Analysis of the factor XI variant Arg184Gly suggests a structural basis for factor IX binding to factor XIa

Area of Science:

• Coagulation cascade biochemistry
• Protein-protein interactions in hemostasis
• Molecular mechanisms of factor activation

Background:

• Factor XI (FXI) deficiency can be associated with specific mutations, such as Arg184Gly in the FXI A3 domain.
• The FXI A3 domain's exosite is crucial for binding activated Factor IX (FIX).

Purpose of the Study:

• To investigate the functional impact of the Arg184Gly substitution on FIX activation by FXI.
• To elucidate the structural and functional characteristics of the FIX-binding site on activated FXI (FXIa).

Main Methods:

• Utilized recombinant FXIa and FIX variants to pinpoint residues involved in FXIa-mediated FIX activation.
• Employed structural analysis of FXI to identify potential binding sites for FIX.

Main Results:

• The Arg184Gly substitution in FXIa increased the Km for FIX activation threefold, indicating Arg184's role in the exosite.
• Residues 183-185 in FXIa are critical for exosite activity, with alanine substitution abolishing function.
• The Ω-loop of FIX (residues 4-11) was identified as the binding site on FXIa, as demonstrated by poor activation when substituted with the FVII Ω-loop.

Conclusions:

• The Ω-loop of FIX binds to a composite site on FXIa, formed by N- and C-terminal residues of the A3 domain.
• These binding residues are concealed in zymogen FXI and become accessible upon activation to FXIa, facilitating FIX binding.