May 6, 2021.
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