RAD6 encodes an E2 ubiquitin-conjugating enzyme required for postreplicational DNA repair, transcriptional activation and repression, protein degradation, and sporulation (1 and reviewed in 6). Rad6p interacts with various E3 ubiquitin ligases, such as Rad18p, Bre1p, and Ubr1p, that recruit Rad6p to the appropriate target protein during repair, transcription, and degradation, respectively (7, 8, 9). In its role in post-replication repair, a process often referred to as the RAD6 pathway, Rad6p promotes replication through DNA lesions located at stalled replication forks. Among the other proteins in the RAD6 epistasis group are RAD18, RAD5, MMS2, UBC13, POL30, REV1, REV3, REV7, and SRS2 (reviewed in 6 and 10).

Rad6p and Rad18p form a stable heterodimer that binds single-stranded DNA and possesses single-stranded DNA-dependent ATPase activity. (7). Upon DNA damage, Rad6p-Rad18p monoubiquitinates Pol30p (PCNA) at lysine residue 164 to activate error-prone and error-free translesion repair via polymerases eta and zeta. Monoubiquitinated PCNA can be subsequently polyubiquitinated in a Rad5p-Mms2p-Ubc13p dependent manner to promote error-free postreplicational repair (11, reviewed in 6).

The E3 ubiquitin ligase Bre1p targets Rad6p to promoter regions and enables the enzyme to attach ubiquitin to lysine 123 in histone H2B (8, 12, 13). This chromatin modification is required for histone H3 methylation and downstream transcriptional silencing (14). In vitro, Rad6p is able to efficiently multiubiquitinate histones H2A, H2B, and H3 in the absence of an E3 enzyme, with the C-terminus of the protein facilitating Rad6p targeting to histones (15).

During amino-end rule degradation, Ubr1p and Rad6p are able to multiubiquinate various protein substrates, thus targeting them for degradation by ubiquitin-dependent protease (9, 16). The amino terminal domain of Rad6p has been shown to be required for Ubr1p interaction as well as E3-dependent protein degradation (17).

rad6 mutants display sensitivity to UV light, X rays, and many chemical mutagens; they have an elevated spontaneous-mutation rate but are incapable of DNA damage-induced mutagenesis. Cells lacking Rad6p also have silencing and sporulation defects but are hyperactive in mitotic recombination (reviewed in 18). Mutations in rhp6, the S. pombe ortholog of RAD6, also are defective in DNA repair, UV mutagenesis and sporulation (19). RAD6 homologs have also been identified in Drosophila and in humans; humans have two RAD6 homologs, the genes HHR6A (OMIM) and HHR6B (OMIM) (20, 21).

Last updated: 2006-04-12