Albany 2015:Book of Abstracts
June 9-13 2015
©Adenine Press (2012)
Aprataxin and the threat of RNA contamination in DNA
The maintenance of eukaryotic genomes is ensured by three-step DNA ligation reactions employed by ATP-dependent DNA ligases. However, when DNA ligases engage DNA or RNA-/DNA structures with non-canonical 5' or 3' termini, the DNA ligase catalytic activity can paradoxically exacerbate DNA damage through a process termed "abortive ligation" which leads to toxic adducted 5'-adenylated (5' -AMP) lesions. The human Aprataxin protein (APTX, aka Hnt3 in budding yeast) is a DNA ligase "proofreader", and catalyzes the direct reversal of AMP-modified nucleic acid to generate 5'-phosphorylated ends. However, the molecular context for deadenylation repair has remained enigmatic. To better define APTX function, we examined the importance of APTX/Hnt3 to the RNaseH2-dependent excision repair (RER) pathway of removal of genome embedded ribonucleotides. We have demonstrated that DNA ligases undergo abortive ligation to generate adenylated 5'-ends containing a ribose characteristic of RNaseH2 incision at mono-ribonucleotides in DNA. APTX/Hnt3 efficiently repairs adenylated RNA-DNA to promote cellular survival and prevent S-phase checkpoint activation in budding yeast undergoing RER. A series of X-ray structures of human APTX/RNA-DNA/AMP/Zn complexes define a mechanism for detecting and reversing adenylation at RNA-DNA junctions. This involves A-form RNA-DNA binding, and an induced fit active site assembly controlled by APTX engagement of nicked RNA-DNA junctions. We discuss how heritable mutations in the APTX gene linked to the debilitating neurological disorder Ataxia with Oculomotor Apraxia 1 (AOA1) impact APTX protein structure and function. Altogether, the emerging results from structural, biochemical and genetic analysis of human and yeast Aprataxins leads to the hypothesis that accumulation of adenylated RNA-DNA may contribute to APTX-linked neurodegenerative disease.
This research is supported the NIH Intramural Research program, National Institute of Environmental Health Sciences, 1Z01ES102765 to R.S.W.
Schellenberg, M. J., Tumbale, P. P., & Williams, R. S. (2015). Molecular underspinnings of Aprataxin RNA/DNA deadenylase function and dysfunction in neurological disease. Prog Biophys Mol Biol. Jan 29. pii: S0079-6107(15)00008-5. doi: 10.1016/j.pbiomolbio.2015.01.007. [Epub ahead of print]
R. Scott Williams
Genome Integrity and Structural Biology Laboratory