PodcastCell Biology

Science Signaling Podcast: 31 January 2012

Science Signaling  31 Jan 2012:
Vol. 5, Issue 209, pp. pc2
DOI: 10.1126/scisignal.2002870


This Podcast features a conversation with authors of a Research Article published in the 31 January 2012 issue of Science Signaling. Jared Rutter and Caleb Cardon discuss their discovery that signaling through PAS kinases can bypass signaling through Tor2 in yeast. Tor2 is one of two Tor kinases that participate in signaling through the yeast target of rapamycin (TOR) pathway, which integrates information about cellular nutrient and energy status to control growth. Cells lacking Tor2 cannot grow, but activation of either of two PAS kinases can restore growth to Tor2-deficient cells by activating a downstream target of Tor2 signaling. The authors demonstrate that PAS kinases are part of a protein complex that may integrate metabolic and stress signaling.

(Length: 17 min; file size: 9.6 B; file format: mp3; location: http://podcasts.aaas.org/science_signaling/ScienceSignaling_120131.mp3)

Technical Details

Length: 17 min

File size: 9.6 MB

File Format: mp3

RSS Feed: http://stke.sciencemag.org/rss/podcast.xml

Listen to Podcast: http://podcasts.aaas.org/science_signaling/ScienceSignaling_120131.mp3

Educational Details

Learning Resource Type: Audio

Context: High school upper division 11-12, undergraduate lower division 13-14, undergraduate upper division 15-16, graduate, professional, general public and informal education

Intended Users: Teacher, learner

Intended Educational Use: Learn, teach

Discipline: Cell biology, microbiology

Keywords: Science Signaling, cell wall synthesis, genetic screen, growth, overexpression, PAS kinase, Psk1, Psk2, Rho1, stress, target of rapamycin, TOR, Tor2, Ugp1, yeast


Host – Annalisa VanHookWelcome to the Science Signaling Podcast for January 31st, 2012. I’m Annalisa VanHook, and today I’m speaking with Jared Rutter and Caleb Cardon about a signaling pathway that can bypass the requirement for signaling through Tor2 in yeast (1).

All cells, whether they’re part of a multicellular organism or lead a unicellular existence, use information from their environment to make basic decisions like whether or not to grow. One of the factors that cells use to make this decision is the availability of nutrients and how much energy the cell has in the form of ATP. The target of rapamycin—or TOR—pathway is one of the pathways that eukaryotic cells use to integrate information about nutrients and energy status with the cellular machinery that drives cell growth. When yeast cells lack TOR signaling, they can’t grow, so mutations that abrogate TOR signaling are lethal. In a new paper published in the current issue of Science Signaling, Rutter and Cardon identified an alternative signaling pathway that can bypass the requirement for signaling through Tor2, which is one of yeast’s two Tor kinases, and therefore rescue the growth defect of yeast that lack Tor2 function.

Rutter and Cardon spoke to me by telephone from the Department of Biochemistry at the University of Utah School of Medicine in Salt Lake City.

Interviewer – Annalisa VanHookWelcome, Dr. Rutter, Mr. Cardon.

Interviewee – Caleb CardonHello.

Interviewee – Jared RutterHi. Thanks for having me on.

Interviewer – Annalisa VanHookIn this paper, you describe the results of a screen for genes that can suppress the growth defect in yeast that lack TOR signaling. Can you give us an overview of the screen and what genes you identified in this screen as suppressors of the TOR phenotype?

Interviewee – Jared RutterSo the screen was essentially designed to identify genes that, when overexpressed, would suppress the temperature-sensitive mutant of the TOR2 gene. And so, actually a fairly simple set-up for the screen, and Caleb can talk more about that. And what we’ve got out of that is somewhat interesting and some things that were expected. One of the necessary effects of Tor2 signaling is the activation of Rho—predominantly Rho1 in yeast cells. One of the things that we got out of that screen is Rho2, which is a paralog of Rho1 with overlapping functions, and when that is overexpressed, that is able to suppress the tor2 mutation, again consistent with the idea that loss of Tor2 causes problems for the cell through loss of Rho1 signaling. So, the observation that we made from the screen that led to this publication in Science Signaling was the recovery of the paralogous PSK1 and PSK2 genes from the screen. So, overexpression of either of those genes rescues the temperature-sensitive phenotype of the tor2ts mutant, and, you know, as I’m sure we’ll talk about later, that then led us to start exploring what’s downstream of Psk1 and 2 that leads to suppression of the tor2ts. The final class of genes that we recovered and aren’t described in this paper—and hopefully will be described in a future paper but are basically unrelated—are genes that are involved in the activation of Psk1 and 2. And so, we think we’ve identified some genes that might, when overexpressed, lead to the activation of Psk1 or Psk2 and thereby activate this pathway that we described in the manuscript and thereby suppress the tor2ts and [that’s] something that we’re quite interested to pursue in future work.

