The PCNA binding domain of Rad2p plays a role in mutagenesis by modulating the cell cycle in response to DNA damage.

The xeroderma pigmentosum group G (XPG) gene, encoding an essential element in nucleotide excision repair (NER), has a proliferating cell nuclear antigen-binding domain (PCNA-BD) at its C-terminal region. However, the role of this domain is controversial because its presence does not affect NER. Using yeast RAD2, a homolog of human XPG, we show that Rad2p interacts with PCNA through its PCNA-BD and the PCNA-BD of Rad2p plays a role in UV-induced mutagenesis. While a mutation of Rad2p endonuclease activity alone causes dramatically increased mutation rates and UV sensitivity, as well as growth retardation after UV irradiation, a mutation of the Rad2p PCNA-BD in the same mutant causes dramatically decreased mutation rates, reduced UV sensitivity and increased growth rate after UV irradiation. After UV irradiation, large-budded cells of Rad2p endonuclease defective mutants wane due to a mutation of the Rad2p PCNA-BD. Besides, the Rad2p PCNA-BD mutant protein exhibits alleviated PCNA-binding efficiency. These results show a hitherto unsuspected role of the Rad2p PCNA-BD that controls mutagenesis via cell cycle modulation together with PCNA. Furthermore, the high mutation rate of cells with other NER gene mutations was also decreased by the mutation of the Rad2p PCNA-BD, which indicates that the Rad2p-PCNA interaction might be responsible for mutagenesis control in the general NER pathway. Our results suggest that the drastically increased incidence of skin cancer in xeroderma pigmentosum patients could arise from the synergistic effects between cell cycle arrest due to the XPG-PCNA interaction and the accumulation of damaged DNA via defects in DNA damage repair.

Yeast RAD2, a homolog of human XPG, plays a key role in the regulation of the cell cycle and actin dynamics.

Mutations in the human XPG gene cause Cockayne syndrome (CS) and xeroderma pigmentosum (XP). Transcription defects have been suggested as the fundamental cause of CS; however, defining CS as a transcription syndrome is inconclusive. In particular, the function of XPG in transcription has not been clearly demonstrated. Here, we provide evidence for the involvement of RAD2, the Saccharomyces cerevisiae counterpart of XPG, in cell cycle regulation and efficient actin assembly following ultraviolet irradiation. RAD2 C-terminal deletion, which resembles the XPG mutation found in XPG/CS cells, caused cell growth arrest, the cell cycle stalling, a defective α-factor response, shortened lifespan, cell polarity defect, and misregulated actin-dynamics after DNA damage. Overexpression of the C-terminal 65 amino acids of Rad2p was sufficient to induce hyper-cell polarization. In addition, RAD2 genetically interacts with TPM1 during cell polarization. These results provide insights into the role of RAD2 in post-UV irradiation cell cycle regulation and actin assembly, which may be an underlying cause of XPG/CS.

Alanine-metabolizing enzyme Alt1 is critical in determining yeast life span, as revealed by combined metabolomic and genetic studies.

Alterations in metabolic pathways are gaining attention as important environmental factors affecting life span, but the determination of specific metabolic pathways and enzymes involved in life span remains largely unexplored. By applying an NMR-based metabolomics approach to a calorie-restricted yeast (Saccharomyces cerevisiae) model, we found that alanine level is inversely correlated with yeast chronological life span. The involvement of the alanine-metabolizing pathway in the life span was tested using a deletion mutant of ALT1, the gene for a key alanine-metabolizing enzyme. The mutant exhibited increased endogenous alanine level and much shorter life span, demonstrating the importance of ALT1 and alanine metabolic pathways in the life span. ALT1's effect on life span was independent of the TOR pathway, as revealed by a tor1 deletion mutant. Further mechanistic studies showed that alt1 deletion suppresses cytochrome c oxidase subunit 2 expression, ultimately generating reactive oxygen species. Overall, ALT1 seems critical in determining yeast life span, and our approach should be useful for the mechanistic studies of life span determinations.