Inhibition of proteasomal degradation by the Gly-Ala repeat of Epstein–Barr virus is influenced by the length of the repeat and the strength of the degradation signal

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Abstract
The Gly-Ala repeat (GAr) of the Epstein-Barr virus nuclear antigen-1 is a transferable element that inhibits in cis ubiquitin/proteasome-dependent proteolysis. We have investigated this inhibitory activity by using green fluorescent protein-based reporters that have been targeted for proteolysis by N end rule or ubiquitin-fusion degradation signals, resulting in various degrees of destabilization. Degradation of the green fluorescent protein substrates was inhibited on insertion of a 25-aa GAr, but strongly destabilized reporters were protected only partially. Protection could be enhanced by increasing the length of the repeat. However, reporters containing the Ub-R and ubiquitin-fusion degradation signals were degraded even in the presence of a 239-aa GAr. In accordance, insertion of a powerful degradation signal relieved the blockade of proteasomal degradation in Epstein-Barr virus nuclear antigen-1. Our findings suggest that the turnover of natural substrates may be finely tuned by GAr-like sequences that counteract targeting signals for proteasomal destruction.