Title

The role of tapasin in MHC class I antigen presentation

Date of Completion

January 2007

Keywords

Chemistry, Biochemistry|Health Sciences, Immunology

Degree

Ph.D.

Abstract

The cell-surface presentation of antigenic peptides by MHC class I molecules is an important event in human immunity that protects individuals against viral infections and cancer. Studies aimed at elucidating molecular and cellular aspects of the MHC class I antigen presentation pathway have consistently pointed to an endoplasmic reticulum (ER)-resident protein, namely tapasin, as playing a critical role in optimizing peptide loading to MHC class I molecules. To date, however, the mechanism of action of tapasin is unexplained. Through my dissertation research, I characterized tapasin and its function in peptide loading. The results of my study, which are summarized as below, allow me to propose a mechanistic model for the function of tapasin.^ The ER-lumenal region of tapasin (referred to as TPN) adopts an asymmetric, elongated structure with relatively high content of β-sheets. TPN has a thermal denaturation curve centered at ∼45°C. TPN consists of two major domains separated by a solvent-exposed linker; these two domains also associate together through non-covalent interactions.^ TPN weakly associates with the ER-lumenal domain of MHC class I molecules. TPN also has weak interactions with the complex of calreticulin and ERp57. MHC Class I molecules and TPN form complexes through Fos/Jun leucine-zipper dimerization motifs. The functions of TPN can be probed in the zippered complexes. ^ TPN stabilizes the active conformation of peptide-deficient MHC class I molecules. TPN increases the rates of peptide dissociation via a peptide-specific manner. TPN also increases the rates of peptide association. Kinetic studies enable us to draw two conclusions: (1) TPN acts as a catalyst and a chaperone by widening the peptide binding groove of MHC class I molecules. (2) The widening effect of TPN involves disruption of both peptide sequence-independent and -dependent interactions between peptide and MHC class I residues. The biological significance of the function of TPN is to maximize the diversity of minimally stable MHC class I/peptide complexes presented at the cell surface. ^ The knowledge and understanding derived from my studies strengthen our abilities to manipulate MHC class I antigen presentation, which has broad medical applications such as for developing vaccines against viral infections and cancer.^