NMR and mutagenesis evidence for an I domain allosteric site that regulates lymphocyte function-associated antigen 1...

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• Leukocyte integrin LFA-1 (CD11a/CD18) is crucial for immune and inflammatory responses.
• LFA-1 activation involves a conformational shift from a resting to a high-affinity state for ligand binding, such as ICAM-1.
• Previous studies on Mac-1 suggested conformational changes in the I domain's C-terminal alpha-helix during activation.

Purpose of the Study:

• To identify residues in the CD11a I domain affected by ICAM-1 binding using NMR spectroscopy.
• To investigate the role of these affected residues in LFA-1 activation and ICAM-1 binding.
• To explore potential allosteric regulation mechanisms of LFA-1 function.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy to monitor changes in CD11a I domain residues upon ICAM-1 binding.
• Site-directed mutagenesis to substitute residues at identified affected sites.
• Assessment of LFA-1 binding activity of mutant proteins to ICAM-1.

Main Results:

• Two distinct regions in the CD11a I domain were perturbed by ICAM-1 binding: one near the MIDAS and a second distal site including the C-terminal alpha-helix.
• Mutations in the distal site conferred constitutive LFA-1 activation for ICAM-1 binding, without forming essential binding interfaces.
• Residues in I domain linker sequences were shown to regulate LFA-1 binding.
• The identified distal site acts as an I domain allosteric site (IDAS) structurally linked to the MIDAS.

Conclusions:

• LFA-1 binding to ICAM-1 is regulated by an allosteric mechanism involving the IDAS.
• The IDAS is structurally connected to the MIDAS, suggesting a coordinated conformational change for integrin activation.
• These findings provide insights into the allosteric regulation of leukocyte integrin function.