The TRC8 E3 ligase ubiquitinates MHC class I molecules before dislocation from the ER

Area of Science:

• Virology
• Immunology
• Cell Biology

Background:

• Human cytomegalovirus (HCMV) utilizes US2 and US11 proteins to disrupt immune surveillance.
• These viral proteins target Major Histocompatibility Complex class I (MHC I) for degradation through the endoplasmic reticulum (ER)-associated degradation (ERAD) pathway.
• The specific E3 ligase responsible for MHC I ubiquitination in this process remained unidentified.

Purpose of the Study:

• To identify the E3 ligase involved in human cytomegalovirus (HCMV)-mediated degradation of Major Histocompatibility Complex class I (MHC I).
• To elucidate the mechanism by which HCMV evades the host immune response by manipulating the ERAD pathway.

Main Methods:

• A functional small interfering RNA (siRNA) library screen was employed to identify key proteins in the ERAD pathway.
• Ubiquitination assays and protein complex analysis were performed to investigate the role of candidate E3 ligases.

Main Results:

• TRC8 (translocation in renal carcinoma, chromosome 8 gene), an ER-resident E3 ligase, was identified as essential for US2-mediated MHC I ubiquitination.
• Depletion of TRC8 inhibited MHC I ubiquitination and dislocation, restoring cell surface MHC I levels.
• TRC8 was found to be part of a novel multiprotein ER complex including MHC I, US2, and signal peptide peptidase.

Conclusions:

• TRC8 is a critical E3 ligase required for the dislocation of MHC I from the ER during HCMV infection.
• This study reveals a new multiprotein complex involved in mammalian ERAD and viral immune evasion.