https://pmc.ncbi.nlm.nih.gov/articles/PMC18459/
The ubiquitin-like proteins SMT3 and SUMO-1 are conjugated by the UBC9 E2 enzyme - PMC
The ubiquitin-like protein SMT3 from Saccharomyces cerevisiae and SUMO-1, its mammalian homolog, can be covalently attached to other proteins...