Elucidation of ubiquitin-related functions by ubiquitin overexpression approach

To identify new ubiquitin-related functions using yeast, we searched for mutants in which ubiquitin overexpression rescued their sensitivity to amino acid analogs and temperature-sensitivities, and identified the mutation of NFI1/SIZ1 encoding a SUMO E3 ligase and SPC2 encoding a subunit of signal peptidase complex (SPC), respectively. Ubiquitin overexpression suppressed a degradation defect of one of the Nfi1 substrates in the nfi1 mutant, providing new insight into the relationship between ubiquitination and sumoylation. The double mutants harboring spc2 and hrd1 showed a synergistic growth defect showing a close functional relationship between ER-associated protein degradation (ERAD) and SPC. Our results show that ubiquitin overexpression approach provides a useful approach to elucidate ubiquitin-related functions.