Beginning at the end with SUMO.

The article focuses on the small ubiquitin-like modifier (SUMO) ligase. Ubiquitin-like protein modifiers (Ubls) regulate a host of processes in eukaryotic cells. Ubiquitin, the original Ubl, signals proteolysis and diverse non-proteolytic outcomes. Functional breadth is also characteristic of SUMO. SUMOylation was discovered because it is required to localize RanGAP1, a regulator of nucleocytoplasmic transport, to nuclear pore complexes. Studies of the SUMOylation of RanGAP1 and other substrates have revealed distinguishing features. SUMOylation, unlike ubiquitylation, is frequently site specific. SUMOylation of RanGAPl is frequently observable with just E1 and Ubc9 (the SUMO E2).