A pivotal role for a conserved bulky residue at the α1-helix of the αI integrin domain in ligand binding

Area of Science:

• Biochemistry
• Structural Biology
• Cellular Biology

Background:

• Integrins are crucial cell surface receptors involved in cell adhesion and signaling.
• The von Willebrand factor A domains (βI and αI) within integrins undergo conformational changes to regulate ligand binding affinity.
• Specific residues within these domains, like glycine in βI and phenylalanine in αI, are critical for maintaining resting and active states.

Purpose of the Study:

• To investigate the functional role of the conserved phenylalanine residue in the α1-helix of integrin αI domains.
• To elucidate the mechanism by which this phenylalanine contributes to integrin activation and ligand binding.
• To explore the conformational cross-talk between the αI and βI domains of integrins.

Main Methods:

• Site-directed mutagenesis to alter the phenylalanine residue in the α1-helix of α_Lβ₂ and α_Mβ₂ integrins.
• Ligand binding assays using soluble and immobilized intercellular cell adhesion molecule 1 (ICAM-1).
• Molecular dynamics simulations to analyze conformational changes and stabilization mechanisms.

Main Results:

• Trimming the phenylalanine residue in the α1-helix abolished α_Lβ₂ and α_Mβ₂ integrin binding to ICAM-1.
• The phenylalanine acts as a pawl, stabilizing the β6-α7 loop and α7-helix movement required for high-affinity binding.
• Conformational changes between the αI and βI domains can be uncoupled, indicating independent activation pathways.

Conclusions:

• The bulky phenylalanine in the αI domain is essential for integrin activation by stabilizing key structural elements for high-affinity ligand binding.
• This pawl-like mechanism is likely conserved across other von Willebrand factor A domains.
• The loose linkage between αI and βI domains allows for independent conformational changes, facilitating leukocyte rolling adhesion.