Interviewer – Annalisa VanHookCaleb, as the graduate student who performed most of the experiments described in the paper, why would you opt to do a screen where you’re looking for genes that have to be overexpressed—genes that have to be present in multiple copy numbers to suppress the phenotype—as opposed to looking for second-site mutations that can rescue the phenotype?

Interviewee – Caleb CardonI think the simplest explanation is just that overexpression of the gene can be thought of, I think, as analogous to activation of the protein. And given that we don’t know exactly what the proper activation conditions are of each of these genes or proteins, it’s hard to really dial in that screen using the exact right conditions, right, because you could have thousands of conditions. And so, by doing the overexpression, that allows you to really simplify things and make kind of the a priori assumption that the overexpression of the gene is going to be functionally equal to activation of that protein.

Interviewer – Annalisa VanHookTwo of the genes that you identify from the screen are PSK1 and PSK2, which are paralogs. They’re very similar genes, and they can both rescue the tor2 phenotype when they’re overexpressed. How does increasing the activity of either of those two proteins bypass the block in TOR signaling?

Interviewee – Caleb CardonWe know that activation of either of those proteins leads to phosphorylation of this metabolic enzyme Ugp1. And this phosphorylation event, interestingly, can do kind of two different things, as described in the paper: It can produce an increase in structural carbohydrate synthesis or cell wall synthesis, and you could imagine that would be needed for growth. As you expand and get bigger, you need more of that. And it also nucleates the formation of a complex, which leads to activation of Rho1, which Mike Hall’s group has shown very nicely that Rho1 is the primary downstream effector of Tor2 signaling. And so, PAS kinase—Psk1 or Psk2—activation leads to Rho1 activation, which is what Tor2 activation leads to. So, it’s able to bypass that by leading the activation of the same downstream effector.

Interviewer – Annalisa VanHookWhat do Psk1 and Psk2 do normally in yeast cells that don’t have a Tor2 signaling defect?

Interviewee – Caleb CardonSo, their primary function really is to respond to various cues within the environment to properly partition glucose. So, in yeast, they partition glucose through phosphorylation of Ugp1. Unphosphorylated Ugp1 uses glucose to produce glycogen to store it, and phosphorylated Ugp1 produces UDP-glucose, which is used for cell wall synthesis. And so, PAS kinase can respond to things like cell integrity stress or to nutrient conditions to then partition glucose to the appropriate place at the appropriate time, which is critical for normal growth—to be able to use the glucose efficiently and effectively in order to grow and to survive. Under conditions like SDS treatment, which would be a form of cell integrity stress, you get activation of PAS kinase, phosphorylation of Ugp1, and activation of the downstream effector Rho1. And so, exactly what conditions are able to stimulate this branch of PAS kinase signaling is still not completely clear, I don’t think, and there’s still ongoing questions within the lab about exactly what are kind of the fundamental molecular signals that, number one, lead to PAS kinase activation, and, number two, lead to Ugp1 phosphorylation and then the downstream effects of that. We don’t really know right now if there is some sort of bifurcation in the pathway where you could activate Ugp1 via phosphorylation, which would only activate Rho1 and not lead to an increase in cell wall synthesis. We don’t really know if that is possible at this time. All of our data so far indicates that you phosphorylate Ugp1, it translocates to cell membrane, and there it’s able to do both things—it’s able to increase cell wall synthesis, and it’s able to lead to Rho1 activation.

Interviewer – Annalisa VanHookDr. Rutter mentioned that some of the other genes that you got out of your screen were things that seemed to function upstream of Psk. Are those the sorts of things that might help you figure out what it is that Psk is doing in a normal cell?

Interviewee – Caleb CardonThat is certainly the hope. So, yeah, some of the genes, they seem to fall into kind of a broad category of metabolic function, which is kind of where we had PAS kinase sitting previous to this, but the hope is that by understanding how these genes and proteins are able to lead to activation of PAS kinase, that we might able to better understand or maybe narrow our approach at looking for the specific signal that leads to PAS kinase activation. And exactly how that will flush out in the end isn’t exactly clear at this point, obviously, but there are indications that active metabolism is able to activate PAS kinase.

Interviewer – Annalisa VanHookPsk2 seems to function in signaling that the cell has undergone some sort of cell wall stress.

Interviewee – Jared RutterRight.

Interviewer – Annalisa VanHookAnd you mention in the paper that cell wall stress can also suppress the tor2 phenotype.

Interviewee – Jared RutterThat’s right.

Interviewer – Annalisa VanHookWhat’s the rationale behind cell integrity stress—or damage to the cell wall—feeding into TOR signaling to promote growth? To a first approximation, that seems a little counterintuitive—that cell wall stress maybe should tell the cell to stop growing, that there’s something wrong.

