Low-affinity integrin states have faster ligand-binding kinetics than the high-affinity state

Area of Science:

• Cell Biology
• Biochemistry
• Biophysics

Background:

• Integrins exist in low-affinity (bent-closed, extended-closed) and high-affinity (extended-open) states.
• Current models propose integrin activation precedes ligand binding.
• Ligand-binding kinetics are crucial for cell adhesion and understanding activation mechanisms.

Purpose of the Study:

• To measure ligand-binding kinetics for integrins α4β1 and α5β1.
• To investigate the role of integrin conformational states in ligand binding.
• To determine if ligand binding precedes or follows integrin activation.

Main Methods:

• Kinetic measurements of integrin-ligand interactions.
• Comparison of binding rates for different integrin conformational states.
• Analysis of on- and off-rates for cell-surface and ectodomain integrins.

Main Results:

• Low-affinity integrin states exhibit substantially faster ligand binding than the high-affinity state.
• The extended-open conformation has a ~20-fold slower on-rate but a ~25,000-fold slower off-rate.
• On- and off-rates are consistent between cell-surface and ectodomain integrins.

Conclusions:

• Integrin binding to ligands may occur before activation via inside-out signaling.
• Rapid binding by low-affinity states and their higher surface population support this model.
• The findings challenge the paradigm that activation must precede ligand engagement.