The interaction between fibrinogen and zymogen FXIII-A2B2 is mediated by fibrinogen residues γ390-396 and the FXIII-B subunits

Coagulation transglutaminase factor XIII (FXIII) exists in circulation as heterotetrameric proenzyme FXIII-A 2 B 2 . Effectively all FXIII-A 2 B 2 circulates bound to fibrinogen, and excess FXIII-B 2 circulates in plasma. The motifs that mediate interaction of FXIII-A 2 B 2 with fibrinogen have been elusive. We recently detected reduced binding of FXIII-A 2 B 2 to murine fibrinogen that has γ-chain residues 390-396 mutated to alanines (Fibγ 390-396A ). Here, we evaluated binding features using human components, including recombinant fibrinogen variants, FXIII-A 2 B 2 , and isolated FXIII-A 2 and -B 2 homodimers. FXIII-A 2 B 2 coprecipitated with wild-type (γA/γA), alternatively-spliced (γ′/γ′), and αC-truncated (Aα251) fibrinogens, whereas coprecipitation with human Fibγ 390-396A was reduced by 75% ( P .0001). Surface plasmon resonance showed γA/γA, γ′/γ′, and Aα251 fibrinogens bound FXIII-A 2 B 2 with high affinity (nanomolar); however, Fibγ 390-396A did not bind FXIII-A 2 B 2 . These data indicate fibrinogen residues γ390-396 comprise the major binding motif for FXIII-A 2 B 2 . Compared with γA/γA clots, FXIII-A 2 B 2 activation peptide release was 2.7-fold slower in Fibγ 390-396A clots ( P 2 (lacking FXIII-B 2 ) was similar in γA/γA and Fibγ 390-396A clots, suggesting fibrinogen residues γ390-396 accelerate FXIII-A 2 B 2 activation in a FXIII-B 2 –dependent mechanism. Recombinant FXIII-B 2 bound γA/γA, γ′/γ′, and Aα251 with similar affinities as FXIII-A 2 B 2 , but did not bind or coprecipitate with Fibγ 390-396A . FXIII-B 2 also coprecipitated with fibrinogen from FXIII-A–deficient mouse and human plasmas. Collectively, these data indicate that FXIII-A 2 B 2 binds fibrinogen residues γ390-396 via the B subunits, and that excess plasma FXIII-B 2 is not free, but rather circulates bound to fibrinogen. These findings provide insight into assembly of the fibrinogen/FXIII-A 2 B 2 complex in both physiologic and therapeutic situations.