New & Noteworthy
March 14, 2013
You can’t teach an old dog new tricks, or so the saying goes. But imagine you found that your old dog knew a complicated trick and had been doing it all her life, right under your nose, without your ever noticing it! You’d be surprised – about as surprised as the Hinnebusch group at NIH when they discovered that some long-studied S. cerevisiae genes had an unexpected trick of their own.
They were working on the VPS* (vacuolar protein sorting) genes. While known for a very long time to be important in protein trafficking within the cell, Gaur and coworkers found that two of these genes, VPS15 and VPS34, play an important role in RNA polymerase II (pol II) transcription elongation too. Now there is an unexpected new trick…like your dog learning to use a litter box!
There had been a few hints in recent years that the VPS genes, especially VPS15 and VPS34, might have something to do with transcription. Following up on these, the researchers tested whether vps15 and vps34 null mutants were sensitive to the drugs 6-azauracil and mycophenolic acid. Sensitivity to these drugs is a hallmark of known transcription elongation factors. Sure enough, they were as sensitive as a mutant in SPT4, encoding a known transcription elongation factor. Further experiments with reporter genes and pol II occupancy studies showed that pol II had trouble getting all the way to the end of its transcripts in the vps mutant strains.
There was a bit of genetic interaction evidence that had suggested that there might be a connection between VPS15, VPS34, and the NuA4 histone acetyltransferase complex. This is important, since NuA4 is known to modify chromatin to help transcription elongation. Looking more closely, the researchers found that Vps34p and Vps15p were needed for recruitment of NuA4 to an actively transcribing reporter gene.
Other lines of investigation all pointed to the conclusion that these VPS proteins have a role in transcription. They were required for positioning of several transcribing genes at the nuclear pore, could be cross-linked to the coding sequences of transcribing genes, and could be seen localizing at nucleus-vacuole junctions near nuclear pores.
One appealing hypothesis to explain this has to do with what both genes actually do. Vps34p synthesizes phosphatidylinositol 3-phosphate (PI(3)P) in membranes, while Vps15p is a protein kinase required for Vps34p function. The idea is that when Vps15p and Vps34p produce PI(3)P at the nuclear pore near transcribing genes, this recruits the NuA4 complex and other transcription cofactors that can bind phosphoinositides like PI(3)P. There are hints that this mechanism may also be at work in mammalian and plant cells.
There’s a lot more work to be done to nail down the exact role of these proteins in transcription. But this story is a good reminder to researchers that new and interesting discoveries may always be hiding in plain sight.
* These genes were also called VPL for Vacuolar Protein Localization and VPT for Vacuolar Protein Targeting
March 5, 2013
Sixty new datasets have been added to our expression analysis tool at SGD, facilitating the rapid identification of co-expressed genes based on patterns of expression shared with query gene(s) across the entire collection. Expression data are now available at SGD from a comprehensive collection of 430 datasets representing 9190 microarrays from a total of 286 publications. The expression analysis tool can be accessed via the Expression tab and Expression Summary histogram located on Locus Summary pages, or using the ‘Expression’ option in the Function pulldown in the menu bar at the top of SGD pages. The new data will by default be included with the previous data when using the ‘New Search’, ‘Show Expression Levels’, or ‘Dataset Listing’ options. Alternatively, the new datasets can be specifically filtered using the dataset tag ‘not yet curated’. All of the RNA expression data are available for download in expression directory. Datasets are grouped by publication and are in PCL format.
March 5, 2013
Cancer often gets going with chromosome instability. Basically a cell gets a mutation that causes its chromosomes to mutate at a higher rate. Now it and any cells that come from it build mutations faster and faster until they hit on the right combination to make the cell cancerous. An accelerating avalanche of mutations has led to cancer.
There are plenty of obvious candidates for the genes that start these avalanches: genes like those involved in segregating chromosomes and repairing DNA, for example. But there are undoubtedly sleeper genes that no one has really thought of. In a new study out in GENETICS, Minaker and coworkers have used the yeast S. cerevisiae to identify three of these genes — GPN1 (previously named NPA3), GPN2, and GPN3.
A mutation in any one of these genes leads to chromosomal problems. For example, mutations in GPN1 and GPN2 cause defects in sister chromatid cohesion and mutations in GPN3 confer a visible chromosome transmission defect. All of the mutants also show increased sensitivity to hydroxyurea and ultraviolet light, two potent mutagens. And if two of the genes are mutated at once, these defects become more severe. Clearly, mutating GPN1, GPN2, and/or GPN3 leads to an increased risk for even more mutations!
What makes this surprising is what these genes actually do in a cell. They are responsible for getting RNA polymerase II (RNAPII) and RNA polymerase III (RNAPIII) into the nucleus and assembled properly. This was known before for GPN1, but here the authors show that in gpn2 and gpn3 mutants, RNAPII and RNAPIII subunits also fail to get into the nucleus. Genetic and physical interactions between all three GPN proteins suggest that they work together in overlapping ways to get enough RNAPII and RNAPIII chugging away in the nucleus.
So it looks like having too little RNAPII and RNAPIII in the nucleus causes chromosome instability. This is consistent with previous work that shows that mutations in many of the RNAPII subunits have similar effects. Still, these genes would not be the first ones most scientists would look at when trying to find causes of chromosomal instability. Score another point for unbiased screens in yeast leading to a better understanding of human disease.