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The CryoAPEX Method for Electron Microscopy Analysis of Membrane Protein Localization Within Ultrastructurally-Preserved Cells

Key cellular events like signal transduction and membrane trafficking rely on proper protein location within cellular compartments. Understanding precise ...

Purdue University

The maintenance of endoplasmic reticulum (ER) homeostasis is a critical aspect of determining cell fate and requires a properly functioning unfolded protein response (UPR). We have discovered a previously unknown role of a post-translational modification termed adenylylation/AMPylation in regulating signal transduction events during UPR induction. A family of enzymes, defined by the presence of a Fic (filamentation induced by cAMP) domain, catalyzes this adenylylation reaction. The human genome encodes a single Fic protein, called HYPE (Huntingtin yeast interacting protein E), with adenylyltransferase activity but unknown physiological target(s). Here, we demonstrate that HYPE localizes to the lumen of the endoplasmic reticulum via its hydrophobic N terminus and adenylylates the ER molecular chaperone, BiP, at Ser-365 and Thr-366. BiP functions as a sentinel for protein misfolding and maintains ER homeostasis. We found that adenylylation enhances BiP's ATPase activity, which is required for refolding misfolded proteins while coping with ER stress. Accordingly, HYPE expression levels increase upon stress. Furthermore, siRNA-mediated knockdown of HYPE prevents the induction of an unfolded protein response. Thus, we identify HYPE as a new UPR regulator and provide the first functional data for Fic-mediated adenylylation in mammalian signaling. 

A novel link between Fic (filamentation induced by cAMP)-mediated adenylylation/AMPylation and the unfolded protein response.

Pasteurella multocida causes respiratory-tract infections in a broad range of animals, as well as opportunistic infections in humans. P. multocida secretes a multidomain toxin called PfhB2, which contains a YopT-like cysteine protease domain at its C-terminus. The YopT domain of PfhB2 contains a well conserved Cys-His-Asp catalytic triad that defines YopT family members, and shares high sequence similarity with the prototype YopT from Yersinia sp. To date, only one crystal structure of a YopT family member has been reported; however, additional structural information is needed to help characterize the varied substrate specificity and enzymatic action of this large protease family. Here, a catalytically inactive C3733S mutant of PfhB2 YopT that provides enhanced protein stability was used with the aim of gaining structural insight into the diversity within the YopT protein family. To this end, the C3733S mutant of PfhB2 YopT has been successfully cloned, overexpressed, purified and crystallized. Diffraction data sets were collected from native crystals to 3.5 Å resolution and a single-wavelength anomalous data set was collected from an iodide-derivative crystal to 3.2 Å resolution. Data pertaining to crystals belonging to space group P3, with unit-cell parameters a = 136.9, b = 136.9, c = 74.7 Å for the native crystals and a = 139.2, b = 139.2, c = 74.7 Å for the iodide-derivative crystals, are discussed.

YopT domain of the PfhB2 toxin from Pasteurella multocida: protein expression, characterization, crystallization and crystallographic analysis.

We describe a method, termed cryoAPEX, which couples chemical fixation and high-pressure freezing of cells with peroxidase tagging (APEX) to allow precise localization of membrane proteins in the context of a well-preserved subcellular membrane architecture. Further, cryoAPEX is compatible with electron tomography. As an example, we apply cryoAPEX to obtain a high-resolution three-dimensional contextual map of the human FIC (filamentation induced by cAMP) protein, HYPE (also known as FICD). HYPE is a single-pass membrane protein that localizes to the endoplasmic reticulum (ER) lumen and regulates the unfolded protein response. Alternate cellular locations for HYPE have been suggested. CryoAPEX analysis shows that, under normal and/or resting conditions, HYPE localizes robustly within the subdomains of the ER and is not detected in the secretory pathway or other organelles. CryoAPEX is broadly applicable for assessing both lumenal and cytosol-facing membrane proteins.

CryoAPEX - an electron tomography tool for subcellular localization of membrane proteins.

During disease, cells experience various stresses that manifest as an accumulation of misfolded proteins and eventually lead to cell death. To combat this stress, cells activate a pathway called unfolded protein response that functions to maintain endoplasmic reticulum (ER) homeostasis and determines cell fate. We recently reported a hitherto unknown mechanism of regulating ER stress via a novel post-translational modification called Fic-mediatedadenylylation/AMPylation. Specifically, we showed that the human Fic (filamentation induced by cAMP) protein, HYPE/FicD, catalyzes the addition of an adenosine monophosphate (AMP) to the ER chaperone, BiP, to alter the cell's unfolded protein response-mediated response to misfolded proteins. Here, we report that we have now identified a second target for HYPE-alpha-synuclein (αSyn), a presynaptic protein involved in Parkinson's disease. Aggregated αSyn has been shown to induce ER stress and elicit neurotoxicity in Parkinson's disease models. We show that HYPE adenylylates αSyn and reduces phenotypes associated with αSyn aggregation invitro, suggesting a possible mechanism by which cells cope with αSyn toxicity.

