Factor XI anion-binding sites are required for productive interactions with polyphosphate

Area of Science:

• Biochemistry
• Hematology
• Thrombosis Research

Background:

• Polymers of inorganic phosphate (polyP) enhance factor XI (FXI) activation to FXIa.
• FXI binding to polyP is essential for this enhancement.
• Anion-binding sites (ABSs) on FXIa are known to be important for heparin-mediated inhibition by antithrombin.

Purpose of the Study:

• To investigate the role of FXI's anion-binding sites (ABSs) in polyP-enhanced FXI activation.
• To determine if ABSs are critical for polyP's cofactor activity in FXI activation.

Main Methods:

• Utilized recombinant FXI variants with absent or single ABSs.
• Assessed FXI activation in purified protein systems and plasma clotting assays.
• Evaluated FXI function in a murine thrombosis model.

Main Results:

• FXI activation by polyP was significantly reduced in FXI variants lacking one or both ABSs.
• PolyP binding to FXIa did not enhance antithrombin inhibition or interfere with FIX activation.
• FXI variants with deficient ABSs showed impaired FXI-dependent coagulation and reduced thrombus formation in vivo.

Conclusions:

• The ABSs on FXIa are essential for polyP's cofactor activity in FXI activation, similar to their role in heparin's effects.
• These FXI ABSs play a significant role in FXI-dependent thrombotic processes.