New & Noteworthy

Mother Yeast Keeps Daughters Tidy

December 18, 2014

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Yeast need translation and organelle tethering to sequester misfolded proteins away from the cytoplasm:

Sometimes parents feel like they’re constantly nagging their kids to clean things up. Eventually, some parents just give in and do it themselves. It turns out that yeast cells aren’t all that different.

And it is even more important for cells to clean up their cellular trash than it is for that sullen teenager. If trash like misfolded or damaged proteins is not sequestered from the rest of the cytoplasm, it can cause other proteins to change their conformation or interfere with metabolic processes. This is obviously much worse than having a smelly room!

One way that cells bin their trash is by compacting it into globs, or aggregates. While this works in the short run, as cells age these aggregates build up, and in human cells, they’re hallmarks of diseases like ALS and Alzheimer’s.

In a new study published in Cell, Zhou and colleagues looked at the process of protein aggregation in S. cerevisiae. A number of other studies had already suggested that just like human parents, mother yeast cells clean up trash for their daughters. The researchers confirmed this and also made a couple of very surprising findings.

Zhou and coworkers discovered that although aggregates are mostly composed of previously translated proteins, they don’t form without new translation. And they found that aggregates aren’t littered around the cytoplasm, but instead are collected in very specific trash bins located on the surface of cellular organelles.

To do these studies, the scientists assembled a toolkit of ways to induce and visualize protein aggregation.  They used stresses like heat shock or various chemicals to stimulate aggregate formation.

They visualized the aggregates by using GFP-labeled Hsp104p, which binds to them specifically. They also had some thermally unstable reporter proteins fused to different colored fluorescent markers that helped them track aggregation. And they created time-lapse videos to watch the whole process.

They first looked in detail at aggregate formation, creating two different kinds of cellular trash (aggregates of distinct sets of marker proteins) at different times to ask whether they would all end up in the same aggregates or in separate ones. These experiments showed that in general, newly aggregated proteins will join existing aggregates rather than creating new ones. And intriguingly, these results also hinted that new translation was needed to start the aggregation process.

Zhou and coworkers tested this directly by adding cycloheximide, an inhibitor of translation, to their aggregation experiments. Sure enough, cycloheximide prevented all of the treatments from causing aggregation, and other treatments and conditions that blocked translation did the same thing.

So just as the threat of taking away the car keys may get that sullen teenager off the couch to start cleaning up, newly synthesized polypeptides are the inducer for the cellular clean-up. Without them, all the trash just stays littered around the cell.

The researchers guessed that if aggregation starts at sites where translation is occurring, it might be concentrated at the surface of the endoplasmic reticulum (ER), where a large proportion of ribosomes are bound. They used several different sophisticated microscopy techniques to confirm that most aggregates were in fact associated with the ER.

But they also got a surprise: in addition to the ER, many aggregates were associated with the mitochondrial surface and some even formed directly on it.* The authors also observed aggregates that formed at the ER but migrated along it to ER-mitochondrial contact points and eventually to mitochondria. So, cells don’t litter their trash just anywhere; they start specific collection points on the surfaces of organelles. And much of the trash seems to end up attached to mitochondria.

The researchers noticed something else when they looked at aggregates in cells that were dividing. When a bud forms, mitochondria are actively transported into it. However, the aggregates that were on mitochondria didn’t go into buds. They stayed on mitochondria that remained in the mother cell. Mom protects her daughter from any trash that mom created!

To figure out what controls this asymmetric segregation, Zhou and colleagues tested a panel of 72 mutant strains, each with a deletion in a mitochondrial outer membrane protein. One strain, the fis1 null mutant, was markedly defective: aggregates often went into the bud. Fis1p is known to be involved in mitochondrial fission, but this result suggests it may have an entirely separate role in making sure that trash-bearing mitochondria stay in the mother cell.

And finally, the authors saw that as mother cells got older, they got less and less able to keep aggregates out of their daughters’ cytoplasm. Towards the end of the mothers’ lives, aggregates were distributed more or less randomly between mother and daughter.

Trash build up is a big problem in teenagers’ rooms and in cells. Just like mom, the cell packs the trash away into bins where it will do less harm. Unfortunately, as the cell (and mom) get older, this gets harder and harder to do. In both cases, the daughter is saddled with more and more trash as mom struggles to keep up. And this is bad for the daughter as well as for the mom.

So, there’s actually a very good reason behind all that nagging to clean up your room. The secret to a long life is to always pick up your trash!

*New data from the Weissman lab, described here in a recent blog post, dovetail nicely with this finding since they establish that a lot of translation takes place on the mitochondrial surface.

by Maria Costanzo, Ph.D., Senior Biocurator, SGD

New Protein Modification and Abundance Data in YeastMine

December 17, 2014

Have you ever wondered what’s happening to your favorite protein as it’s hanging out in the cell? SGD’s advanced search tool, YeastMine, now includes four new templates that can be used to find protein modification and abundance data.

The Gene -> Protein Modifications template retrieves phosphorylation, ubiquitination, succinylation, acetylation and methylation data, currently curated from the following 11 publications: Peng et al. 2003, Hitchcock et al. 2003, Seyfried et al. 2008, Vogtle et al. 2009, Ziv et al. 2011, Mommen et al. 2012, Henriksen et al. 2012, Swaney et al. 2013, Kolawa et al. 2013, Weinert et al. 2013, and Wang et al. 2014.

The Gene -> Experimental N-termini and N-terminal modifications template retrieves experimentally-determined amino-terminal sequence and acetylation data, currently curated from Vogtle et al. 2009 and Mommen et al. 2012.

