Multiple binding sites in fibrinogen for integrin alphaMbeta2 (Mac-1)

Area of Science:

• Immunology
• Biochemistry
• Molecular Biology

Background:

• Leukocyte integrin alphaMbeta2 (Mac-1) is crucial for inflammatory responses, mediating leukocyte adhesion.
• It links thrombosis and inflammation by binding the coagulation protein fibrinogen.
• Previous models of alphaMbeta2-fibrinogen interaction involving specific gammaC domain sequences were incomplete.

Purpose of the Study:

• To elucidate the binding mechanism between the alphaMbeta2 integrin and fibrinogen.
• To identify alternative binding sites beyond previously known sequences.
• To understand the molecular basis of alphaMbeta2's ligand recognition.

Main Methods:

• Surface plasmon resonance was used to study the interaction between the alphaMbeta2 alphaMI-domain and fibrinogen's D fragment.
• Peptide libraries covering gammaC and betaC domains of fibrinogen were screened for alphaMI-domain binding.
• Substitutional peptide libraries were analyzed to determine recognition specificity.

Main Results:

• The alphaMbeta2 alphaMI-domain binds to multiple sites within fibrinogen's D fragment, not just previously identified sequences (P1 and P2).
• Synthetic peptides from gammaC and betaC domains effectively inhibited alphaMbeta2-mediated cell adhesion and supported direct adhesion.
• Analysis revealed that basic and hydrophobic residues are critical for alphaMbeta2 alphaMI-domain binding specificity.

Conclusions:

• A new model proposes that alphaMbeta2's alphaMI-domain interacts with numerous sites on fibrinogen.
• This multi-site binding mechanism explains the promiscuous ligand recognition of alphaMbeta2.
• Findings have implications for understanding integrin-ligand interactions in inflammation and thrombosis.