One sequence, two folds: a metastable structure of CD2

Area of Science:

• Structural biology
• Molecular biology
• Immunology

Background:

• Lymphocyte cell adhesion molecule CD2 is crucial for immune cell interactions.
• The NH2-terminal domain of CD2 typically adopts an immunoglobulin superfamily (IgSF) fold.
• Previous studies determined the monomeric CD2 structure using X-ray crystallography and NMR.

Purpose of the Study:

• To characterize the structure and properties of a dimeric form of the CD2 NH2-terminal domain.
• To investigate the relationship between the monomeric and dimeric CD2 structures.
• To understand the implications of alternative protein folding for evolution.

Main Methods:

• X-ray crystallography of the dimeric CD2 form at 2.0-A resolution.
• Denaturation and refolding experiments to assess the stability of the dimeric form.
• Site-directed mutagenesis to probe the dimerization interface.

Main Results:

• A second, dimeric, folded state of CD2 was identified, comprising approximately 15% of recombinant molecules.
• The dimeric form represents a metastable state, convertible to the monomeric form upon refolding.
• The dimeric structure features domains formed by the intercalation of two polypeptide chains, distinct from the monomeric IgSF fold.
• The dimerization interface involves hydrophilic interactions mimicking cell adhesion, and mutations here regulate dimer formation.

Conclusions:

• The CD2 NH2-terminal domain can adopt alternative, metastable folds.
• The dimeric form highlights how inter-chain interactions can lead to novel quaternary structures.
• This study demonstrates the potential for protein oligomers to evolve from single-domain protein sequences.