Rad6-mediated ubiquitination regulates the formation of Sam1-containing stress granules during nutrient stress in...

Stress granules (SGs) are dynamic cytoplasmic structures that form in response to environmental stress and contribute to cellular adaptation. Recent studies have shown that metabolic enzymes localize to SGs under nutrient stress; however, the molecular mechanisms regulating the assembly and organization of metabolic enzyme-containing SGs remain poorly understood. In this study, we examined the role of ubiquitination in the regulation of SGs containing the S-adenosylmethionine synthase Sam1 in budding yeast. We demonstrate that Sam1 is stable but undergoes non-proteolytic ubiquitination in vivo. We identified Rad6 as one of the major ubiquitin-conjugating enzymes that catalyze Sam1 ubiquitination. Deletion of RAD6 caused pronounced alterations in Sam1-containing SGs during nutrient stress. Specifically, rad6Δ cells exhibited increases in both the proportion of cells forming Sam1-positive SGs and the number of SGs per cell. These results demonstrate that Rad6-mediated ubiquitination restricts the number and organization of Sam1-containing stress granules under nutrient stress. Our findings reveal a non-proteolytic role for ubiquitination in regulating stress granule architecture and provide new insight into the regulation of metabolic enzyme assemblies during nutrient stress.