Expression, purification and assembly of soluble multimeric MHC class II-invariant chain complexes

Area of Science:

• Immunology
• Molecular Biology
• Protein Biochemistry

Background:

• Major histocompatibility class (MHC) II molecules are crucial for adaptive immune responses.
• MHC class II and invariant chain (Ii) complexes are assembled in the endoplasmic reticulum and trafficked to late endosomes.
• Ii trimers, potentially via their transmembrane (TM) domain, mediate MHC class II assembly.

Purpose of the Study:

• To investigate the self-assembly properties of the invariant chain (Ii).
• To understand the role of the Ii transmembrane (TM) domain in trimerization.
• To elucidate the stoichiometry of MHC class II-Ii complex formation.

Main Methods:

• Expression of soluble forms of MHC class II-Ii complexes.
• Biochemical analysis of Ii trimerization and MHC class II binding.

Main Results:

• Invariant chain (Ii) spontaneously trimerizes in the absence of its transmembrane (TM) domain.
• Ii trimerization occurs independently of alpha/beta chain binding.
• MHC class II-Ii complexes can exhibit variable stoichiometry, with Ii trimers binding one or two MHC class II molecules, not exclusively trimers of trimers.

Conclusions:

• Ii trimerization is an intrinsic property, not solely dependent on the TM domain or MHC class II binding.
• The assembly of MHC class II-Ii complexes is more flexible than previously thought.
• These findings provide insights into the structural basis of antigen presentation and adaptive immunity.