Supine orientation of a murine MHC class I molecule on the membrane bilayer

Structural studies of cellular immune receptors such as MHC molecules, T cell receptors (TCR), and TCR/ MHC complexes have been carried out with recombinant, soluble forms of the extracytoplasmic domain of these glycoproteins [1]. The important role of the membrane bilayer in T cell recognition and antigen presentation has become increasingly obvious with the description of lipid microdomains [2, 3]. These rafts appear to regulate recognition and signaling by clustering receptors and facilitating the formation of the immune synapse [4]. However, the interactions and orientation of these receptors at the lipid bilayer are unknown. We have used H-2K(b), a major-histocompatibility (MHC) class I molecule, and tethered its soluble domain to a lipid bilayer via a surrogate connecting peptide to reveal the disposition of MHC molecule on the membrane surface. We demonstrate that the long axis of the MHC molecule is approximately parallel to the plane of the membrane with the peptide binding pocket close to the membrane surface. This result was determined by analyzing 4.5Angstrom resolution electron crystallographic projection data from frozen-hydrated 2-dimensional crystals. Ionic interactions between the lipid headgroup and the protein appear to be responsible for this orientation, which could establish a "fourth dimension" during MHC/T cell receptor interactions critical for activation.