Researchers at the Hopkins School of Medicine have shed new light on a pathway of protein activity that allows a specific strain of yeast cells to survive far longer than usual under starvation conditions. The findings could have implications for studies on human aging.
Through their work, the group has identified several new substrates for an enzyme called NuA4, further clarified how the mutation allows the mutants to live longer, and drawn a potential parallel from yeast to human aging.
Previous research has shown that normal yeast actually live longer when starved in water than in a nutrient-rich environment.
Additionally, yeast with a nonfunctional form of an enzyme called Sir2 live longer when starved than do wild type, or genetically normal, yeast. However, until this study it was not known how deactivating Sir2 led to an increased lifespan.
The study by the Hopkins team focused on two different enzymes, NuA4 and Pck1p. NuA4 is a protein acetylase, an enzyme that controls protein activity by attaching an acetyl group (-COCH3) to its substrate. Before, the only known substrates for NuA4 were histone proteins, which are located in the cell nucleus and help to compact DNA.
The researchers confirmed 13 other proteins that NuA4 acts upon in yeast, with many more potential targets. "Our work suggests that there are dozens if not hundreds of other substrates," Jef Boeke, a professor of molecular biology and genetics at the School of Medicine, said.
One of the newly discovered substrates for NuA4 was a protein called Pck1p. In starvation conditions, no outside source of protein cells make their own glucose out of simpler molecules, such as ethanol.
This process, called gluconeogenesis, is tightly controlled, and Pck1p is the enzyme at the rate limiting step. When NuA4 attaches an acetyl group to Pck1p, the enzyme becomes activated and allows gluoconeogenesis to progress.
Another protein, Sir2, which is the mutated protein in longer living mutants, takes the acetyl group off of Pck1p and deactivates the enzyme. When Sir2 can no longer do its job, gluoconeogenesis remains on and the cell is constantly scavenging for ethanol in the surrounding water and making its own glucose.
"In the Sir2 mutant, Pck1 is going full blast so gluconeogenesis is very efficient in those cells, and they're better at sucking up every ethanol molecule in the water," Boeke said. "Our work reveals the long-lost substrate of Sir2."
Sir2 mutants can live up to 20 percent longer in starvation conditions than can normal yeast cells. In contrast, mutants in which Sir2 is always turned off have a lifespan about 80 percent shorter than normal yeast.
These yeast proteins have very close counterparts in human cells, but the implications for humans are unknown. "It's hotly debated if [Pck1p] is relevant for human aging," Boeke said. There are probably several other proteins involved in the starvation response in human cells.
However, the research team plans to conduct a similar study on the human gluconeogenesis system. They also plan to do follow-up studies on the many new substrates they discovered for the NuA4 enzyme, some of which could also be involved in the starvation response.
The study has demonstrated that protein acetylases and deacetylases such as NuA4 and Sir2 play a much larger role in cellular pathways than once thought. "Like protein kinases and phosphatases which control development, the cell cycle and DNA repair, the more we dig into acetylases and deacetylases the more we see they are also intimately involved in these processes," Boeke said.
Boeke led the work together with Heng Zhu, assistant professor of pharmacology at the School of Medicine, and Shelley Berger of the Wistar Institute in Philadelphia. Their findings were published in Cell.
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