Ubiquitin and ubiquitin-like proteins (Ubls), including small ubiquitin-like modifier (SUMO), are covalently attached to target proteins through a coordinated series of reactions. These posttranslational modifications are important in regulating many signaling pathways and cellular processes, including DNA repair and maintenance of chromosome structure. Whereas ubiquitination is often associated with targeting proteins for degradation, SUMOylation enables new protein-protein interactions to occur and was thought to perhaps increase protein stability by blocking ubiquitination. To the contrary, however, two papers now identify a family of eukaryotic ubiquitin ligases that target SUMOylated proteins and proteins containing SUMO-like domains (SLDs). Prudden et al. used yeast two-hybrid studies to identify Rfp1 [really interesting new gene (RING) finger protein 1] and Rfp2 and found that both proteins formed heterodimers with another RING finger protein, the ubiquitin E3 ligase, Slx8. Sequence analysis showed that Rfp1 and Rfp2 also contained N-terminal SUMO-interacting motifs (SIMs). Yeast deficient in both Rfp1 and Rfp2 exhibited poor cell growth and had fragmented chromosomes, a phenotype similar to that seen in Slx8-deficient yeast. Cells deficient in Slx8-Rfp showed genomic instability, accumulated SUMOylated proteins, and had defects in DNA repair. Expression of the mouse Rfp ortholog, RNF4, rescued this phenotype. Further, the yeast DNA repair factor Rad60, which contains an SLD, was a target for Slx8-Rfp. Sun et al. showed that whereas the Rfps must recruit Slx8 in order to have ubiquitin ligase activity, RNF4 is itself a ubiquitin ligase that targets SUMOylated proteins through its SIM, consistent with the previous study. In vitro ubiquitin ligase assays showed that target proteins were ubiquitinated in a SUMO binding-dependent fashion. The discovery of ubiquitin ligases that target SUMOylated proteins provides a link between these two important pathways of posttranslational modification.
J. Prudden, S. Pebernard, G. Raffa, D. A. Slavin, J. J. P. Perry, J. A. Tainer, C. H. McGowan, M. N. Boddy, SUMO-targeted ubiquitin ligases in genome stability. EMBO J. 26, 4089-4101 (2007). [PubMed]
H. Sun, J. D. Leverson, T. Hunter, Conserved function of RNF4 family proteins in eukaryotes: Targeting a ubiquitin ligase to SUMOylated proteins. EMBO J. 26, 4102-4112 (2007). [PubMed]