Structure of an integrin with an alphaI domain, complement receptor type 4

Area of Science:

• Structural Biology
• Immunology
• Biochemistry

Background:

• Integrins are critical cell surface receptors involved in cell adhesion and signaling.
• Integrin alpha(X)beta(2), also known as complement receptor type 4 (CR4), plays a key role in immune cell function.
• Previous models suggested a fixed orientation between alphaI and beta-propeller domains for allosteric signal transmission.

Purpose of the Study:

• To determine the high-resolution structure of integrin alpha(X)beta(2).
• To investigate the role of the alphaI domain's flexibility in integrin function.
• To elucidate the mechanisms of allosteric signal transmission and activation in alpha(X)beta(2).

Main Methods:

• X-ray crystallography was used to determine the structure of alpha(X)beta(2).
• Analysis of multiple crystal forms provided insights into interdomain flexibility.
• Comparison with integrins lacking alphaI domains was performed.

Main Results:

• The alphaI domain of alpha(X)beta(2) is highly flexible, not fixed in orientation.
• Two betaI domain states couple with three alphaI domain states, enhancing ligand accessibility.
• The orientation of terminal domains (calf-2 and beta-tail) differs from alphaI-less integrins.
• Unstructured linkers connect to transmembrane domains.
• Activating mutations and antibody epitopes highlight the importance of extension and headpiece conformation changes.

Conclusions:

• Integrin alpha(X)beta(2) exhibits significant interdomain flexibility, particularly in the alphaI domain.
• This flexibility is essential for modulating ligand binding and allosteric signal transmission.
• The findings support a model where extension, rather than a 'deadbolt' mechanism, is critical for integrin activation.