New & Noteworthy
Smoothing Over an Extra Chromosome
August 22, 2013
Let’s say you had a rock you had to move that was way too heavy for you to lift. You could either start lifting weights until you could move it yourself or get someone to help you. Most of us would start texting our friends pretty quick.
Jon Bon Jovi used scissors to go from fluffy to smooth. Yeast uses an extra chromosome XVI.
Turns out our friend S. cerevisiae can be the same way. Many strains of this yeast can exist as either a fluffy colony or a smooth one. In a new study, Tan and coworkers show that some of these strains switch between the two by gaining or losing one of their chromosomes. They’d rather “get” an extra chromosome than try to gain a mutation that activates the necessary gene(s).
In this study, the authors found a strain where around one in a thousand yeast switched between fluffy and smooth colonies. As the smooth colonies grew, they developed “blebs” – little bumps on the smooth colonies. Turns out these were yeast that switched back to the fluffy morphology. The authors set out to explore why this strain switches at such a high rate and why it would want to.
A first look showed that when this yeast strain went from fluffy to smooth, it gained an extra copy of chromosome XVI. When the new smoother yeast lost this extra chromosome, it reverted back to its fluffy look. A harder look showed that an extra chromosome XVI wasn’t the only way to a smoother yeast. Occasionally the fluffy to smooth change could be caused by an extra copy of chromosome III, X, or XV, and an extra copy of V caused a slightly smoother colony.
These results suggest a couple of different ways that an extra chromosome might be leading to a smooth colony. One is that just having extra DNA around causes the change. The other is that a variety of genes can cause the change when present in higher than normal doses. The researchers show pretty convincingly that the second reason is probably the right mechanism.
First off they show that not all extra chromosomes are created equal. Some lead to a very sickly yeast while others have no effect on fluffiness. Just having extra DNA around is probably not the culprit.
The authors next set out to figure out exactly what was going on with chromosome XVI. Through a series of deletion studies, they found a single gene responsible for the fluffy to smooth shift – DIG1. Overexpression of this single gene caused fluffy colonies to turn smooth. Presumably there are other genes on some of the other chromosomes that serve a similar function.
They next set out to determine why yeast would ever want to do this. Turns out that, as you might expect, each phenotype has an advantage in a different situation. On a solid surface the fluffy strain did better, while the smooth one did better in liquid media.
The “extra chromosome option” is actually a great way for a sedentary beast like yeast to quickly deal with a new situation. Gaining an extra chromosome is much simpler than gaining a new mutation that up-regulates a gene under certain situations.
Figuring out this mechanism of fluffy to smooth transitions isn’t just fun biology either. It may also point us in new directions for treatments for a variety of diseases, including drug-resistant cancers and microbial infections.
In many cases, these cells become resistant because their chromosome number has changed from what is considered the norm. If we could find a way to force cells to maintain the correct number of chromosomes, we might be able to make them more susceptible to drugs. As usual, yeast studies are much more than fluff…they smooth the way to the future.
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight
Tags: aneuploidy, Saccharomyces cerevisiae
New data tracks added to GBrowse
August 20, 2013
SGD has added new data tracks to the GBrowse genome viewer covering differential expression RNA-seq data from Waern & Snyder (2013) and Transcript Leader RNA-seq data from Arribere & Gilbert (2013).
Download data tracks, metadata and supplementary data by clicking on the ‘?’ icon on each data track within GBrowse or directly from the SGD Downloads site. We welcome new data submissions pre- or post-publication and invite authors to work with us to integrate their data into our GBrowse and PBrowse viewers. Please contact us if you are interested in participating or have questions and comments. Happy browsing!
Categories: New Data
Tags: differential expression, GBrowse, RNA-seq, TATL-seq, TL-seq
Anchors Aweigh for Peroxisomes
August 15, 2013
Suppose you had a fleet of rowboats that you wanted to split up evenly between two shores. You probably wouldn’t just set them free and hope they drifted to the right places. A better way might be to run some ropes from the distant shore and pull half the rowboats across. That way you’d be sure to get the distribution of boats that you wanted.
Cells anchor their peroxisomes as carefully as fishermen anchor their boats.
Dividing cells face a similar problem with their organelles. When cells divide, they need to make sure mother and daughter receive the right number of organelles. Otherwise one or both could die!
