Zeman et al. describe how deubiquitination of the ubiquitin ligase Rad18 fine-tunes the cell’s response to specific types of DNA damage.
Rad18 helps the cell’s DNA replication machinery “tolerate” DNA damage and duplicate the genome with a minimal number of mutations. The ubiquitin ligase induces slightly different tolerance pathways in response to different types of DNA lesions, and it may even promote mutagenesis if activated in the absence of any damage at all. Zeman et al. therefore investigated how Rad18 is regulated.
Around a quarter of a cell’s Rad18 molecules are usually ubiquitinated, probably by the enzyme itself. Rad18 was deubiquitinated, however, in response to the DNA alkylating agent MMS or H2O2 but not in response to other insults such as UV irradiation. MMS and H2O2 promote Rad18’s interaction with another ubiquitin ligase, SHPRH, that is crucial for the cell’s ability to tolerate these sources of DNA damage. Accordingly, ubiquitinated Rad18 didn’t bind SHPRH and instead preferred to dimerize with other, nonubiquitinated Rad18 molecules, potentially sequestering them in an inactive state. Ubiquitinated Rad18 failed to localize to sites of DNA damage and was unable to ubiquitinate its downstream targets. Cells overexpressing a constitutively ubiquitinated form of Rad18 were less able to tolerate MMS and thus showed an increased rate of mutagenesis.
Ubiquitination therefore inhibits Rad18’s activity in undamaged cells, but this posttranslational modification is quickly removed in response to MMS or H2O2, allowing the ubiquitin ligase to bind SHPRH and promote tolerance of the ensuing DNA damage. The authors now want to investigate how Rad18 is deubiquitinated in response to these DNA damage agents.
Text by Ben Short