Cryo-EM structure of the human native plasma coagulation factor XIII complex

• The native plasma FXIII complex structure reveals the intra- and intermolecular arrangement between/within the FXIII-A and -B subunits.

• The impact of disease-causing, clinically relevant FXIII mutations is visualized in the assembly of the heterotetrameric coagulation complex.

The structure of human coagulation factor XIII (FXIII), a heterotetrameric plasma protransglutaminase that covalently cross-links preformed fibrin polymers, remains elusive until today. The heterotetrameric complex is composed of 2 catalytic FXIII-A and 2 protective FXIII-B subunits. Structural etiology underlying FXIII deficiency has so far been derived from crystallographic structures, all of which are currently available for the FXIII-A₂ homodimer only. Here, we present the cryogenic electron microscopy (cryo-EM) structure of a native, human plasma–derived FXIII-A₂B₂ complex at 2.4 Å resolution. The structure provides detailed information on FXIII subunit interacting interfaces as the 2 subunits interact strongly in plasma. The native FXIII-A₂B₂ complex reveals a pseudosymmetric heterotetramer of 2 FXIII-B monomers intercalating with a symmetric FXIII-A₂ dimer forming a “crown”-like assembly. The symmetry axes of the A₂ and B₂ homodimers are twisted relative to each other such that Sushi domain 1 interacts with the catalytic core of the A subunit, and Sushi domain 2 with the symmetry related A′ subunit, and vice versa. We also report 4 novel mutations in the F13A1 gene encoding the FXIII-A subunit from a cohort of patients with severe FXIII deficiency. Our structure reveals the etiological basis of homozygous and heterozygous pathogenic mutations and explains the conditional dominant negative effects of heterozygous mutations. This atomistic description of complex interfaces is consistent with previous biochemical data and shows a congruence between the structural biochemistry of the FXIII complex and the clinical features of FXIII deficiency.