Project description:Stress granules are dynamic non-membrane bound organelles made up of untranslating messenger ribonucleoproteins (mRNPs) that form when cells integrate stressful environmental cues resulting in stalled translation initiation complexes. Although stress granules dramatically alter mRNA and protein localization, understanding these complexes has proven to be challenging through conventional imaging, purification, and crosslinking approaches. We therefore developed an RNA proximity labeling technique, APEX-Seq, which uses the ascorbate peroxidase APEX2 to probe the spatial organization of the transcriptome. We show that APEX-Seq can resolve the localization of RNAs within the cell and determine their enrichment or depletion near key RNA-binding proteins. Matching both the spatial transcriptome using APEX-seq, and the spatial proteome using APEX-mass spectrometry (APEX-MS) provide new insights into the organization of translation initiation complexes on active mRNAs, as well as revealing unanticipated complexity in stress granule contents, and provides a powerful approach to explore the spatial environment of macromolecules.
Project description:Although stress granules (SGs) are composed of a stable core structure, they are surrounded by a dynamic shell with assembly, disassembly, and transitions of proteins and RNA between the core and shell. Different SGs have distinct proteins and RNA constituents, which raises the possibility that different SGs might perform different biological functions. The RNA compositions of DDX3X- or DDX3Y-positive SGs are not known, nor is it known what differential effects DDX3X or DDX3Y may exert on these RNA components. To understand the biological function and the compositional differences between DDX3X- or DDX3Y-positive SGs, we first determined the RNA constituents in these SGs using an adapted Ascorbate-peroxidase (APEX)-based proximity labeling method. APEX converts exogenously supplied biotin-phenol (BP) to biotin-phenoxyl radicals, which in turn covalently labels protein and RNA in nanometers. APEX-mediated biotinylation has been widely used in studies of protein-protein interactions and recently in studying protein-RNA interactions. Here, we applied the APEX-mediated biotinylation in DDX3X and DDX3Y SGs to dissect their different RNA composition.
Project description:We present MERR APEX-seq, a method for newly transcribed RNAs subcellular profiling combined metabolic incorporation of electron-rich ribonucleosides, 6-thioguanosine and 4-thiouridine, with the peroxidase-mediated RNA labeling method, APEX-seq. MERR APEX-seq offers both high spatial specificity and high coverage in the mitochondrial matrix and at the endoplasmic reticulum membrane. Application of MERR APEX-seq at nuclear lamina of human cells reveals that the mRNA components tend to encode for transcripts processing related proteins. MERR APEX-seq with high spatial specificity and high coverage could be widely used to expand our knowledge of RNA localization and function at subcellular compartments.
Project description:We introduce APEX-seq, a method for RNA sequencing based on direct proximity labeling of RNA using the peroxidase enzyme APEX2. APEX-seq in nine distinct subcellular locales produced a nanometer-resolution spatial map of the human transcriptome as a resource, revealing extensive patterns of localization for diverse RNA classes and transcript isoforms. We uncover a radial organization of the nuclear transcriptome, which is gated at the inner surface of the nuclear pore for cytoplasmic export of processed transcripts. We identify two distinct pathways of messenger RNA localization to mitochondria, each associated with specific sets of transcripts for building complementary macromolecular machines within the organelle. APEX-seq should be widely applicable to many systems, enabling comprehensive investigations of the spatial transcriptome.
Project description:We sought to map RBM6 interactome using ascorbate peroxidase (APEX2)-based proximity labelling combined with mass spectrometry. Toward this end, we used CRISPR-cas9 methodology to establish MCF10A cell line expressing APEX2 fused to the N-terminal of RBM6, hereafter called MCF10AAPEX2-RBM6. The peroxidase activity of APEX2-RBM6 fusion was confirmed. Next, RBM6 proximal proteins that were biotinylated by APEX2 activation using hydrogen peroxidase (H2O2) in the presence of biotin-phenol were isolated and subjected to mass spectrometry. We identified 173 (P-value<0.05) RBM6 proximity-interaction partners.
Project description:To address specificity of ALaP-seq for PMLwt, we performed genome-wide profiling of genomic regions associated with NLS-APEX or PMLca-APEX. Cells were treated with H2O2 to trigger labeling of chromatin with biotin (H2O2+). Experimental negative control for PMLca (H2O2-), where the H2O2 treatment was omitted, was also analyzed.
Project description:The intimate association between the endoplasmic reticulum (ER) and mitochondrial membranes at ER-mitochondria contact sites (ERMCS) serves as a platform for several critical cellular processes, particularly lipid synthesis. How contacts are remodeled and the subsequent biological consequences of altered contacts such as perturbed lipid metabolism remains poorly understood. Here we show that the p97 AAA-ATPase and its ER-tethered ubiquitin-X domain adaptor 8 (UBXD8) regulate the prevalence of ERMCS. The p97-UBXD8 complex localizes to contacts and its loss increases contacts in a manner that is dependent on p97 catalytic activity. Quantitative proteomics and lipidomics of ERMCS demonstrates alterations in proteins regulating lipid metabolism and a significant change in saturated or monounsaturated lipid species in UBXD8 knockout cells. We show that loss of p97-UBXD8 results in perturbed contacts due to an increase in membrane lipid saturation via SREBP1 and the lipid desaturase SCD1. These aberrant contacts can be rescued by supplementation with unsaturated fatty acids or overexpression of SCD1. Notably, we find that the SREBP1-SCD1 pathway is negatively impacted in the brains of mice with p97 mutations that cause neurodegeneration. Our results suggest that contacts are exquisitely sensitive to alterations to membrane lipid composition and saturation in a p97-UBXD8 dependent manner.
Project description:The cytosol-facing membranes of cellular organelles contain proteins that enable signal transduction, regulation of morphology and trafficking, protein import and export, and other specialized processes. Discovery of these proteins by traditional biochemical fractionation can be plagued with contaminants and loss of key components. Using peroxidase-mediated proximity biotinylation, we captured and identified endogenous proteins on the outer mitochondrial membrane (OMM) and endoplasmic reticulum membrane (ERM) of living human fibroblasts. The proteomes of 137 and 634 proteins, respectively, are highly specific and highlight 94 potentially novel mitochondrial or ER proteins. Dataset intersection identified protein candidates potentially localized to mitochondria-ER contact sites. We found that one candidate, the tail-anchored, PDZ-domain-containing OMM protein SYNJ2BP, dramatically increases mitochondrial contacts with rough ER when overexpressed. Immunoprecipitation-mass spectrometry identified ribosome-binding protein 1 (RRBP1) as SYNJ2BPs ERM binding partner. Our results highlight the power of proximity biotinylation to yield insights into the molecular composition and function of intracellular membranes.
Project description:NIMA-related kinase 9 (NEK9) was identified to interact with the essential myosin light chain (ELC). To validate these findings ascorbate peroxidase (APEX) catalyzed proximity labeling (Hung et al. Nat Protoc. 2016) was performed. To do so, human ELC fused to APEX was overexpressed in HEK293 cells. Empty pcDNA3-APEX2-NES vector serves as negative control. Half of each sample was treated with H2O2, which catalyzes protein labeling in close proximity of ELC. After streptavidin pull down of labeled proteins, the samples were analyzed by mass spectrometry.