Autophagic Punctum
Autophagy 6:2, 294-295; February 16, 2010; © 2010 Landes Bioscience
The Tor and cAMP-dependent protein kinase signaling pathways coordinately control autophagy in Saccharomyces cerevisiae Joseph S. Stephan,1,2 Yuh-Ying Yeh,1,2 Vidhya Ramachandran,1,2 Stephen J. Deminoff1 and Paul K. Herman1,2,* Department of Molecular Genetics; 2Program in Molecular, Cellular and Developmental Biology; The Ohio State University; Columbus, OH USA
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Key words: Atg1, macroautophagy, phagophore assembly structure, Atg13, nutritional control Submitted: 12/18/09 Revised: 12/30/09 Accepted: 01/06/10 Previously published online: www.landesbioscience.com/journals/ autophagy/article/11129 *Correspondence to: Paul K. Herman; Email:
[email protected] Punctum to: Stephan JS, Yeh YY, Ramachandran V, Deminoff SJ, Herman PK. The Tor and PKA signaling pathways independently target the Atg1/Atg13 protein kinase complex to control autophagy. Proc Natl Acad Sci USA 2009; 106:17049–54; PMID: 19805182; DOI: 10.1073/ pnas.0903316106
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acroautophagy (hereafter autophagy) is a conserved membrane trafficking pathway responsible for the turnover of cytosolic protein and organelles during periods of nutrient deprivation. This pathway is also linked to a number of processes important for human health, including tumor suppression, innate immunity and the clearance of protein aggregates. As a result, there is tremendous interest in autophagy as a potential point of therapeutic intervention in a variety of pathological states. To achieve this goal, it is imperative that we develop a thorough understanding of the normal regulation of this process in eukaryotic cells. The Tor protein kinases clearly constitute a key element of this control as Tor activity inhibits this degradative process in all organisms examined, from yeast to man. Here, we discuss recent work indicating that the cAMP-dependent protein kinase (PKA) also plays a critical role in controlling autophagy in the budding yeast, Saccharomyces cerevisiae. A model describing how PKA activity might influence this degradative process, and how this control might be integrated with that of the Tor pathway, is presented. Previous work indicates that autophagy in S. cerevisiae is regulated by a number of signaling pathways responsible for coordinating cell growth with nutrient availability. These pathways all appear to target a complex of proteins that contains that Atg1 protein kinase and a key regulator of this enzyme, Atg13. For example, Ohsumi and colleagues have shown that a specific Tor complex, known as TORC1, directly
Autophagy
phosphorylates Atg13 and prevents its association with Atg1. This interaction is important for the activation of Atg1 and thus, the induction of autophagy. In the present study, we show that PKA also targets this complex of proteins and thereby inhibits the autophagy process. In particular, we find that PKA directly phosphorylates Atg13 and that this phosphorylation inhibits the ability of this protein to associate with the phagophore assembly site, or PAS. The PAS is the cytoplasmic nucleation site from which the autophagosome transport intermediate is formed during periods of nutrient deprivation. In contrast, this PKA phosphorylation does not have an effect upon the Atg13 interaction with Atg1. These results therefore suggest that the PKA and Tor signaling pathways have distinct effects upon the autophagy process. Whereas Tor signaling appears to influence both PAS association and the Atg1-Atg13 interaction, PKA activity affects only the former. Therefore, upon the inactivation of Tor signaling, Atg1 would be found in an active form at the PAS and autophagy would be induced. It is less apparent how Atg1 is activated following the loss of PKA signaling, although it is possible that the high concentration of Atg1 and Atg13 at the PAS might override the inhibitory effects of Tor activity or that Atg13 might be selectively dephosphorylated at this site. Further work is needed to distinguish between these and other possibilities. Despite these differences, we find that the inactivation of the PKA pathway results in an induction of autophagy that is similar in magnitude to that brought on by a shutdown of Tor signaling.
Volume 6 Issue 2
Autophagic Punctum
Autophagic Punctum
Figure 1. A model proposing that the PKA and Tor pathways independently control autophagy via the Atg1 complex of proteins in S. cerevisiae. For the sake of simplicity, the Tor and PKA pathways are shown to be responding specifically to the levels of the nitrogen and carbon sources in the environment, respectively. However, it is important to point out that the situation in vivo is likely to be somewhat more complicated. See the text for more details.
Interestingly, these pathways appear to function independently of each other as the simultaneous inactivation of both leads to an increased and more rapid autophagy response. This interpretation is supported by observations indicating that the PKA and Tor kinases both phosphorylate Atg13 but appear to do so at distinct sites within
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this protein. Therefore, the Atg1 complex appears to represent an important point of signal integration within the autophagy pathway (Fig. 1). Since this complex is also targeted by the Tor pathway in metazoans, it is possible that a central role for Atg1 in the regulation of autophagy has been conserved throughout eukaryotic evolution. Previous studies suggest that the flux through the autophagy pathway can be rather high when this process is fully engaged. As a result, it is likely imperative that autophagy is repressed in dividing cells so as to prevent the unwanted turnover of material needed for the ongoing proliferation. It is not obvious, however, why the cell might need multiple pathways to accomplish this task. One intriguing explanation is that each of these pathways is responding to a different nutritional signal and is therefore responsible for coordinating autophagy activity with the relative availability of the corresponding metabolite (Fig. 1). The effects of deprivation will likely vary for different metabolites and might therefore require distinct types of autophagy responses. This possibility is consistent with present data indicating that PKA is primarily responding to the carbon source present in the environment and that the Tor pathway is largely
Autophagy
responsive to nitrogen levels. Although this depiction is almost certainly an oversimplification, it is very likely that these pathways are responding to different signals in yeast cells. One advantage of having a system with multiple regulatory inputs is that the cell could specifically tailor its autophagic response to the particular environmental conditions present. This control could be exclusively affecting the traditional macroautophagy pathway that we are discussing here, or could also involve other autophagy-based processes, such as microautophagy or a novel form of macroautophagy like that which was described recently in mammals. The key is that the cell could use this system to bring about rather nuanced responses to the different nutritional conditions it might encounter. Therefore, to fully understand this control in S. cerevisiae, it will be important to examine the autophagy response elicited by the loss of PKA activity with additional assays, including electron microscopy. Such analyses will hopefully provide new insights into the control of the autophagy process. Acknowledgements
This work was supported by National Institutes of Health Grant GM65227 to P.K.H.
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