A role for the transmembrane domain in the trimerization of the MHC class II-associated invariant chain

MHC class II and invariant chain (Ii) associate early in biosynthesis to form a nonameric complex. Ii first assembles into a trimer and then associates with three class II αβ heterodimers. Although the membrane- proximal region of the Ii luminal domain is structurally disordered, the C- terminal segment of the luminal domain is largely α-helical and contains a major interaction site for the Ii trimer. In this study, we show that the Ii transmembrane domain plays an important role in the formation of Ii trimers. The Ii transmembrane domain contains an unusual patch of hydrophilic residues near the luminal interface. Substitution of these polar residues with nonpolar amino acids resulted in a decrease in the efficiency of Ii trimerization and subsequent class II association. Moreover, N-terminal fragments of Ii were found to trimerize independently of the luminal α- helical domain. Progressive C-terminal truncations mapped a homotypic association site to the first 80 aa of Ii. Together, these results implicate the Ii transmembrane domain as a site of trimer interaction that can play an important role in the initiation of trimer formation.