This is obviously too important to just leave to chance, which is why cells have devised ways to precisely control how many organelles end up in mother and daughter. But not every organelle is divided in the same way. In our boat analogy, some are pulled over with ropes, others with chains, some with winches and so on.
We don’t know a lot about how some organelles are distributed between mother and daughter cells. For example, the details have not been worked out for peroxisomes, the organelles that contain enzymes for beta-oxidation of fatty acids. Until now, that is…
In a recent study in The EMBO Journal, Knoblach and colleagues looked in great detail at how yeast cells distribute their peroxisomes. During budding, some peroxisomes stay tied up in the mother cell while others are transported into the bud and re-tied there.
The authors found that the structure that ties up peroxisomes is like a rope with hooks at both ends. One hook attaches to the peroxisome, while the other hook attaches to the cortical endoplasmic reticulum (ER) near the cell wall. Surprisingly, the same protein, Pex3p, acts as the hook at both ends of the rope, connecting it to both ER and peroxisomes.
The authors already knew that some peroxisomes stayed anchored around the edges of the mother cell while others were “mobile” and moved to the daughter when yeast cells divided. They also knew that the protein Inp1p was important for anchoring peroxisomes. In the inp1 null mutant all the peroxisomes are mobile and end up in the bud, while overexpressing Inp1p causes all the peroxisomes to be anchored in the mother cell and stay there.
Knoblach and colleagues suspected that Inp1p might act as the rope that tethers peroxisomes. To test this, they fused Inp1p to a protein that sits in the mitochondrial outer membrane, Tom70p. Now peroxisomes in this strain were attached to mitochondria! This established that Inp1p is the tether.
Another major molecular player in this process is Pex3p. The pex3 null mutant phenotype looks a lot like the inp1 mutant phenotype: the mother cell loses all its peroxisomes and they end up in the bud. Pex3p is an integral membrane protein that is channeled through the ER on its way to the peroxisomal membrane – so it can be present in both places. The authors found that both the N terminus and C terminus of Inp1p bind to Pex3p. All this suggested that together, Inp1p and Pex3p might form a structure that links peroxisomes to the ER.
They were able to show that Inp1p and Pex3p interact directly both at the peroxisome and at the ER using a neat trick called bimolecular fluorescence complementation. This simply means that if the two halves of green fluorescent protein (GFP) are brought close to each other, they can fluoresce like the intact protein. The basic idea is that they fused the first half of GFP with Inp1p and the second half with Pex3p and looked for green spots to turn up in the right place of the cell. Of course this is easier said than done!
To pull this off, the authors had to first make two haploid strains of opposite mating types. The first had a pex3 mutation that caused all Pex3p to be stuck in the ER, anchoring all its peroxisomes there. It also carried a version of INP1 that was fused to half of GFP.
The second strain had a different pex3 mutation that set all peroxisomes adrift, and this mutant pex3 gene was fused to the other half of GFP. This strain also had an additional marker that made peroxisomes glow red.
When the cytoplasms of these two strains had a chance to mix after mating, the zygote had red peroxisomes with glowing green spots, showing that Inp1p-half GFP from the first strain was interacting with Pex3p-half GFP from the second strain. Because the Inp1p-half GFP of the first strain was bound to ER-localized Pex3p, and the Pex3p-half GFP of the second strain was localized to peroxisomes, this result showed that Inp1p connects peroxisomes with the ER.
The authors studied the kinetics of this process in a lot more detail and even tracked the wanderings of individual peroxisomes. The model that comes from all this work is that at start of budding, some peroxisomes are bound to Inp1p and others aren’t. Those that aren’t bound to Inp1p move to the bud, and the others stay anchored to the mother cell’s ER. Meanwhile, during budding Pex3p passes through the ER and re-emerges at the ER membrane at the bud cortex. It can then bind Inp1p, which in turn binds to the Pex3p on the surface of the migrating peroxisomes to dock them in the bud.
Not only is this cool just from a basic biology perspective, but it may also help us deal with some human peroxisome biogenesis disorders. For example, Zellweger syndrome and infantile Refsum disease are associated with specific mutations in the human ortholog of PEX3. Once again, little S. cerevisiae is helping us navigate the inner workings of eukaryotic cells.
by Maria Costanzo, Ph.D., Senior Biocurator, SGD
Categories: Research Spotlight, Yeast and Human Disease
Tags: peroxisomes, Saccharomyces cerevisiae, yeast model for human disease
When One Spark Isn’t Enough
August 07, 2013
A single mutation, just like a single spark, is more likely to fizzle out.
