The Cytoplasmic Domain of Rhesus Cytomegalovirus Rh178 Interrupts Translation of Major Histocompatibility Class I Leader Peptide-Containing Proteins prior to Translocation

Abstract

Cytomegalovirus (CMV) efficiently evades many host immune defenses and encodes a number of proteins that prevent antigen presentation by major histocompatibility complex class I (MHC-I) molecules in order to evade recognition and killing of infected cells by cytotoxic CD8؉ T cells. We recently showed that rhesus CMV-specific Rh178 intercepts MHC-I protein translation before interference of MHC-I maturation by homologues of the human CMV US6 family. Here, we demonstrate that Rh178 localizes to the membrane of the endoplasmic reticulum, displaying a short luminal and large cytosolic domain, and that the membraneproximal cytosolic portion is essential for inhibition of MHC-I expression. We further observed that Rh178 does not require synthesis of full-length MHC-I heavy chains but is capable of inhibiting the translation of short, unstable amino-terminal fragments of MHC-I. Moreover, the transfer of amino-terminal fragments containing the MHC-I signal peptide renders recipient proteins susceptible to targeting by Rh178. The cytosolic orientation of Rh178 and its ability to target protein fragments carrying the MHC-I signal peptide are consistent with Rh178 intercepting partially translated MHC-I heavy chains after signal recognition particledependent transfer to the endoplasmic reticulum membrane. However, interference with MHC-I translation by Rh178 seems to occur prior to SEC61-dependent protein translocation, since inhibition of MHC-I translocation by eeyarestatin 1 resulted in a full-length degradation intermediate that can be stabilized by proteasome inhibitors. These data are consistent with Rh178 blocking protein translation of MHC-I heavy chains at a step prior to the start of translocation, thereby downregulating MHC-I at a very early stage of translation.Cytomegaloviruses (CMV), members of the betaherpesviridae, are masters at evading the host immune system. All CMV genomes dedicate many of their open reading frames (ORFs) to escaping various mechanisms of immune defense. CMVencoded immunomodulators function to circumvent cellautonomous defenses such as apoptosis and the interferon (IFN) response, as well as to prevent innate and adaptive immune responses by natural killer (NK) cells and T cells (5,13,34,61). These proteins allow the virus to establish primary infection, maintain persistent infection, and support repeated superinfection of chronically infected hosts. The study of cytomegaloviral immunomodulatory proteins not only has underscored the important and delicate relationship between virus and host but also has revealed novel proteins of the host immune system like the UL16-protein binding family of NKG2D ligands and the UL18-binding NK cell inhibitory receptor LIR-1 (7, 8).In addition, CMV immunomodulators have been employed to decipher basic cell biological principles such as protein quality control. Glycoproteins within the US6 family of human CMV (HCMV) (25) block endoplasmic reticulum (ER)-associated degradation of major histocompatibility complex class I (MHC-I) proteins and thereby prevent...

A Novel Bat Herpesvirus Encodes Homologues of Major Histocompatibility Complex Classes I and II, C-Type Lectin, and a Unique Family of Immune-Related Genes

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Genomic Sequencing and Characterization of Cynomolgus Macaque Cytomegalovirus

“…CyCMV encodes HCMV homologues for major histocompatibility complex class I (MHC-I) downregulation genes (US2, US3, and US11), viral interleukin-10 (UL111.5A), a tumor necrosis factor receptor homologue (UL144), and antiapoptotic genes (UL36, UL37 ex1, and UL38). In addition to the HCMV genes, cy203 carries a homologue of the RhCMV-specific gene (rh178) involved in MHC-I downregulation by interference with the translation of the heavy chain portion of the MHC-I molecule (53,56). Although it was originally thought to be unique to RhCMV (53), it appears that this immunomodulatory gene may in fact be an NHP-specific immunevasin.…”

Reevaluation of the Coding Potential and Proteomic Analysis of the BAC-Derived Rhesus Cytomegalovirus Strain 68-1

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