Abstract
In eukaryotes many players in the DNA-damage response (DDR) catalyze protein sumoylation or ubiquitylation. Emphasis has been placed on how these modifications orchestrate the sequential recruitment of repair factors to sites of DNA damage or stalled replication forks. Here, we shed light on a pathway in which sumoylated factors are eliminated through the coupled action of Sumo-targeted ubiquitin ligases (STUbLs) and the ubiquitin-fusion degradation protein 1 (Ufd1). Ufd1 is a subunit of the Cdc48-Ufd1-Npl4 complex implicated in the sorting of ubiquitylated substrates for degradation by the proteasome. We find that in fission yeast, Ufd1 interacts physically and functionally with the Sumo-targeted ubiquitin ligase (STUbL) Rfp1, homologous to human RNF4, and with the Sumo E3 ligase Pli1, homologous to human PIAS1. Deleting a C-terminal domain of Ufd1 that mediates the interaction of Ufd1 with Rfp1, Pli1, and Sumo (ufd1ΔCt 213-342) lead to an accumulation of high-molecular-weight Sumo conjugates and caused severe genomic instabilities. The spectrum of sensitivity of ufd1ΔCt 213-342 cells to genotoxins, the epistatic relationships of ufd1ΔCt 213-342 with mutations in DNA repair factors, and the localization of the repair factor Rad22 in ufd1ΔCt 213-342 cells point to ufd1ΔCt 213-342 cells accumulating aberrant structures during replication that require homologous recombination (HR) for their repair. We present evidence that HR is however often not successful in ufd1ΔCt 213-342 cells and we identify Rad22 as one of the high-molecular-weight conjugates accumulating in the ufd1ΔCt 213-342 mutant consistent with Rad22 being a STUbL/Ufd1 substrate. Suggesting a direct role of Ufd1 in the processing of Sumo-conjugates, Ufd1 formed nuclear foci colocalizing with Sumo during the DDR, and Sumo-conjugates accumulated in foci in the ufd1ΔCt 213-342 mutant. Broader functional relationships between Ufd1 and STUbLs conceivably affect numerous cellular processes beyond the DDR.
Highlights
The small modifiers SUMO and ubiquitin are effectors of many regulatory changes occurring in eukaryotic cells
We conducted large-scale two-hybrid screens using respectively the fission yeast SUMO E3 ligase Pli1 and the Sumo-targeted ubiquitin ligases (STUbLs) protein Rfp1 as baits to identify factors acting in concert with these proteins (Figure 1)
ubiquitin-fusion degradation protein 1 (Ufd1) mediates the interactions of the Cdc48/Ufd1/Npl4 complex with ubiquitylated proteins, permitting the extraction of these proteins from higher-order complexes in an energydriven process catalyzed by the Cdc48 ATPase [38,39]
Summary
The small modifiers SUMO and ubiquitin are effectors of many regulatory changes occurring in eukaryotic cells. In reactions catalyzed by E1, E2 and E3 enzymes that act in a cascade, SUMO or ubiquitin can each be conjugated to lysine residues of target proteins. Conjugation often results in a change in the interaction properties of the target. In contrast to the ubiquitin pathway where substrate selection is mediated by large sets of E2 and E3 enzymes, sumoylation appears restricted to the use of very few E2 and E3 enzymes in all organisms examined to date [1,2,3]. In fission yeast the only known SUMO E3 ligases are Nse and the PIAS family member Pli. Nse and Pli are both of the SP-RING type, functioning together with a single E2 enzyme, Ubc ( called Hus5; [4,5,6])
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