As anyone who has ever tried to start a fire with flint knows, a single spark is rarely enough. You need to get a bunch of sparks all working at once to end up with that roaring campfire. And with the wrong kindling or wood, even lots of powerful sparks just can’t get it done.
A new study in the yeast S. cerevisiae by Lang and coworkers suggests that evolution may be similar. A single helpful DNA change may not be enough to give an individual yeast that leg up it needs to spread through the population. Turns out that more often than not it needs something like 5-7 mutations. And again, even that may not be enough if the rest of its DNA isn’t up to par. A set of powerful sparks on soggy wood still won’t light a fire.
Lang and coworkers followed 40 different yeast populations for a thousand generations as the yeast adapted to a new environment (rich medium). They sequenced each population to 100-fold depth at 12 different time points. Not only did this allow the researchers to watch mutations rise and fall over time, it also let them screen out sequencing errors. Real mutations will correlate over time, sequencing errors won’t.
The key finding of their research was that mutations that increased in the population over time almost always came in bunches (or cohorts) and that not all the mutations were beneficial. Neutral mutations invariably hitchhiked along with strongly beneficial ones.
A great example of this involves the ELO1 and GAS1 genes. These two mutations arose together in a yeast population but when the researchers looked at each individually, only GAS1 was beneficial. ELO1 appeared to go along for the ride.
Another key point of this study is that mutations do not happen in a vacuum…beneficial mutations only “catch” in the context of a good background. This is clearly shown in one of the populations they followed.
In this population, yeast with a mutation in the SPC3 gene began to spread through the population. After about 300 generations, though, a second yeast with mutations in the WHI2 and ROT2 genes began to outcompete the SPC3 mutant. If things stayed like this, the SPC3 mutation would disappear from the population even though it was obviously helpful.
What happened instead was that a yeast with the SPC3 mutation developed a useful mutation in the YUR1 gene. This combination was strong enough for this yeast to stay in the game until one of them developed a third mutation in the WHI2 gene. This triple threat proved too much for the yeast with the mutations in the WHI2 and ROT2 genes – they were driven to extinction.
No wonder people refer to evolution as a dynamic process! This example shows just how tumultuous it actually is. Even helpful mutations like the ones in ROT2 and WHI2 can disappear over time if they happen in a weaker background. And presumably even potentially harmful mutations can spread if they hitchhike along with a cohort of strongly beneficial mutations.
These results not only shed light on how evolution works, but could also spark other discoveries on how cancer progresses, how bacteria become resistant to antibiotics, and how viruses deal with our immune system, just to name three. And that could help kindle a brighter future.
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight
Tags: evolution, Saccharomyces cerevisiae
The Baby Bear of Proteins
July 31, 2013
In the story of Goldilocks and the Three Bears, Goldilocks always likes Baby Bear’s stuff best. Baby Bear has the most comfortable bed, the best porridge, and so on.
When it comes to protein levels, yeast cells can be just as picky as Goldilocks.
The reason Goldilocks likes Baby Bear’s things are that they are just right. They are neither too hard nor too soft, too hot nor too cold, too big nor too little.
Turns out that when it comes to certain proteins, the yeast S. cerevisiae is sort of like Goldilocks…it likes to have them at just the right levels. Too much or too little protein can throw things out of whack.
This idea is supported in a new study in GENETICS, where Sasanuma and coworkers find that a key helicase in yeast, Srs2p, needs to be present in just the right amounts for meiotic recombination to go off without a hitch. In particular, they show that this protein affects meiotic recombination by interfering with the assembly of filaments containing another protein, Rad51p.
Meiotic recombination starts off with Spo11p making a double stranded (DS) break in the DNA. This DS DNA is then trimmed back so that there is a 3′ overhang of single stranded DNA which is then coated with replication protein A (RPA), Rad51p, and Dmc1p. The coated single stranded DNA then invades a stretch of homologous DNA and recombination can begin.
One of the first things Sasanuma and coworkers did was to show that toying with Srs2p levels has a negative effect on meiosis in general. Too little Srs2p brings spore viability down to 36.8% of wild type, and overexpressing it brings spore viability down to 22.4%. Clearly Srs2p is a bit like Goldilocks…the amount has to be just right.
