Sumoylation as a signal for polyubiquitylation and proteasomal degradation

The small ubiquitin-related modifier (SUMO) is a versatile cellular tool to modulate a protein's function. SUMO modification is a reversible process analogous to ubiquitylation. The consecutive actions of E1, E2 and E3 enzymes catalyze the attachment of SUMO to target proteins, while deconjugation is promoted by SUMO specific proteases. Contrary to the long-standing assumption that SUMO has no role in proteolytic targeting and rather acts as an antagonist of ubiquitin in some cases, it has recently been discovered that sumoylation itself can function as a secondary signal mediating ubiquitin-dependent degradation by the proteasome. The discovery of a novel family of RING finger ubiquitin ligases bearing SUMO interaction motifs implicated the ubiquitin system in the control of SUMO modified proteins. SUMO modification as a signal for degradation is conserved in eukaryotes and ubiquitin ligases that specifically recognize SUMO-modified proteins have been discovered in species ranging from yeasts to humans. This chapter summarizes what is known about these ligases and their role in controlling sumoylated proteins.