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

Major histocompatibility class (MHC) II molecules are essential for running adaptive immune response. They are produced in the ER and targeted to late endosomes with the help of invariant chain (Ii) trimers. Ii trimerization may be induced by the Ii TM domain. To enable mechanistic and structural studies of MHC class II–Ii assembly, soluble forms of the complexes were expressed. We show that Ii trimerizes in the absence of the transmembrane part, prior to binding of α/β chains. The biochemical analysis supports the suggestion that the MHC class II–Ii complexes are not necessarily trimers of trimers, but that the Ii trimer can also be occupied by one or two MHC class II complexes.

► Soluble invariant chain lacking the transmembrane region forms a trimer.
► MHC class II dimers of various stoichiometry associate with soluble invariant chain trimer.
► Soluble MHC class II-Invariant chain complexes are secreted outside the cells.