Tissue factor residues that putatively interact with membrane phospholipids.

Area of Science:

• Biochemistry
• Molecular Biology
• Hematology

Background:

• Blood clotting is a complex process initiated by the tissue factor (TF)/factor VIIa complex.
• TF acts as a membrane-bound cofactor, binding factor VIIa to activate downstream clotting factors.
• Previous simulations suggested TF ectodomain residues interact with phosphatidylserine (PS) for optimal complex positioning.

Purpose of the Study:

• To investigate the role of putative PS-interactive residues in the TF ectodomain.
• To determine the impact of these mutations on TF cofactor function and factor VIIa binding.
• To identify specific TF residues critical for PS interaction and cofactor activity.

Main Methods:

• Site-directed mutagenesis of putative PS-interactive residues in the TF ectodomain to alanine.
• Assays measuring TF cofactor function: activation of factors IX and X, and plasma clotting.
• Liposome-based assays to assess the effect of varying PS content.
• Yeast surface display screening of TF mutants for enhanced factor VIIa binding.

Main Results:

• Mutations in specific TF residues, particularly in the Lys159-Gly164 loop, significantly reduced TF cofactor activity.
• The impaired activity of these mutants could be partially or fully restored by increasing PS concentration.
• A Lys165Glu mutation enhanced factor VIIa binding affinity by 3-fold, identified via yeast surface display.

Conclusions:

• Residues in the C-terminal TF ectodomain are crucial for interacting with phosphatidylserine headgroups.
• This interaction enhances TF cofactor activity, potentially through allosteric modulation of substrate-binding sites.
• TF-PS interactions are vital for efficient blood coagulation initiation.