Ubiquitin-like protein which can be covalently attached to target lysines as a monomer. Does not seem to be involved in protein degradation and may function as an antagonist of ubiquitin in the degradation process. Required for the massive protein sumoylation in the nucleus induced by heat shock and controlled by SIZ1. Involved in the regulation of the heat stress transcription factor HSFA2 in acquired thermotolerance. Interacts with SAE2, SCE1, SIZ1 and MMS21 (By similarity). Interacts with HSFA2. Covalently attached to ABI5, FLD, GTE3, HSFA2 and ICE1. No visible phenotype. Stress conditions rapidly and substantially elevates the amount of SUMO1 and SUMO2 conjugates with a concomitant reduction in the amount of free SUMO proteins. The SUMO conjugation system plays an important function in stress protection and/or repair. Belongs to the ubiquitin family. SUMO subfamily. Sequencing errors.