The native structure of intercellular adhesion molecule-1 (ICAM-1) is a dimer. Correlation with binding to LFA-1

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• Intercellular adhesion molecule-1 (ICAM-1) shows low affinity for its receptor LFA-1 in solution.
• Conflicting reports exist regarding ICAM-1's ability to inhibit cell adhesion.
• Oligomeric state may influence ICAM-1's interaction with integrins.

Purpose of the Study:

• To investigate the oligomeric state of ICAM-1 in solution.
• To determine if ICAM-1 dimerization affects its binding to LFA-1.
• To clarify the biochemical basis of ICAM-1-LFA-1 interactions.

Main Methods:

• Metabolic labeling (35S-methionine/cysteine) of A549 cells and HUVECs.
• Chemical cross-linking and immunoprecipitation to identify ICAM-1 forms.
• Affinity purification and gel filtration chromatography of ICAM-1.
• Binding assays using purified LFA-1 and isolated ICAM-1 fractions.

Main Results:

• A high molecular weight (200 kDa) dimeric form of ICAM-1 was identified in cross-linked cell lysates and supernatants.
• The majority of shed ICAM-1 exists as a non-covalent dimer, unlike recombinant soluble ICAM-1.
• Dimer-rich fractions of ICAM-1 exhibited significantly enhanced binding affinity to LFA-1 compared to monomeric fractions.

Conclusions:

• ICAM-1 exists natively as a non-covalent dimer in both membrane-bound and soluble forms.
• Dimerization of ICAM-1 is directly correlated with its enhanced binding capacity to LFA-1.
• This finding provides a molecular explanation for ICAM-1's adhesive functions and potential therapeutic targeting.