Stuck between a ROS and a hard place: Analysis of the ubiquitin proteasome pathway in selenocysteine treated Brassica napus reveals different toxicities during selenium assimilation.

During the selenium assimilation pathway, inorganic selenate and selenite are reduced to form selenocysteine (Sec). Tolerance to selenium in plants has long been attributable to minimizing the replacement of cysteine with selenocysteine, which can result in nonspecific selenoproteins that are potentially misfolded. Despite this widely accepted assumption, there is no evidence in higher plants demonstrating that selenocysteine induces toxicity by resulting in malformed proteins. In this study, we use Brassica napus to analyze the ubiquitin-proteasome pathway, which is capable of removing misfolded proteins. Sec rapidly increased proteasome activity and levels of ubiquitinated proteins, strongly indicating that selenocysteine induces protein misfolding. Proteasome inhibition increased the amount of selenium in protein in Sec-treated plants. Collectively, these data provide a mechanism that accounts for Sec toxicity. Additionally, Sec did not cause oxidative stress as judged by examining levels of superoxide using fluorescent microscopy. Therefore, the cellular response to Sec is different compared to selenite, which was recently shown to increase antioxidant metabolism in response to elevated mitochondrial superoxide that ultimately impaired proteasome activity. Therefore, plants must contend with two divergent modes of cytotoxicity during selenium assimilation. Selenite can result in oxidative stress, but increased flux of selenite reduction can yield Sec that in turn can cause protein misfolding.