The authors next set out to determine how overexpressing Srs2p affects meiosis so profoundly. They showed that too much Srs2p delays the start of meiosis, causes chromosomes to end up in the wrong places, and stunts the repair of DS DNA breaks. Basically, extra Srs2p inhibits meiotic recombination.
They next looked at areas on the DNA where both Rad51p AND Dcm1p were bound, and found that too much Srs2p keeps Rad51p but not Dcm1p off the DNA. When either of these proteins binds to DNA, it forms foci that are visible as dots when the proteins are detected with fluorescent antibodies. While a wild type strain had roughly equal numbers of Dcm1p and Rad51p foci, there were four fold fewer Rad51p foci when Srs2p was overexpressed. Clearly Srs2p was keeping Rad51p-DNA complexes from forming.
Srs2p can act as a translocase, and it can also bind Rad51p. Sasanuma and coworkers asked which of these functions is essential to its ability to disassemble Rad51p filaments on DNA. Using srs2 mutants that were blocked in just one of these functions, they showed that the translocase mutant was completely unable to remove Rad51p from DNA during meiosis. The Rad51p-binding mutant could still cause Rad51p to dissociate from chromosomes, although at a reduced rate compared to wild type. So the translocase activity is essential, while Rad51p binding is not.
Although it was known that in vitro Srs2p can cause Rad51p-DNA filaments to disassemble, this study is the first to establish that it actually happens in vivo during meiosis. The requirement for the translocase activity suggests that Srs2p may actually move along the filaments as it disassembles them. And this work also shows that just like Goldilocks with her bowl of porridge, the cell needs an amount of Srs2p that is not too big, not too little, but just right.
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight
Tags: helicase, meiotic recombination, Saccharomyces cerevisiae
Size Isn’t All That Matters
July 23, 2013
Stressed people can lose a marble or two, stressed cells lose a chromosome instead.
Sometimes the pressures and stresses of everyday life can make some people go a little crazy…they lose a few of their marbles. The same thing can happen to a cell too. The only difference is that instead of losing their marbles, cells can lose their chromosomes!
There are all sorts of mechanisms in place to make sure that a cell has just the right number of chromosomes. Still, sometimes all these systems fail and a chromosome is lost. This can be catastrophic for a cell and, as an important part of cancer, catastrophic for the whole body too.
Given how important having the right number of chromosomes is, it is surprising how little we know about what makes a particular chromosome more likely to fly the coop. Kumaran and coworkers set out to change this in their new study in PLOS ONE.
First off, they showed that some chromosomes in S. cerevisiae are indeed more likely to be lost than other ones and that there was a surprisingly wide range of stabilities. For example, chromosomes XIII and XIV were thousands of times more stable than chromosome III.
One key factor in stability was chromosome size—the smaller the chromosome the more likely it was to be lost. But chromosome III showed that size was not the whole story. It was five times more likely to be lost than the smallest chromosome.
The authors next set out to determine what about chromosome III made it so flighty. By creating a hybrid of chromosomes III and IX, they were able to show that there was no single site that made chromosome III so unstable. They were also able to rule out the idea that HML, HMR, and the MAT locus made chromosome III more likely to be lost.
They next focused on the centromere because it is such an important player in chromosome segregation. They created a series of plasmids using the centromeres from chromosomes III, IX, XII, XIV, and XV and found that the ones from chromosome III and chromosome XV were around 5-fold less stable than the other chromosomes. While they do not have a good explanation for why chromosome III and XV fared the same in their assay, the result did suggest that at least part of the instability of chromosome III could be explained by its centromere.
As a final experiment, they determined the frequency of chromosome loss in mad2 deletion mutants. They did this because MAD2 is involved in the spindle checkpoint and so is a key mediator of chromosome stability. They found that deleting this gene significantly increased the loss of other chromosomes, but chromosome III was 3-6 fold less affected by the loss of MAD2. It was almost as if the centromere of chromosome III was already somewhat compromised for its interaction with the spindle.
The authors aren’t sure yet how chromosome III got to be so unstable. It could be that random mutations just made its centromere less effective. But another interesting possibility is that it might be under selective pressure. Carrying the mating type loci, chromosome III could be considered to be equivalent to a sex chromosome in larger eukaryotes, and we know that those chromosomes are under different evolutionary constraints from other chromosomes. Maybe S. cerevisiae just can’t take the pressure!
