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Abstract Maintenance of water homeostasis is a fundamental cellular process required by all living organisms. Here, we use the single-celled green algaChlamydomonas reinhardtiito establish a foundational understanding of osmotic-stress signaling pathways through transcriptomics, phosphoproteomics, and functional genomics approaches. Comparison of pathways identified through these analyses with yeast and Arabidopsis allows us to infer their evolutionary conservation and divergence across these lineages. 76 genes, acting across diverse cellular compartments, were found to be important for osmotic-stress tolerance in Chlamydomonas through their functions in cytoskeletal organization, potassium transport, vesicle trafficking, mitogen-activated protein kinase and chloroplast signaling. We show that homologs for five of these genes have conserved functions in stress tolerance in Arabidopsis and reveal a novel PROFILIN-dependent stage of acclimation affecting the actin cytoskeleton that ensures tissue integrity upon osmotic stress. This study highlights the conservation of the stress response in algae and land plants, and establishes Chlamydomonas as a unicellular plant model system to dissect the osmotic stress signaling pathway. 

Abstract Over the past two decades, mass spectrometric (MS)-based proteomics technologies have facilitated the study of signaling pathways throughout biology. Nowhere is this needed more than in plants, where an evolutionary history of genome duplications has resulted in large gene families involved in posttranslational modifications and regulatory pathways. For example, at least 5% of the Arabidopsis thaliana genome (ca. 1,200 genes) encodes protein kinases and protein phosphatases that regulate nearly all aspects of plant growth and development. MS-based technologies that quantify covalent changes in the side-chain of amino acids are critically important, but they only address one piece of the puzzle. A more crucially important mechanistic question is how noncovalent interactions—which are more difficult to study—dynamically regulate the proteome’s 3D structure. The advent of improvements in protein 3D technologies such as cryo-electron microscopy, nuclear magnetic resonance, and X-ray crystallography has allowed considerable progress to be made at this level, but these methods are typically limited to analyzing proteins, which can be expressed and purified in milligram quantities. Newly emerging MS-based technologies have recently been developed for studying the 3D structure of proteins. Importantly, these methods do not require protein samples to be purified and require smaller amounts of sample, opening the wider proteome for structural analysis in complex mixtures, crude lysates, and even in intact cells. These MS-based methods include covalent labeling, crosslinking, thermal proteome profiling, and limited proteolysis, all of which can be leveraged by established MS workflows, as well as newly emerging methods capable of analyzing intact macromolecules and the complexes they form. In this review, we discuss these recent innovations in MS-based “structural” proteomics to provide readers with an understanding of the opportunities they offer and the remaining challenges for understanding the molecular underpinnings of plant structure and function. 

Coronaviruses, like severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), encode a nucleotidyl transferase in the N-terminal (NiRAN) domain of the nonstructural protein (nsp) 12 protein within the RNA dependent RNA polymerase. Here we show the detection of guanosine monophosphate (GMP) and uridine monophosphate-modified amino acids in nidovirus proteins using heavy isotope-assisted mass spectrometry (MS) and MS/MS peptide sequencing. We identified lysine-143 in the equine arteritis virus (EAV) protein, nsp7, as a primary site of in vitro GMP attachment via a phosphoramide bond. In SARS-CoV-2 replicase proteins, we demonstrate nsp12-mediated nucleotidylation of nsp7 lysine-2. Our results demonstrate new strategies for detecting GMP-peptide linkages that can be adapted for higher throughput screening using mass spectrometric technologies. These data are expected to be important for a rapid and timely characterization of a new enzymatic activity in SARS-CoV-2 that may be an attractive drug target aimed at limiting viral replication in infected patients. 

Abstract Coronaviruses, like severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), encode a nucleotidyl transferase in the N-terminal (NiRAN) domain of thenonstructuralprotein (nsp) 12 protein within the RNA dependent RNA polymerase. Here we show the detection of guanosine monophosphate (GMP) and uridine monophosphate-modified amino acids in nidovirus proteins using heavy isotope-assisted mass spectrometry (MS) and MS/MS peptide sequencing. We identified lysine-143 in the equine arteritis virus (EAV) protein, nsp7, as a primary site of in vitro GMP attachment via a phosphoramide bond. In SARS-CoV-2 replicase proteins, we demonstrate nsp12-mediated nucleotidylation of nsp7 lysine-2. Our results demonstrate new strategies for detecting GMP-peptide linkages that can be adapted for higher throughput screening using mass spectrometric technologies. These data are expected to be important for a rapid and timely characterization of a new enzymatic activity in SARS-CoV-2 that may be an attractive drug target aimed at limiting viral replication in infected patients. 

Review article in a peer reviewed book.