Repeat sequence of Epstein-Barr virus-encoded nuclear antigen 1 protein interrupts proteasome substrate processing

Area of Science:

• Molecular Biology
• Immunology
• Virology

Background:

• Epstein-Barr virus (EBV) evades immune detection by inhibiting antigen presentation.
• The viral protein EBV Nuclear Antigen 1 (EBNA1) contains a Gly-Ala repeat (GAr) that inhibits proteasome processing.
• The precise mechanism of GAr-mediated proteasome inhibition remains elusive.

Purpose of the Study:

• To elucidate the mechanism by which the EBV GAr inhibits proteasome function.
• To determine if GAr inhibition of the proteasome involves ubiquitin conjugation or other cellular factors.
• To investigate the impact of the GAr on substrate processing and degradation.

Main Methods:

• Engineered a fusion protein by embedding the GAr into ornithine decarboxylase, a natural proteasome substrate.
• Utilized a purified proteasome system to assess GAr-mediated inhibition.
• Introduced the GAr into a destabilized green fluorescent protein construct to study in vivo proteolysis.

Main Results:

• Demonstrated GAr-mediated proteasome inhibition in a purified system, independent of ubiquitin conjugation.
• Showed that the GAr functions as a stop-transfer signal during proteasome substrate processing.
• Observed accumulation of partially degraded protein products in vivo when the GAr was present.

Conclusions:

• The EBV GAr directly inhibits proteasome degradation by acting as a stop-transfer signal.
• Proteasome ATP motor stalling at the GAr is the proposed mechanism for halted degradation.
• This mechanism contributes to EBV's immune evasion strategy by preventing antigen presentation.