Lastly, two new templates pull protein abundance data curated from Ghaemmaghami et al. 2003. Gene -> Protein Abundance retrieves molecules/cell counts for a gene or list of genes. The same data can be quickly filtered using the Retrieve -> Proteins in a given molecules/cell abundance range template.

Please explore these new YeastMine protein data templates, and send us your feedback.

Telomerator is the Chromosomal Terminator

December 10, 2014

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A simple tool for adding telomeres to linear DNA:

First there was the Terminator. Now, in a new study published in PNAS by Mitchell and Boeke, we have the telomerator.

Instead of being a homicidal robot from the future bent on killing Sarah Connor, the telomerator is a tool that lets scientists easily turn circular DNA into stable chromosomes in the yeast Saccharomyces cerevisiae. While less splashy, this bit of synthetic biology is definitely cool in its own way (and much less dangerous!).

The system that Mitchell and Boeke created is very clever. They first inserted the intron from the ACT1 gene into the middle of the URA3 gene. The URA3 gene was still functional, as ura3 mutants could use it to grow on medium lacking uracil.  

They next inserted a sequence into the middle of the intron that consisted of an 18 base pair I-SceI cleavage site flanked on each side by around 40 base pairs of yeast telomere repeats (called Telomere Seed Sequences or TeSSs). This construct still allowed ura3 mutants to grow in the absence of added uracil.

The final step was to introduce the homing endonuclease I-SceI to the cell so that it cut the circular DNA precisely between the two TeSSs. The idea is that when you add the homing endonuclease, the newly linearized piece of DNA ends up with the telomere seeds on each end. Telomerase adds more repeats to the seeds until the DNA has proper telomeres. Voilà, a chromosome is born.

The URA3 gene part of the plasmid is important for selecting cells with the linearized DNA. Basically a circularized DNA will grow on medium lacking uracil but fail to grow on medium with 5-FOA, while the linearized DNA will do the opposite. In other words, the process of linearization should destroy the URA3 gene. And that’s just what they found.

Previous work had shown that to be stable in yeast, a chromosome needs to be at least 90 kilobases (kb) or so long. This is why they tested their new telomerator in synIXR, a synthetic yeast chromosome that is about 100 kb in length. This chromosome has 52 genes from the right arm of chromosome 9, two genes from the left arm, around 10 kb of nonessential BAC DNA, the native centromere CEN9, and a LEU2 marker. 

Mitchell and Boeke inserted the telomerator sequence into two different locations in the BAC part of the circularized synIXR and found that adding I-SceI appeared to linearize the DNA. In both cases they found that around 100 out of 200 cells were resistant to 5-FOA and unable to grow in the absence of uracil but could still grow in the absence of leucine. This is just what we would predict if we cut the DNA in the middle of the URA3 gene and created a stable piece of linear DNA.

They next wanted to use this tool to study the effects of telomeric DNA on nearby genes. We would predict that because of telomeric silencing, genes near a telomere will be downregulated. Any genes that affect growth when turned down should quickly become evident.

To accomplish this they inserted the telomerator three base pairs downstream of each of the 54 genes on synIXR, generating 54 new plasmids. After activating the telomerator by expressing the I-SceI nuclease, they used pulsed field gel electrophoresis to confirm that 51 of the 54 synthetic chromosomes had indeed been linearized.

As expected, they found that putting a telomere near a gene sometimes has profound effects. For example, when they linearized DNA where the telomerator was 3’ of either YIR014W, MRS1, or YIR020C-B, they got no growth. They also found many more effects on the growth rate at both 30° C and 37° C at many different, “telomerized” genes. The implication is that when these genes are near telomeric DNA, they no longer function at a high enough level for the yeast to grow well or in some cases to even survive.

To confirm that the effects they saw were due to telomeric silencing, Mitchell and Boeke tested each linearized DNA in a sir2 mutant, a key player in this form of silencing. Mutating sir2 reversed the effects of placing a telomere near the gene, further supporting the idea that the newly created chromosome ends are like normal telomeres because they undergo the same Sir2-mediated silencing.

Finally, the researchers tested the stability of the newly created chromosomes by selecting for Ura⁺ revertants from six individual cultures with different linearized molecules. They failed to select any revertants in which the DNA had recircularized, showing that the linear chromosomes are stable.

So in contrast to the Terminator, who sliced and diced his victims randomly, the telomerator will allow synthetic biologists to create linear chromosomes with precisely positioned telomeres. This study proved the concept, and this tool will be incredibly useful in the future, both in yeast and potentially in other eukaryotes. Both the Terminator and the telomerator can say, “I’ll be back”!

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics

New Alternative Reference Genomes

December 8, 2014

At SGD, we are expanding our scope to provide annotation and comparative analyses of all major budding yeast strains, and are making progress in our move toward providing multiple reference genomes. To this end, the following new S. cerevisiae genomes have been incorporated into SGD as “Alternative References”: CEN.PK, D273-10B, FL100, JK9-3d, RM11-1a, SEY6210, SK1, Sigma1278b, W303, X2180-1A, Y55. These genomes are accessible via Sequence, Strain, and Contig pages, and are the genomes for which we have curated the most phenotype data, and for which we aim to curate specific functional information. It is important to emphasize that we are not abandoning a standard sequence; S288C is still in place as “The Reference Genome”. However, we do recognize that it is helpful for students and researchers to be able to ‘shift the reference’, selecting the genome that is most appropriate and informative for a specific area of study.

These new genome sequences have been also been added to SGD’s BLAST datasets, multiple sequence alignments, the Pattern Matching tool, and the Downloads site. Please explore these new genomes, and send us your feedback.