Overexpression of Rad23 confers resistance to methylmercury in Saccharomyces cerevisiae via inhibition of the degradation of ubiquitinated proteins

We report here that overexpression of Rad23, a protein related to the ubiquitin-proteasome system, renders yeast cells resistant to methylmercury. Rad23 has three domains: two ubiquitin-associated (UBA) domains that bind to the multiubiquitin chain of ubiquitinated proteins and a single ubiquitin-like (UbL) domain that binds to proteasomes. To examine the mechanism of acquisition of methylmercury resistance that is induced by overexpression of Rad23, we expressed variants of Rad23 in which one or the other of the two types of domain was defective in yeast cells. In cells that overexpressed full-length intact Rad23, we detected elevated levels of intracellular ubiquitinated proteins, and the cells were resistant to methylmercury. In contrast, cells that overexpressed Rad23 with a defective UBA domain were not resistant to methylmercury and contained control levels of ubiquitinated proteins. Yeast cells that overexpressed Rad23 with a defective UbL domain exhibited enhanced resistance to methylmercury and contained even higher levels of ubiquitinated proteins than cells that overexpressed intact full-length Rad23. Rad23 is known to have two mutually contradictory functions. It suppresses the degradation of ubiquitinated proteins by proteasomes via a mechanism mediated by the UBA domains, and it enhances the degradation of ubiquitinated proteins via a mechanism that is mediated by the UbL domain. Therefore, our findings suggest that Rad23 might induce resistance to methylmercury in yeast cells by suppressing the degradation of proteins that reduce the toxicity of methylmercury via a UBA domain-mediated mechanism.