Interviewee – Jared RutterRight. I think this is that’s a very interesting question and, frankly, it’s one that Caleb and I have argued about for a couple of years now. So, I think there are a couple of possibilities. One potential explanation for this is that, obviously, after the cell wall stress happens—after the damage to the cell wall happens—if you’re a yeast cell, at some point your options are to die or to begin to undergo vegetative growth again. And one possibility is that this activation by cell wall stress of PAS kinase is promoting the decision to grow subsequently to the stress. So, in that case, it would be sort of a pro-growth function that would be coordinating with TOR in the broad sense but would be acting in a temporally separated manner, probably. One other possibility—and this is one that I tend to like a little bit more—is that one of the functions of PAS kinase is directly to stimulate cell wall biosynthesis probably through multiple mechanisms—probably direct feeding of glucose to cell wall production, as Caleb mentioned, and potentially also through a signaling mechanism involving Rho that is described in this manuscript—and both of those functions contributing toward cell wall synthesis could be acting to compensate for the damage by exaggerating the cell wall structure generally to hopefully compensate for any damage that might be there. So, in that case, it would not necessarily be a growth phenomenon occurring in the classical sense. It would obviously be the accumulation of biomass, but it would be in more of a stress response role rather than a pro-growth role. And the experiments that we’ve done to date, unfortunately, have not really differentiated between those two. You know, in one case you’re talking about survival, and in another case you’re talking about ongoing growth, and we haven’t really done any experiments to differentiate between those two. And I think there are fairly good arguments for both of them, and we haven’t yet been able to do an experiment that distinguishes between them.

Interviewer – Annalisa VanHookThose two possibilities are—essentially one is sort of like a wound healing response…

Interviewee – Jared RutterExactly.

Interviewer – Annalisa VanHook…and then the other possibility would be a, you know, the cell’s at this crisis point where it’s experienced stress, and it may as well try to grow because it’s going to die if …

Interviewee – Jared RutterRight.

Interviewer – Annalisa VanHook… if the stress is so severe.

Interviewee – Jared RutterRight. I mean that’s one thing that’s important to remember about yeast is that survival doesn’t matter unless you reproduce, right? It’s different from a cell in the human body where there’s a greater purpose, which is to keep the organism alive. In yeast, the organism is the cell. And, if that cell doesn’t eventually start growing again, survival doesn’t matter because the genetic information is not passed on to daughter cells. And so, you know, eventually no matter how bad the stress is, you can devote all the resources you’d like to survival, but if at the end of the day you’re not able to replicate your genome and pass it onto daughter cells, all of that was irrelevant.

Interviewer – Annalisa VanHookThe TOR pathway is sort of viewed as “the” pathway that governs cell size. And, in multicellular organisms, it also governs the size of the whole organism. And so, this work suggests that signaling from these PAS kinases is also really important, at least in yeast, and that it feeds into some of the same responses as TOR signaling.

Interviewee – Jared RutterRight.

Interviewer – Annalisa VanHookWhat’s known about these kinases in animals? Are they cooperating with TOR signaling in mice or humans? Is that known?

Interviewee – Jared RutterYeah, that’s a good question, as well. So, we have published previously that a mouse lacking PAS kinase shares many of the phenotypes as a mouse lacking a kinase called S6 kinase, which is one of the major effectors of mammalian TOR signaling (2). And, specifically, there are effects on the pancreatic beta cells and insulin production and secretion. In the case of both of these animal models, that’s defective. We also observed in our mouse, as was observed in the S6 kinase knockout mouse, that they had an increased metabolic rate and, probably as a result of that, were protected from obesity in response to high-fat diet feeding. So, it’s a rather striking convergence, I think, of the phenotypes. In that paper, we were not able to discern any direct connection between the two pathways—it’s just a correlation of the phenotypes. In subsequent work, which we hope to publish soon, we’ve actually seen more direct connections where we think that these two pathways are directly connecting with one another, integrating in a pretty substantial way. One of the outputs of that is controlling the synthesis of lipids, which is important for cell growth. Every time a cell divides, it needs to completely regenerate its lipid content and membrane content. We think that both PAS kinase and TOR are involved in controlling that pathway and probably do that in an integrated way. And, almost certainly, there’s going to be other ways in which they crosstalk. We’ve previously published in yeast—and we’ve seen some indication of this in mammalian systems, as well—that PAS kinase is involved in controlling protein synthesis, which is obviously a very critical and one of the most well studied outputs of TOR signaling. And so there are probably going to be multiple areas in which the two pathways at least contribute to the same output and, perhaps, in some cases, do so in a manner that’s concerted where they are actually talking to one another.

Interviewer – Annalisa VanHookThank you, Jared and Caleb, for talking with me.

Interviewee – Jared RutterThank you.

Interviewee – Caleb CardonThank you, Annalisa.

Host – Annalisa VanHookThat was Jared Rutter and Caleb Cardon, authors of a Research Article published in the January 31st issue of Science Signaling. That paper, by Cardon and colleagues, is titled “PAS Kinase Promotes Cell Survival and Growth Through Activation of Rho1” (1).


And that wraps up this Science Signaling Podcast. If you have any questions or suggestions, please write to us at sciencesignalingeditors{at}aaas.org. This show is a production of Science Signaling and of AAAS—Advancing Science, Serving Society. I'm Annalisa VanHook, and on behalf of Science Signaling and its publisher, the American Association for the Advancement of Science, thanks for listening.

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