Abstract: Posttranslational protein modification by the ubiquitin-like SUMO protein is critical to eukaryotic cell regulation, but much remains unknown regarding its operation and substrates. Here we report that specific mutations in the Saccharomyces cerevisiae Ulp1 SUMO protease, including its coiled-coil (CC) domain, lead to the accumulation of distinct sumoylated proteins in vivo. A prominent ~50-kDa sumoylated protein accumulates in a Ulp1 CC mutant. The protein was identified as Scs2, an endoplasmic reticulum (ER) membrane protein that regulates phosphatidylinositol synthesis and lipid trafficking. Mutation of lysine 180 of Scs2 abolishes its sumoylation. Notably, impairment of either cellular sumoylation or cellular desumoylation mechanisms inhibits cell growth in the absence of inositol and exacerbates the inositol auxotrophy caused by deletion of SCS2. Mutants lacking the Ulp2 SUMO protease are the most severely affected, and this defect was traced to the mutants' impaired ability to induce transcription of INO1, which encodes the rate-limiting enzyme of inositol biosynthesis. Conversely, inositol starvation induces a striking change in the profiles of total cellular SUMO conjugates. These results provide the first evidence of cross-regulation between the SUMO and inositol pathways, including the sumoylation of an ER membrane protein central to phospholipid synthesis and phosphoinositide signaling.