A structural and molecular dynamics approach to understanding the peptide-receptive transition state of MHC-I molecules.

ABSTRACT

The mature conformation of major histocompatibility complex class I (MHC-I) proteins depends on the presence of bound peptides, permitting recognition at the cell surface by CD8(+) T lymphocytes. Newly synthesized MHC-I molecules in the endoplasmic reticulum are maintained in a peptide-receptive (PR) transition state by several chaperones until they are released concomitant with the loading of peptides. By determining the crystallographic structure of a region of an MHC-I molecule that is recognized by a unique monoclonal antibody and comparing this with docking and molecular dynamics simulations with the whole molecule, we demonstrate the movement of a hinged unit supporting the part of the binding groove that interacts with the amino terminal residues of the bound peptide. This unit contains a conserved 310 helix that flips from an exposed "open" position in the PR form to a "closed" position in the peptide-loaded (PL) mature molecule. These analyses indicate how this segment of the MHC-I molecule moves to help establish the A and B pockets critical for tight peptide binding and the stable structure required for antigen presentation and T cell recognition at the cell surface.