New & Noteworthy
May 22, 2013
Living your life puts a lot of wear and tear on you. A big reason is that as your cells go about their business, they churn out lots of damaging chemicals.
One of the worst offenders is the free radical superoxide, O2-. Cells can’t help producing this powerful oxidant during normal metabolism, but it’s so toxic that it can destroy proteins and damage DNA.
Cells have come up with a two-step process to deal with this toxic waste. In the first step, they use the enzyme superoxide dismutase (Sod1p is the cytosolic form in yeast) to convert superoxide into the less harmful hydrogen peroxide (H2O2) and water. The cells then use catalases to take care of the H2O2, converting it to water and molecular oxygen.
We’ve known about the first enzyme, superoxide dismutase, for decades. It has always been thought to have a simple role, sitting in the cytoplasm and detoxifying O2-. But new research shows that its job is considerably more interesting than that: it also has a role in a regulatory process known as the Crabtree effect.
The Crabtree effect is named after the scientist who first described it way back in 1929. Some types of cells are able to produce energy by either fermentation or respiration in the presence of oxygen. Since these two processes have different metabolic costs and consequences, which one to use is a critically important choice.
If lots of glucose is around, yeast cells choose fermentation. They prevent respiration by repressing production of the necessary enzymes, and this glucose-dependent repression is the Crabtree effect. It happens not only in yeast, but also in some types of proliferating cancer cells.
A new study by Reddi and Culotta shows that Sod1p is actually a key player in the Crabtree effect. In response to oxygen, glucose, and superoxide levels, it stabilizes two key kinases that are involved in glucose repression.
It was recently found that the sod1 null mutant can’t repress respiration when glucose is around. This is different from the wild type, which is subject to the Crabtree effect.
Reddi and Culotta started by investigating this observation and found that SOD1 is part of the glucose repression pathway that also involves the two homologous protein kinases Yck1p and Yck2p. They found that Sod1p binds to Yck1p, which wasn’t totally unexpected since this interaction had been seen before in a large-scale screen. The unexpected part was that Sod1p binding actually stabilizes Yck1p and Yck2p. These stabilized kinases can now phosphorylate targets that propagate the glucose signal down the pathway and ultimately repress respiration.
Now the question is why does Sod1p binding stabilize the kinases? It turns out that its enzymatic activity is crucial for stabilization. One idea is that the hydrogen peroxide that Sod1p makes in the neighborhood of the kinases could inactivate ubiquitin ligases that would target them for degradation. Ubiquitin ligases are rich in cysteine residues, and so could be especially sensitive to oxidation by H2O2.
This regulation might also feed into other pathways: these kinases are also involved in response to amino acid levels, and the sod1 null mutant was seen to affect the amino acid sensing pathway in this study.
Most excitingly, this mechanism is not just a peculiarity of yeast Sod1p. The authors mixed and matched yeast, worm, and mammalian superoxide dismutases and casein kinase gamma (the mammalian equivalent of Yck1p/Yck2p), and found that binding and stabilization works in the same way across all these species.
Superoxide dismutases may have been drafted into this regulatory role during evolution because they are the only molecules that sense superoxide, whose levels reflect both glucose and oxygen conditions. A radical idea indeed!