Recognition of fibronectin by the platelet integrin alpha IIb beta 3 involves an extended interface with multiple...

Area of Science:

• Molecular Biology
• Biochemistry
• Cellular Biology

Background:

• Normal platelet function requires integrin alpha IIb beta 3 (glycoprotein IIb/IIIa) to bind subendothelial matrix components like fibronectin (Fn) at vascular injury sites.
• Previous studies suggested roles for the 9th (3fn9) and 10th (3fn10) type III repeats of Fn in alpha IIb beta 3 binding.

Purpose of the Study:

• To evaluate the importance of charged residues within the recombinant Fn 3fn9-10 module for binding to integrin alpha IIb beta 3.
• To identify specific Fn residues critical for alpha IIb beta 3 interaction using a charge-to-alanine mutagenesis approach.

Main Methods:

• Utilized a charge-to-alanine mutagenesis strategy on recombinant 3fn9-10 modules, substituting 38 charged residues.
• Assayed mutant 3fn9-10 modules for their ability to inhibit Fn binding to purified alpha IIb beta 3.
• Mapped identified residues and antibody epitopes on the 3D structure of the Fn module.

Main Results:

• Substitutions at Fn residues Arg(1493) and Asp(1495) within the RGD sequence significantly affected alpha IIb beta 3 binding.
• Fn residues Arg(1369), Arg(1371), Arg(1379), Arg(1445), and Arg(1448) were essential for optimal 3fn9-10 module interaction with alpha IIb beta 3.
• All identified critical Fn residues are located on the same molecular face, and antibody epitopes overlap these binding sites.

Conclusions:

• Integrin alpha IIb beta 3 binding to fibronectin involves multiple electrostatic interactions, not solely the RGD sequence.
• The identified charged residues on the Fn 3fn9-10 module are critical for optimal ligand-receptor engagement.
• These findings elucidate the molecular basis of platelet adhesion and aggregation.