Direct peptide regulatable interactions between tapasin and MHC class I molecules

Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI 48109-0620, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 12/2006; 103(48):18220-5. DOI: 10.1073/pnas.0605131103
Source: PubMed


Tapasin (Tpn) has been implicated in multiple steps of the MHC class I assembly pathway, but the mechanisms of function remain incompletely understood. Using purified proteins, we could demonstrate direct binding of Tpn to peptide-deficient forms of MHC class I molecules at physiological temperatures. Tpn also bound to M10.5, a pheromone receptor-associated MHC molecule that has an open and empty groove and that shares significant sequence identity with class I sequences. Two types of MHC class I-Tpn complexes were detectable in vitro depending on the input proteins; those depleted in beta(2)m, and those containing beta(2)m. Both were competent for subsequent assembly with peptides, but the latter complexes assembled more rapidly. Thus, the assembly rate of Tpn-associated class I was determined by the conditions under which Tpn-MHC class I complexes were induced. Peptide loading of class I inhibited Tpn-class I-binding interactions, and peptide-depletion enhanced binding. In combination with beta(2)m, certain peptides induced efficient dissociation of preformed Tpn-class I complexes. Together, these studies demonstrate direct Tpn-MHC class I interactions and preferential binding of empty MHC class I by Tpn, and that the Tpn-class I interaction is regulated by both beta(2)m and peptide. In cells, Tpn is likely to be a direct mediator of peptide-regulated binding and release of MHC class I from the TAP complex.

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    • "Other researchers have suggested that the increase of the affinity of the pool of peptides bound to class I in the presence of tapasin is due to an exchange of low-affinity peptides against high-affinity peptides over time, which tapasin promotes. Such 'peptide editing' is supported by experiments both in vitro and in vivo (Chen and Bouvier, 2007; Howarth et al., 2004; Rizvi and Raghavan, 2006; Thirdborough et al., 2008; Wearsch and Cresswell, 2007; Williams et al., 2002). One hypothesis suggests that peptide editing occurs because tapasin binds preferentially to the peptide-free form of class I, generally decreasing peptide affinity (Wright et al., 2004). "
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    ABSTRACT: To understand the mechanism of action of the chaperone protein tapasin, which mediates loading of high-affinity peptides onto major histocompatibility complex (MHC) class I molecules in the antiviral immune response, we have performed numerical simulations of the class I-peptide binding process with four different mechanistic hypotheses from the literature, and tested our predictions by laboratory experiments. We find - in agreement of experimental and theoretical studies - that class I-peptide binding in cells is generally under kinetic control, and that tapasin introduces partial thermodynamic control to the process by competing with peptide for binding to class I. Based on our results, we suggest further experimental directions.
    Full-text · Article · May 2009 · Molecular Immunology
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    • "Although both molecules operate in separate pathways, it appears that they employ similar strategies to catalyze and edit antigenic peptide loading onto MHC class I and class II molecules respectively. Both can act as chaperones to empty MHC molecules and help maintain them in a state that peptides can readily bind (Zarutskie et al., 2001; Vogt et al., 1997b; Kropshofer et al., 1997a; Chou et al., 2008; Cabrera, 2007; Rizvi and Raghavan, 2006; Schoenhals et al., 1999). Both tapasin and DM affect a series of hydrogen bonds between the antigenic peptide and the MHC molecule. "
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    ABSTRACT: The adaptive immune response relies on the ability of T-lymphocytes to recognize small antigenic peptides presented on the cell surface by specialized receptors of the Major Histocompatibility Complex (MHC). These peptides are either generated by the degradation of intracellular proteins (MHC class I pathway) or by the degradation of internalized extracellular proteins (MHC class II pathway and cross-presentation pathway). The number of proteins that can be degraded by these pathways runs to the thousands leading to a staggering number of possible peptide fragments. A small subset of these peptides is selected by the cell's processing and presentation mechanisms to be presented on the cell surface by MHC molecules and has been defined as the immunopeptidome. The peptide sequences that comprise the immunopeptidome control the immune response and variations of this peptide repertoire are key to understanding the host's ability to fight pathogens, immune response to cancer as well as predisposition to autoimmunity and allergies. In the last few years it has been established that the composition of the immunopeptidome is regulated by specific cellular mechanisms that influence qualitative and quantitative aspects of the cellular immune response in a process that has been described as antigenic peptide editing. This review explores the current knowledge on these cellular mechanisms and discusses the parallels between editing the MHC class I and class II immunopeptidomes.
    Full-text · Article · Mar 2009 · Current Proteomics
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    • "MHC class I molecules assemble with peptide in the endoplasmic reticulum by the assistance of a multi-component protein complex (Ortmann et al. 1997; Pamer and Cresswell 1998; Dick et al. 2002; Paquet et al. 2004; Park et al. 2006). Within this complex, tapasin binds directly to the MHC class I heavy chain (Farmery et al. 2000; Rizvi and Raghavan 2006). Experiments with an insect cell model have shown that tapasin stabilizes the peptide-binding site of the transporter associated with antigen processing (TAP) and increases the thermostability of both of the TAP subunits (Raghuraman et al. 2002). "
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    ABSTRACT: Several endoplasmic reticulum proteins, including tapasin, play an important role in major histocompatibility complex (MHC) class I assembly. In this study, we assessed the influence of the tapasin cytoplasmic tail on three mouse MHC class I allotypes (H2-K(b), -K(d), and -L(d)) and demonstrated that the expression of truncated mouse tapasin in mouse cells resulted in very low K(b), K(d), and L(d) surface expression. The surface expression of K(d) also could not be rescued by human soluble tapasin, suggesting that the surface expression phenotype of the mouse MHC class I molecules in the presence of soluble tapasin was not due to mouse/human differences in tapasin. Notably, soluble mouse tapasin was able to partially rescue HLA-B8 surface expression on human 721.220 cells. Thus, the cytoplasmic tail of tapasin (either mouse or human) has a stronger impact on the surface expression of murine MHC class I molecules on mouse cells than on the expression of HLA-B8 on human cells. A K408W mutation in the mouse tapasin transmembrane/cytoplasmic domain disrupted K(d) folding and release from tapasin, but not interaction with transporter associated with antigen processing (TAP), indicating that the mechanism whereby the tapasin transmembrane/cytoplasmic domain facilitates MHC class I assembly is not limited to TAP stabilization. Our findings indicate that the C terminus of mouse tapasin plays a vital role in enabling murine MHC class I molecules to be expressed at the surface of mouse cells.
    Full-text · Article · Nov 2008 · Immunogenetics
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