Additional functions of selected proteins involved in DNA repair.

Protein moonlighting is a phenomenon in which a single polypeptide chain can perform a number of different unrelated functions. Here we present our analysis of moonlighting in the case of selected DNA repair proteins which include G:T mismatch-specific thymine DNA glycosylase (TDG), methyl-CpG-binding domain protein 4 (MBD4), apurinic/apyrimidinic endonuclease 1 (APE1), AlkB homologs, poly (ADP-ribose) polymerase 1 (PARP-1) and single-strand selective monofunctional uracil DNA glycosylase 1 (SMUG1). Most of their additional functions are not accidental and clear patterns are emerging. Participation in RNA metabolism is not surprising as bases occurring in RNA are the same or very similar to those in DNA. Other common additional function involves regulation of transcription. This is not unexpected as these proteins bind to specific DNA regions for DNA repair, hence they can also be recruited to regulate transcription. Participation in demethylation and replication of DNA appears logical as well. Some of the multifunctional DNA repair proteins play major roles in many diseases, including cancer. However, their moonlighting might prove a major difficulty in the development of new therapies because it will not be trivial to target a single protein function without affecting its other functions that are not related to the disease.