Invariant chain transmembrane domain trimerization: a step in MHC class II assembly.

Area of Science:

• Molecular Biology
• Biophysics
• Immunology

Background:

• The transmembrane (TM) domain of the major histocompatibility complex (MHC) class II-associated invariant chain (Ii) is crucial for MHC class II complex stability and function.
• Ii must form a trimer for correct function, with the TM domain hypothesized to stabilize this state.
• Previous studies linked TM domain mutations to impaired MHC class II functions, suggesting disrupted helix-helix interactions, but lacked direct structural evidence.

Purpose of the Study:

• To investigate the self-assembly and structural properties of the Ii TM domain in isolation.
• To experimentally determine the oligomeric state and stability of the Ii TM domain.
• To analyze the structural impact of functionally relevant mutations within the Ii TM domain.

Main Methods:

• Biophysical techniques including analytical ultracentrifugation.
• Computational methods to study protein folding and assembly.
• Analysis of Ii TM domain self-association in DPC micelles.

Main Results:

• The Ii TM domain exhibits strong self-association, primarily forming a stable trimer.
• Analytical ultracentrifugation revealed a dissociation constant of approximately 120 nM for the TM domain trimer in DPC micelles.
• Mutations in key glutamine and threonine residues disrupted TM helix interactions, correlating with previously observed functional deficits.

Conclusions:

• The Ii TM domain intrinsically forms stable trimers, confirming its role in stabilizing the MHC class II complex.
• The study provides direct experimental evidence linking TM domain structural integrity, specifically helix-helix interactions, to invariant chain function.
• These findings elucidate the molecular basis for how TM domain mutations compromise MHC class II complex formation and antigen presentation.