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight
Tags: chromosome stability, Saccharomyces cerevisiae
Regulation Information Integrated into SGD
July 18, 2013
The Locus Summary pages of 147 DNA-binding transcription factors (TFs; retrieve the list) now include a new tabbed page, Regulation. This page contains information on the regulatory targets of the TF, its binding sites, and its domains and motifs, as well as a free-text paragraph summarizing its biological context. Take a look at a brief video, below, that explains the different kinds of data found on the Regulation tab. In addition to viewing these data page by page, you can download them all using SGD’s data search and retrieval tool, YeastMine. Click on “Regulation” in the YeastMine menu bar to view the predefined templates for regulation data searches.
New Regulation Data at SGD from yeastgenome on Vimeo.
Categories: New Data, Website changes
Oxidation: Maybe Not SO Bad After All
July 11, 2013
You don’t have to be a scientist to get the message that oxidation is bad and antioxidants are good. Just go to the vitamin aisle of your local supermarket, or listen to the ads on late-night TV. You’ll quickly find out that oxidation caused by free radicals is the reason for aging, and antioxidants are the fountain of youth. Of course you shouldn’t believe everything you hear…
It ain’t the rust that will get you, it’s the engine breaking down.
Things just aren’t that clear when you take a good hard look at aging. Yes, oxidation happens, but there actually isn’t solid experimental proof that it causes aging. In mice, this connection has not panned out at all: lowering the ability to sop up oxidants, by knocking out an antioxidant enzyme, does not shorten the mouse’s life.
In a recent eLife paper, joint first authors Brandes and Tienson and their coworkers used our favorite experimental subject, Saccharomyces cerevisiae, to see if oxidation is a cause or just a consequence of aging. They generated a ton of data about oxidation during aging and did not find any evidence for causation. Instead they came to the surprising conclusion that the trigger for aging may actually be a sudden drop in the levels of the coenzyme NADPH.
The first step, published previously by this group, was to come up with a very sensitive assay for protein oxidation. The amino acid cysteine can act as a sensor for levels of oxidation, as its sulfur-containing thiol group can be oxidized and reduced. Their technique, known as OxICAT, detects the ratio of reduced to oxidized thiol groups on cysteine residues for individual proteins. They can do this for hundreds of proteins at the same time.
In the current study, they looked at the oxidation state of cysteine residues in about 300 different proteins and also measured the levels of several different metabolites related to the redox state of the cell. All of these data were collected over time in aging yeast cells, both under normal conditions and under conditions simulating caloric restriction or starvation. These last conditions were included because a lower-calorie diet has been shown to slow down aging, in yeast as well as in animals.
Oxidation of proteins definitely did increase over time. But if oxidation were the cause of cell death, you would expect that it would increase steadily and at some maximum point, the cells would die. Surprisingly, that didn’t happen.
Instead, different groups of proteins were oxidized with different kinetics. The most sensitive proteins (about 10% of the set that they studied) were oxidized 48 hours before the cells started to lose viability. This set included some conserved proteins that are important in maintaining oxidation-reduction balance in the cell, such as the thioredoxin reductase Trr1p.
But it wasn’t only those especially sensitive proteins that were oxidized. In a second wave of oxidation, almost all the remaining proteins (80%) were oxidized at 24 hours before death. And even with so many proteins oxidized the cells were still metabolically active, with ATP levels near normal. So massive oxidation did not equal instant death for these cells.
As predicted, a low-calorie diet slowed down the whole process. The pattern looked a lot like it did in cells on a normal diet, but there was more time between the waves of oxidation and before the end of viability.
The authors also looked at what happened to different metabolites during aging. One key metabolite is the coenzyme NADPH: it donates electrons to the thioredoxin system that helps balance oxidation and reduction. They found that even before any changes in oxidation are detectable, levels of NADPH decrease very suddenly. The authors speculate that this decrease starts the collapse in redox potential that ends in the death of the cell. The oxidation of protein thiols is an effect rather than a cause, and could actually be a way for the cell to sense its redox state and possibly regulate it. NADPH levels have been seen to decrease in aging rats as well, suggesting that this could be a universal part of the aging process.
