The intracellular localization and oligomerization of chicken invariant chain with major histocompatibility complex class II subunits.

Invariant chain (Ii) binds to MHC class II (MHCII) to assemble a nonamer in the endoplasmic reticulum. Major histocompatibility complex class II-associated Ii peptide (CLIP) that occupies the peptide binding groove of MHCII prevents MHCII molecules from loading with endogenous antigens. We used the green or red fluorescent protein-fused Ii or MHCII subunits to detect the intracellular localization and oligomerization of chicken Ii with single chicken MHCII subunits. Our results indicated that chicken Ii associates with single MHCII subunits and formed oligomers with MHCII subunits. The Ii mutant with a deleted CLIP sequence blocks the association with single MHCII subunits, but exchanging CLIP with the Newcastle disease virus F(343) epitope restores this association. Thus, MHCII polymer assembly is not blocked as long as the basic steric molecular structure of Ii is maintained, and different binding models exist in different species or MHCII isotypes.