Ultrastructure and function of dimeric, soluble intercellular adhesion molecule-1 (ICAM-1)

Area of Science:

• Cellular Biology
• Immunology
• Structural Biology

Background:

• Intercellular adhesion molecule-1 (ICAM-1) is known to dimerize on cell surfaces.
• Previous research suggested roles for the immunoglobulin superfamily domain 5 and transmembrane domain in ICAM-1 dimerization.
• Crystallographic data indicated potential dimerization mediated by domain 1.

Purpose of the Study:

• To investigate ICAM-1 dimerization at domain 1 and its impact on binding to integrin alpha(L)beta(2).
• To characterize different topological forms of ICAM-1 dimers.
• To compare the binding affinities of various ICAM-1 dimer topologies to the integrin alpha(L)beta(2) I domain.

Main Methods:

• Construction of soluble C-terminally dimerized ICAM-1 using an alpha-helical coiled coil.
• Introduction of cysteine mutations at the domain 1 dimer interface to create disulfide-bonded domain 1 dimers.
• Generation of combined domain 1/C-terminal dimers.
• Electron microscopy for structural analysis of dimer topologies.
• Surface plasmon resonance (SPR) to assess binding affinities.

Main Results:

• Soluble C-terminal dimers exhibited straight, bent, U-shaped, ring-like, and W-shaped tetramer forms.
• Mutations at the domain 1 interface successfully generated soluble disulfide-bonded ICAM-1 domain 1 dimers.
• Combining domain 1 and C-terminal dimerization yielded significant amounts of ring-like dimers and W-shaped tetramers.
• All tested ICAM-1 dimer forms (domain 1, C-terminal, and domain 1/C-terminal) bound to the integrin alpha(L)beta(2) I domain with 1.5-3 fold higher affinity than monomeric ICAM-1.

Conclusions:

• ICAM-1 can adopt at least three distinct dimeric topologies.
• Dimerization at domain 1 does not impede ICAM-1's binding to the integrin alpha(L)beta(2) I domain.
• ICAM-1 dimerization, particularly at domain 1, enhances binding affinity to its integrin ligand.