March 10, 2016
You’ve probably heard the old saying, “Don’t put all your eggs in one basket.” The idea of course is that the wise thing to do is to spread out your possessions so when something happens to one set, you still have the rest. (See what Homer and Marge Simpson think of this saying.)
If it really is wise to follow this saying, then according to the results of a new study just published in GENETICS by Kennedy and coworkers, the budding yeast S. cerevisiae is wiser than the fission yeast S. pombe. Well, at least as far as for one part of entry into mitosis.
To enter mitosis, every eukaryote tested so far needs to increase the activity of cyclin dependent kinase 1 (Cdk1). Dephosphorylation of a key tyrosine residue in Cdk1 is an important part of this increased activity.
One of the big players in this dephosphorylation is the phosphatase Cdc25 in S. pombe or Mih1p in S. cerevisiae. In fact, it is so important in S. pombe, that deleting it is lethal. These poor cells arrest in G2 and eventually die.
The same is not true for S. cerevisiae. Deleting MIH1 has only mild effects—a slight delay in entering mitosis and starting anaphase. The phosphorylation on the key tyrosine on Cdk1p, Y19, remains for a longer period of time in this strain, but does eventually clear, explaining the delayed mitotic entry.
One interpretation of this result is that S. cerevisiae has spread its Cdk1 phosphatase activity over multiple proteins. Knocking out MIH1 still leaves enough Cdk1 activity to allow the cell to enter mitosis, albeit more slowly.
Kennedy and coworkers found that deleting both MIH1 and PTP1 in S. cerevisiae had a more severe effect on mitotic entry and exit from anaphase compared to deleting only MIH1. In addition, the level of Y19 phosphorylation on Cdk1p remained for an even longer period in the mih1 ptp1 deletion strain. But it was still not lethal and the cells did eventually manage to pass through mitosis.
These results suggest there is still another player involved. The next suspect these researchers focused on was protein phosphatase 2A (PP2A). Previous work had shown that mutation of the B-regulatory subunits of PP2A, Cdc55p and Rts1p, both affect Cdk1p phosphorylation.
Because of the multiple routes by which PP2A can affect entry into mitosis, the authors designed an in vivo phosphatase assay to accurately measure the level of phosphorylation of Y19 of Cdk1p. The results of this assay suggested that PP2ARts1 and not PP2ACdc55 affected the phosphorylation state of Y19.
Kennedy and coworkers finally managed to kill off their yeast by deleting MIH1, PTP1, and PP2ARts1! They had finally found enough of this yeast’s phosphatase activity to mimic the effects of just Cdc25 in the fission yeast S. pombe.
Using immunopurified protein complexes, Kennedy and coworkers were able to show that both Mih1p and Ptp1p could dephosphorylate Y19 of Cdk1. They could not, however, see dephosphorylation by PP2ARts1. It could be that their in vitro assay did not detect it for this protein or that PP2ARts1 works on a different phosphatase that affects Cdk1.
Bottom line is that the budding yeast has evolved such that the phosphatase activity needed to enter mitosis is spread out over multiple proteins. The fission yeast evolved in a way that kept all of its phosphatase eggs in the same basket, Cdc25. We’ll let you decide which yeast you think is the wiser.
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight