Role of PP2A and TOR signaling in the regulation of translation

Cells in our body respond to many signals from the environment that surrounds them, and they listen to those signals in order to decide what to do: to divide, to change or to stay the same. One of the more basic things that the cell has to pay attention to is the availability of nutrients in the medium, as well as other stressful conditions. One of the main processes that will be affected in those cases will be the production of new proteins, what we call translation. This regulation is very important, as cells that do not adapt to these changes grow without control, which can result in the development of cancer. Two of the main systems that are in charge of finding out if there are enough nutrients available is the TOR signaling and the MAPK signaling. These systems are present even in the simplest cell and we can take advantage of this fact to understand how they work. In this project, we investigate how the TOR and MAPK systems together with another protein called PP2A modulates translation in yeast. There are different classes of PP2A and we have created mutant strains where we can control their activity. We have looked at how loss of PP2A activity affects TOR and MAPK signaling, as well as the phosphorylation of proteins involved in protein translation. Importantly, we have seen that different phosphatases control different regulators of translation depending on the stress condition. Therefore, our work highlights the importance of phosphatase activity in the adaptation to environmental stress and nutrient deprivation. An appropriate regulation of the rate of protein production in response to stress has implications in aging and cell proliferation. Hence, this project provides insight into diseases that have in their root deregulation of these processes.

The control of translation is an important part of the process of protein homeostasis or proteostasis, with implications in aging and cancer development. We have found that different PP2A subcomplexes, PP2A-B56 and PP2A-B55, and the PP2A-related phosphatase PP6 control different events in the regulation of protein translation. Ppe1 prevents the untimely activation of eIF2alpha kinases, whereas PP2A-B56 directly counteracts eIF2alpha phosphorylation. PP2A-B55 regulates the phosphorylation status of a newly described 4E-BP (Feb1). During this period, we have published a review and we have a manuscript recently accepted for publication in iScience (not included in the performance indicators). We are also preparing a manuscript on our findings on Feb1. This project will have an impact in the field of pretostasis as we highlight the importance of phosphatase activity for the timely and accurate control of protein translation, adding a new layer of regulation to this complex process.

This project aims at studying the implications of PP2A activity as a mediator of TOR signaling in the regulation of translation. The two TOR complexes play a central role integrating growth-promoting signals and conveying them to the cellular machinery regulating metabolism, transcription and protein translation as well as cell cycle progression. Protein translation, being a high-energy demanding process, needs to be tightly regulated when nutrient sources are scarce, and silencing of TORC1 activity is a key step for this control. In this proposal we use fission yeast as a model system, because it is a simpler and more amenable organism compared to higher eukaryotes and it still conserves all the components of the TOR signaling. Our main interest in the laboratory is the study of protein phosphatases (especially PP2A) in the regulation of cell growth and division. Previous studies in the laboratory have shown that PP2A-B55 functions downstream of TORC1 regulating the transcriptional response to nutritional stress. We have also preliminary data suggesting that PP2A also regulates protein translation in this context. Our main goal is to understand how PP2A and TORC1 work to modulate protein synthesis in the response to nutrient deprivation. For this aim, we have divided this project in three parts. The first one focuses on the generation of a toolset to conditionally modulate the activity of the different TOR complexes as well as of the different PP2A subcomplexes. In this part we also aim at creating a translation reporter system to evaluate general changes in translation. In the second part we want to perform ribosome profiling of the mutants generated in the first point in order to get a full overview of the changes in translation. The last part is a proteomic study of PP2A interacting proteins with the goal to identify the mechanisms leading to translation regulation.