Mukherjee Lab UCSF

Research

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LEGIONELLA PNEUMOPHILA

a master cell biologist

 

Legionella pneumophila is a gram-negative, intracellular bacterial pathogen. Although species within the Legionella genus typically use water-dwelling amoebae and other phagocytic protists as hosts, several species of Legionella, including Legionella pneumophila, are capable of forming a replicative niche within human alveolar (lung) macrophages when aerosolized. This can lead to a severe atypical pneumonia called Legionnaires’ disease in immunocompromised or elderly individuals. It is worth noting that non-immunocompromised individuals rapidly detect Legionella and clear them before an infection takes root.

Due to its ability to infect a wide variety of eukaryotic host cells, Legionella pneumophila is an excellent eukaryotic cell biologist. It uses an impressive arsenal of over 300 effector proteins, secreted through a Type IV secretion system, to subvert various host cell processes. In a matter of seconds after internalization, L. pneumophila blocks trafficking down the endo-lysosomal pathway and begins recruiting ER-derived vesicles to form a niche for itself, deemed the Legionella-containing vacuole (LCV). Over the next few hours of infection, Legionella turns the LCV into a highly specialized organelle, capable of supporting bacterial replication. This is a remarkable feat, and many aspects of how Legionella accomplishes this have yet to be discovered. Read about our active projects below!

 
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Active Projects in the Lab

The UPR is mediated by three major sensors on the surface of the ER: IRE1, PERK, and ATF6. In the presence of unfolded proteins, the ER resident chaperone, BiP, dissociates from these UPR sensors, which contributes to their activation and downstream cellular responses, which include expression of protein chaperones and ERAD. Bacterial pathogens both activate (green) and inhibit (red) all three branches of the UPR. Source: Cornejo E, Schlaermann P, Mukherjee, S. How to rewire the host cell: A home improvement guide for intracellular bacteria. J Cell Biol. 2017 Nov 2. doi:10.1083/jcb.201701095.

LEgionella and the Unfolded Protein Response (UPR)

During the course of infection, Legionella establishes a replicative niche derived from the endoplasmic reticulum (ER) of the host cell, a vital organelle regulating protein folding and lipid biosynthesis. The maintenance of ER homeostasis is crucial for cell survival. In response to accumulation of unfolded proteins or lipid stress, the ER activates the unfolded protein response (UPR), a set of responses that evolved to restore homeostasis in the cell. We and others recently reported that Legionella manipulates this important homeostatic stress signaling pathway to its benefit. Several projects in the lab focus on understanding the mechanisms and consequences of the manipulation of the UPR by Legionella.

Relevant Papers published by the Mukherjee Lab:

Treacy-Abarca S & Mukherjee S. Legionella suppresses the host unfolded protein response via multiple mechanisms. Nat. Commun. 2015 July 29. doi: 10.1038/ncomms.8887. 


Legionella and Trafficking

In order to form its intracellular, membrane-enclosed niche, Legionella must manipulate many aspects of vesicle traffic in the cell. Legionella famously intercepts ER-Golgi traffic through the manipulation of Rab1 and Arf1 small GTPases. For both proteins, Legionella encodes LCV-bound effectors with guanine exchange factor (GEF) activity, allowing GDP-bound Rab1 and Arf1 to be extracted from the cytosol and activated with GTP. Soon after internalization, Legionella also inhibits fusion of the early endosome with lysosomes. There are many open questions as to how Legionella manipulates trafficking and membrane fusion processes, and several projects in the lab are focused on understanding this subversion better.

Relevant papers by the Mukherjee Lab:

Levin RS, Hertz NT, Burlingame AL, Shokat KM*, Mukherjee, S*. Innate immunity kinase TAK1 phosphorylates Rab1 on a hotspot for posttranslational modifications by host and pathogen. Proc Natl Acad Sci USA. 2016 Aug 16. doi:10.1073/pnas.1608355113. * Co-corresponding author

Horenkamp FA*, Mukherjee S*, Alix E*, Schauder CM, Hubber AM, Roy CR, Reinisch KM. Legionella pneumophila subversion of host vesicular transport by SidC effector proteins. Traffic. 2014 May;15(5):488-99. doi: 10.1111/tra12158. * Equal Contribution

Campanacci V, Mukherjee S, Roy CR, Cherfils J. Structure of the Legionella effector AnkX reveals the mechanism of phosphocholine transfer by the FIC domain. EMBO J. 2013 May 15;32(10):1469-77. doi: 10.1038/emboj.2013.82. 

Mukherjee S*, Liu X*, Arasaki K, McDonough J, Galán JE, Roy CR. Modulation of Rab GTPase function by a protein phosphocholine transferase. Nature. 2011 Aug 7;477(7362):103-6. doi: 10.1038/nature10335. * Equal Contribution 

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Legionella and Post-Translational Modifications

Of the ~300 secreted effectors, many are capable of adding targeted post-translational modifications (PTMs) to host proteins. Legionella encodes many kinases, phosphatases, ubiquitin E3 ligases, deubiquitinases, glucosyltranferases, phosphocholine transferases, AMPylases, deAMPylases, methyltransferases, and many other PTM modifiers that have yet to be discovered. Legionella-mediated PTMs manipulate proteins involved in all aspects of cell biology, including trafficking, protein translation, chromatin packaging, RNA transcription/splicing, metabolism, cytoskeletal structure, and immunity. In some cases, studying the PTMs added by Legionella has led to the discovery of host proteins capable of similar PTMs in an uninfected context.

Several projects in the lab involve investigating these PTMs and discerning their roles in manipulation of cellular biology. Collaboration with the Krogan and Shokat labs at UCSF has helped us understand these PTMs on a broader scale.

Relavant Papers by the Mukherjee Lab:

Moss MS, Taylor IR, Ruggero D, Gestwicki JE, Shokat KM*, and Mukherjee S*. A Legionella pneumophila kinase phosphorylates the Hsp70 chaperone family to inhibit eukaryotic protein synthesis. Cell Host Microbe2019 Feb 6. pii: S1931-3128(19)30043-5. doi: 10.1016/j.chom.2019.01.006. [Epub ahead of print] * Co-corresponding author

Levin RS, Hertz NT, Burlingame AL, Shokat KM*, Mukherjee, S*. Innate immunity kinase TAK1 phosphorylates Rab1 on a hotspot for posttranslational modifications by host and pathogen. Proc Natl Acad Sci USA. 2016 Aug 16. doi:10.1073/pnas.1608355113. * Co-corresponding author

Treacy-Abarca S & Mukherjee S. Legionella suppresses the host unfolded protein response via multiple mechanisms. Nat. Commun. 2015 July 29. doi: 10.1038/ncomms.8887. 

Horenkamp FA*, Mukherjee S*, Alix E*, Schauder CM, Hubber AM, Roy CR, Reinisch KM. Legionella pneumophila subversion of host vesicular transport by SidC effector proteins. Traffic. 2014 May;15(5):488-99. doi: 10.1111/tra12158. * Equal Contribution

Campanacci V, Mukherjee S, Roy CR, Cherfils J. Structure of the Legionella effector AnkX reveals the mechanism of phosphocholine transfer by the FIC domain. EMBO J. 2013 May 15;32(10):1469-77. doi: 10.1038/emboj.2013.82. 

Mukherjee S*, Liu X*, Arasaki K, McDonough J, Galán JE, Roy CR. Modulation of Rab GTPase function by a protein phosphocholine transferase. Nature. 2011 Aug 7;477(7362):103-6. doi: 10.1038/nature10335. * Equal Contribution