Identifying the ERAD ubiquitin E3 ligases for viral and cellular targeting of MHC class I.

D.J.H Van Den Boomen,P.J Lehner

doi:10.1016/j.molimm.2015.07.005

Abstract

The human cytomegalovirus (HCMV) US2 and US11 gene products hijack mammalian ER-associated degradation (ERAD) to induce rapid degradation of major histocompatibility class I (MHC-I) molecules. The rate-limiting step in this pathway is thought to be the polyubiquitination of MHC-I by distinct host ERAD E3 ubiquitin ligases. TRC8 was identified as the ligase responsible for US2-mediated MHC-I degradation and shown to be required for the cleavage-dependent degradation of some tail-anchored proteins. In addition to MHC-I, plasma membrane profiling identified further immune receptors, which are also substrates for the US2/TRC8 complex. These include at least six α integrins, the coagulation factor thrombomodulin and the NK cell ligand CD112. US2's use of specific HCMV-encoded adaptors makes it an adaptable viral degradation hub. US11-mediated degradation is MHC-I-specific and genetic screens have identified TMEM129, an uncharacterised RING-C2 E3 ligase, as responsible for US11-mediated degradation. In a unique auto-regulatory loop, US11 readily responds to changes in cellular expression of MHC-I. Free US11 either rebinds more MHC-I or is itself degraded by the HRD1/SEL1L E3 ligase complex. While virally encoded US2 and US11 appropriate mammalian ERAD, the MHC-I complex also undergoes stringent cellular quality control and misfolded MHC-I is degraded by the HRD1/SEL1L complex. We discuss the identification and central role of E3 ubiquitin ligases in ER quality control and viral degradation of the MHC-I chain.

Highlights

The essential role of the major histocompatibility class I (MHC-I) antigen presentation pathway in immune detection of intracellular pathogens is emphasised by the different strategies used by many viruses to interrupt this pathway (Hansen and Bouvier, 2009; Randow and Lehner, 2009)
In a subsequent proteomics study we found the tail anchored (TA) protein heme oxygenase-1 (HO-1) accumulates in the absence of signal peptide peptidase (SPP) (Boname et al, 2014)
In vitro we showed the TMEM129 RING has autoubiquitination activity, the hallmark of ubiquitin RINGs and TMEM129-deficient cells show a loss of US11-induced MHC-I ubiquitination as well as retrotranslocation and proteasomal degradation

Summary

Introduction

The essential role of the major histocompatibility class I (MHC-I) antigen presentation pathway in immune detection of intracellular pathogens is emphasised by the different strategies used by many viruses to interrupt this pathway (Hansen and Bouvier, 2009; Randow and Lehner, 2009). The ongoing battle between viruses and the immune system has resulted in many viruses co-evolving one or more gene products which inhibit MHC-I presentation, with the presumed aim of preventing viral peptide presentation by MHC-I to cytotoxic T-lymphocytes (CTL) This is exemplified in the herpesvirus family, with human cytomegalovirus (HCMV) providing one of the major paradigms for viral immune evasion. As the largest known human herpesvirus, HCMV encodes ∼170 canonical open reading frames of which only 45 are required for viral replication, with many of the remaining genes involved in immune evasion. The downside of this extensive range of viral gene products is that they provide a smorgasbord of potential viral epitopes for MHC-I presentation. ERAD is not just responsible for the degradation of misfolded proteins, and plays a critical role in the regulated turn-over of ER-resident proteins, such as the activated IP3 receptor and the HMG-CoA reductase, squalene monooxygenase and heme oxygenase-1 (HO-1) enzymes

A central role for E3 ubiquitin ligases in the ERAD pathway

The short tail of US11 allows its escape from TMEM129-mediated degradation

HRD1 mediates endogenous quality control of misassembled MHC-I heavy chains