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 (By similarity). Interacts with SAE2, SCE1, SIZ1 and MMS21 Covalently attached to a number of proteins. 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.