Antigen presentation is a cellular process that involves a number of steps, beginning with the production of peptides by proteolysis or aberrant synthesis and the delivery of peptides to cellular compartments where they are loaded on MHC class I (MHC-I) or MHC class II (MHC-II) molecules. The selective loading and editing of high-affinity immunodominant antigens is orchestrated by molecular chaperones: tapasin/TAP-binding protein, related for MHC-I and HLA-DM for MHC-II. Once peptide/MHC (pMHC) complexes are assembled, following various steps of quality control, they are delivered to the cell surface, where they are available for identification by αβ receptors on CD8+ or CD4+ T lymphocytes. In addition, recognition of cell surface peptide/MHC-I complexes by natural killer cell receptors plays a regulatory role in some aspects of the innate immune response. Many of the components of the pathways of antigen processing and presentation and of T cell receptor (TCR)-mediated signaling have been studied extensively by biochemical, genetic, immunological, and structural approaches over the past several decades. Until recently, however, dynamic aspects of the interactions of peptide with MHC, MHC with molecular chaperones, or of pMHC with TCR have been difficult to address experimentally, although computational approaches such as molecular dynamics (MD) simulations have been illuminating. Studies exploiting X-ray crystallography, cryo-electron microscopy, and multidimensional nuclear magnetic resonance (NMR) spectroscopy are beginning to reveal the importance of molecular flexibility as it pertains to peptide loading onto MHC molecules, the interactions between pMHC and TCR, and subsequent TCR-mediated signals. In addition, recent structural and dynamic insights into how molecular chaperones define peptide selection and fine-tune the MHC displayed antigen repertoire are discussed. Here, we offer a review of current knowledge that highlights experimental data obtained by X-ray crystallography and multidimensional NMR methodologies. Collectively, these findings strongly support a multifaceted role for protein plasticity and conformational dynamics throughout the antigen processing and presentation pathway in dictating antigen selection and recognition.

The U21 open reading frame from human herpesvirus-7 encodes a membrane protein that associates with and redirects class I MHC molecules to the lysosomal compartment. The mechanism by which U21 accomplishes this trafficking excursion is unknown. Here we have examined the contribution of localization, glycosylation, domain structure, and the absence of substrate class I MHC molecules on the ability of U21 to traffic to lysosomes. Our results suggest the existence of a cellular protein necessary for U21-mediated rerouting of class I MHC molecules.Human herpesvirus-6 and -7 (HHV-6 and -7)4 are ␤-herpesviruses most closely related to human cytomegalovirus. HHV-6 and -7 possess almost entirely collinear genomes and share many biological properties; both viruses infect T-lymphocytes, although they can infect other cell types as well, and both can cause high fever and exanthem subitum (roseola), although HHV-6 is the most common cause of this childhood disease (1, 2). Almost all of the population is seropositive for both viruses. Infection with HHV-6 occurs by the age of 2, whereas HHV-7 infection usually occurs slightly later (3). The cells infected with HHV-6 and -7 exhibit cytomegaly and are prone to syncytium formation, features reminiscent of those seen in human cytomegalovirus infection.Like all other herpesviruses, HHV-6 and -7 remain latent or establish persistent infections. To do so, they must avoid detection and elimination by the immune system. Viral immune evasion strategies include restriction of viral gene expression, infection at immunoprivileged sites, obstruction of antiviral cytokine function, and interference with antigen presentation (for review see Ref. 4). Notably, all of the herpesviruses thus far examined employ the latter strategy of interfering with viral antigen presentation to cytotoxic T lymphocytes. Some herpesviral proteins interfere with proteolysis of antigens or peptide transport into the ER (5-7). Others retain class I molecules in the ER, mediate their destruction through ER-associated degradation, enhance the internalization of class I molecules, or divert class I molecules to lysosomes for degradation (8 -17). Judging from the number and molecular diversity of these strategies, the removal of class I MHC-peptide complexes from the cell surface must be evolutionarily advantageous to these viruses as a means of escaping immune detection and thriving in a host.Because so many of the viral immunoevasins affect trafficking or stability of class I MHC molecules, in previous work, we took a biochemical approach to examine the maturation and stability of class I molecules in HHV-7-infected T cells (11). We found that class I MHC stability was indeed decreased in HHV-7-infected T cells and that a 55-kDa viral glycoprotein coimmunoprecipitated with class I MHC molecules. We identified this associated protein as the product of the HHV-7 U21 open reading frame. HHV-7 U21 lacks homology to any other gene product or domain, with the exception of the U21 encoded by HHV-6. U21 is a type I membrane...

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The U21 gene product from human herpesvirus 7 binds to and redirects class I major histocompatibility complex (MHC) molecules to a lysosomal compartment. The molecular mechanism by which U21 reroutes class I MHC molecules to lysosomes is not known. Here, we have reconstituted the interaction between purified soluble U21 and class I MHC molecules, suggesting that U21 does not require additional cellular proteins to interact with class I MHC molecules. Our results demonstrate that U21, itself predicted to contain an MHC class I-like protein fold, interacts tightly with class I MHC molecules as a tetramer, in a 4:2 stoichiometry. These observations have helped to elucidate a refined model describing the mechanism by which U21 escorts class I MHC molecules to the lysosomal compartment. IMPORTANCEIn this report, we show that the human herpesvirus 7 (HHV-7) immunoevasin U21, itself a class I MHC-like protein, binds with high affinity to class I MHC molecules as a tetramer and escorts them to lysosomes, where they are degraded. While many class I MHC-like molecules have been described in detail, this unusual viral class I-like protein functions as a tetramer, associating with class I MHC molecules in a 4:2 ratio, illuminating a functional significance of homooligomerization of a class I MHC-like protein.