Alpha-Synuclein Is a Target of Fic-Mediated Adenylylation/AMPylation: Possible Implications for Parkinson's Disease.

Cortactin is a Src tyrosine phosphorylation substrate that regulates multiple actin-related cellular processes. While frequently studied in nonneuronal cells, the functions of cortactin in neuronal growth cones are not well understood. We recently reported that cortactin mediates the effects of Src tyrosine kinase in regulating actin organization and dynamics in both lamellipodia and filopodia of  growth cones. Here, we identified a single cortactin tyrosine phosphorylation site (Y499) to be important for the formation of filopodia. Overexpression of a 499F phospho-deficient cortactin mutant decreased filopodia length and density, whereas overexpression of a 499E phospho-mimetic mutant increased filopodia length. Using an antibody against cortactin pY499, we showed that tyrosine-phosphorylated cortactin is enriched along the leading edge. The leading edge localization of phosphorylated cortactin is Src2-dependent, F-actin-independent, and important for filopodia formation. In vitro kinase assays revealed that Src2 phosphorylates cortactin at Y499, although Y505 is the preferred site in vitro. Finally, we provide evidence that Arp2/3 complex acts downstream of phosphorylated cortactin to regulate density but not length of filopodia. In conclusion, we have characterized a tyrosine phosphorylation site in  cortactin that plays a major role in the Src/cortactin/Arp2/3 signaling pathway controlling filopodia formation.

A single tyrosine phosphorylation site in cortactin is important for filopodia formation in neuronal growth cones.

Studies have successfully elucidated the mechanism of action of several effector domains that comprise the multifunctional-autoprocessing repeats-in-toxins (MARTX) toxins of Vibrio vulnificus. However, the biochemical linkage between the cysteine proteolytic activity of Makes Caterpillars Floppy (MCF)-like effector and its cellular effects remains unknown. In this study, we identify the host cell factors that activate in vivo and in vitro MCF autoprocessing as adenosine diphosphate (ADP)-Ribosylation Factor 1 (ARF1) and ADP-Ribosylation Factor 3 (ARF3). Autoprocessing activity is enhanced when ARF1 is in its active [guanosine triphosphate (GTP)-bound] form compared to the inactive [guanosine diphosphate (GDP)-bound] form. Subsequent to auto-cleavage, MCF is acetylated on its exposed N-terminal glycine residue. Acetylation apparently does not dictate subcellular localization as MCF is found localized throughout the cell. However, the cleaved form of MCF gains the ability to bind to the specialized lipid phosphatidylinositol 5-phosphate enriched in Golgi and other membranes necessary for endocytic trafficking, suggesting that a fraction of MCF may be subcellularly localized. Traditional thin-section electron microscopy, high-resolution cryoAPEX localization, and fluorescent microscopy show that MCF causes Golgi dispersal resulting in extensive vesiculation. In addition, host mitochondria are disrupted and fragmented. Mass spectrometry analysis found no reproducible modifications of ARF1 suggesting that ARF1 is not post-translationally modified by MCF. Further, catalytically active MCF does not stably associate with ARF1. Our data indicate not only that ARF1 is a cross-kingdom activator of MCF, but also that MCF may mediate cytotoxicity by directly targeting another yet to be identified protein. This study begins to elucidate the biochemical activity of this important domain and gives insight into how it may promote disease progression.

N-terminal autoprocessing and acetylation of multifunctional-autoprocessing repeats-in-toxins (MARTX) Makes Caterpillars Floppy-like effector is stimulated by adenosine diphosphate (ADP)-Ribosylation Factor 1 in advance of Golgi fragmentation.

Seema Mattoo

Department of Biological Sciences,
Bindley Biosciences Center,
Purdue Institute of Integrative Neuroscience,
Purdue Center for Cancer Research

Purdue University

The CryoAPEX Method for Electron Microscopy Analysis of Membrane Protein Localization Within Ultrastructurally-Preserved Cells

Seema Mattoo

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Biological Sciences,Bindley Biosciences Center,Purdue Institute of Integrative Neuroscience,Purdue Center for Cancer Research

Purdue University

The CryoAPEX Method for Electron Microscopy Analysis of Membrane Protein Localization Within Ultrastructurally-Preserved Cells

Department of Biological Sciences,
Bindley Biosciences Center,
Purdue Institute of Integrative Neuroscience,
Purdue Center for Cancer Research