Major histocompatibility complex (MHC) class II molecules exhibit conformational heterogeneity, which influences their ability to stimulate CD4 T cells and drive immune responses. Previous studies suggest a role for the transmembrane domain of the class II αβ heterodimer in determining molecular structure and function. Our previous studies identified an MHC class II conformer that is marked by the Ia.2 epitope. These Ia.2(+) class II conformers are lipid raft-associated and able to drive both tyrosine kinase signaling and efficient antigen presentation to CD4 T cells. Here, we establish that the Ia.2(+) I-A(k) conformer is formed early in the class II biosynthetic pathway and that differential pairing of highly conserved transmembrane domain GXXXG dimerization motifs is responsible for formation of Ia.2(+) versus Ia.2(-) I-A(k) class II conformers and controlling lipid raft partitioning. These findings provide a molecular explanation for the formation of two distinct MHC class II conformers that differ in their inherent ability to signal and drive robust T cell activation, providing new insight into the role of MHC class II in regulating antigen-presenting cell-T cell interactions critical to the initiation and control of multiple aspects of the immune response.