The results of this study are too voluminous to describe fully here, but they raise a lot of intriguing questions. Some proteins never got oxidized – what protected them? Are NADPH levels really the trigger for aging, and if so, what causes the sudden decrease? Is oxidation of cysteines actually part of a sensory mechanism? And if that’s true, would preventing oxidation really be such a good thing? This may be another good reason to turn off late-night TV.
by Maria Costanzo, Ph.D., Senior Biocurator, SGD
Categories: Research Spotlight
Tags: aging, oxidation, redox, Saccharomyces cerevisiae
Links to LoQate
July 09, 2013
SGD now provides links to LoQate (the localization and quantitation atlas of the yeast proteome) from the Protein Information section of the Locus Summary Pages. The LoQate database provides localization and abundance data for 5300 yeast proteins at single-cell resolution under three different stress conditions: DTT, H2O2, and nitrogen starvation (Breker et al, 2013, J Cell Biol. 200(6), 839-850). Thanks to Maya Schuldiner for helping us set up the links.
Categories: New Data, Website changes
Hanging on by a Thread
July 02, 2013
When Nik Wallenda recently made his incredible tightrope walk over a 1500 foot-deep gorge, the attachment of the cable he walked on was critical. If that had failed, it would have been a very unhappy ending for Nik.
If the rope pulls away from the side of the gorge, this tightrope walker is in trouble. The same thing is true if the kinetochore pulls away from a chromosome.
Something equally dramatic can happen in a cell. If the attachment of spindle microtubules to chromosomes during cell division fails, then the chromosomes don’t end up in the right place. When this happens, the cell can end up dead, or even worse, cancerous. This is as bad as falling off a tightrope without a net!
In a cell, the chromosome is attached to the spindle with something called the kinetochore. It is like the spike driven into the side of the gorge the tightrope walker is going over. One end is attached to the chromosome (the side of the gorge) and the other is attached to the spindle (the rope that is tied to the spike).
This is where the analogy ends though…a kinetochore is way more complicated than a metal spike. It is a huge, multi-protein complex with lots of specialized parts. The way in which the whole complex assembles still isn’t completely understood.
In a new paper in GENETICS, Akiyoshi and coworkers unraveled a bit of the mystery behind it. They found that phosphorylation by a highly conserved protein kinase known as Aurora B (Ipl1p in S. cerevisiae) of one kinetochore subunit, Dsn1p, provides some of the glue that holds the structure together. More specifically, they found that phosphorylated Dsn1p does a better job at keeping inner kinetochore proteins attached to the complex. It drives the spike deeper into the gorge.
The researchers mutated two residues in Dsn1p that are sites for Ipl1p phosphorylation. They mutated one or both to alanine, which prevents phosphorylation, or to aspartic acid, which mimics the phosphorylated state. They found that preventing phosphorylation of these sites loosened the complex and keeping them “phosphorylated” tightened it.
First, to try to look at what happens when Dsn1p isn’t phosphorylated by Ipl1p, they mutated the two sites to alanine. Either site could be mutated with no apparent effects, but mutating both was lethal. Clearly these sites are doing something!
The researchers got around this lethality issue by mutating a third site in Dsn1p. This site is a target for phosphorylation by a different kinase, Cdk kinase (Cdc28p). The idea is that preventing phosphorylation by Ipl1p makes Dsn1p unstable, but then preventing phosphorylation by Cdc28p can stabilize the mutant protein.
Now that they had a living yeast strain in which Dsn1p wasn’t phosphorylated by Ipl1p, they could look to see what was different about the kinetochore in this mutant. When they pulled down the mutant Dsn1p using antibody and a Flag-tag, it brought down normal levels of outer kinetochore proteins but reduced 
levels of inner kinetochore proteins. So this suggested that Ipl1p phosphorylation promotes interactions between Dsn1p and inner kinetochore proteins.
Supporting this idea, an ipl1 mutant that phosphorylated Dsn1p to a lower extent showed lower-than-wild-type levels of inner kinetochore proteins associated with Dsn1p. And, when they looked at a mutant where those Dsn1p residues were changed to aspartic acid, mimicking constant phosphorylation, higher levels of inner kinetochore proteins were pulled down. All of this evidence, and more, points to Ipl1p phosphorylation of Dsn1p as critical for attachment of inner kinetochore proteins to the kinetochore complex.
In yeast there is just one Aurora kinase, and Dsn1p is just one of its substrates. In human cells there are multiple versions of Aurora, and they are implicated in cancer development. Clearly, yeast will be a helpful model in understanding all the details of how Aurora influences kinetochore structure and chromosome segregation. And that will be a much more impressive and useful feat than a tightrope walk!
by Maria Costanzo, Ph.D., Senior Biocurator, SGD
Categories: Research Spotlight, Yeast and Human Disease