The macro domain protein family: structure, functions, and their potential therapeutic implications

Macro domains are ancient, highly evolutionarily conserved domains that are widely distributed throughout all kingdoms of life. The 'macro fold' is roughly 25kDa in size and is composed of a mixed α-β fold with similarity to the P loop-containing nucleotide triphosphate hydrolases. They function as binding modules for metabolites of NAD(+), including poly(ADP-ribose) (PAR), which is synthesized by PAR polymerases (PARPs). Although there is a high degree of sequence similarity within this family, particularly for residues that might be involved in catalysis or substrates binding, it is likely that the sequence variation that does exist among macro domains is responsible for the specificity of function of individual proteins. Recent findings have indicated that macro domain proteins are functionally promiscuous and are implicated in the regulation of diverse biological functions, such as DNA repair, chromatin remodeling and transcriptional regulation. Significant advances in the field of macro domain have occurred in the past few years, including biological insights and the discovery of novel signaling pathways. To provide a framework for understanding these recent findings, this review will provide a comprehensive overview of the known and proposed biochemical, cellular and physiological roles of the macro domain family. Recent data that indicate a critical role of macro domain regulation for the proper progression of cellular differentiation programs will be discussed. In addition, the effect of dysregulated expression of macro domain proteins will be considered in the processes of tumorigenesis and bacterial pathogenesis. Finally, a series of observations will be highlighted that should be addressed in future efforts to develop macro domains as effective therapeutic targets.

Poly(ADP-ribose) glycohydrolase silencing protects against H2O2-induced cell death

PAR [poly(ADP-ribose)] is a structural and regulatory component of multiprotein complexes in eukaryotic cells. PAR catabolism is accelerated under genotoxic stress conditions and this is largely attributable to the activity of a PARG (PAR glycohydrolase). To overcome the early embryonic lethality of parg-knockout mice and gain more insights into the biological functions of PARG, we used an RNA interference approach. We found that as little as 10% of PARG protein is sufficient to ensure basic cellular functions: PARG-silenced murine and human cells proliferated normally through several subculturing rounds and they were able to repair DNA damage induced by sublethal doses of H2O2. However, cell survival following treatment with higher concentrations of H2O2 (0.05-1 mM) was increased. In fact, PARG-silenced cells were more resistant than their wild-type counterparts to oxidant-induced apoptosis while exhibiting delayed PAR degradation and transient accumulation of ADP-ribose polymers longer than 15-mers at early stages of drug treatment. No difference was observed in response to the DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine, suggesting a specific involvement of PARG in the cellular response to oxidative DNA damage.