No abstract

While eukaryotic cells have a myriad of membrane-bound organelles enabling the isolation of different chemical environments, prokaryotic cells lack these defined reaction vessels. Biomolecular condensates—organelles that lack a membrane—provide a strategy for cellular organization without a physical barrier while allowing for the dynamic, responsive organization of the cell. It is well established that intrinsically disordered protein domains drive condensate formation via liquid–liquid phase separation; however, the role of globular protein domains on intracellular phase separation remains poorly understood. We hypothesized that the overall charge of globular proteins would dictate the formation and concentration of condensates and systematically probed this hypothesis with supercharged proteins and nucleic acids in E. coli . Within this study, we demonstrated that condensates form via electrostatic interactions between engineered proteins and RNA and that these condensates are dynamic and only enrich specific nucleic acid and protein components. Herein, we propose a simple model for the phase separation based on protein charge that can be used to predict intracellular condensate formation. With these guidelines, we have paved the way to designer functional synthetic membraneless organelles with tunable control over globular protein function.

Target of rapamycin (TOR) kinase is a conserved regulator of cell growth whose activity is modulated in response to nutrients, energy and stress. Key proteins involved in the pathway are conserved in the model photosynthetic microalga Chlamydomonas reinhardtii, but the substrates of TOR kinase and downstream signaling network have not been elucidated. Our study provides a new resource for investigating the phosphorylation networks governed by the TOR kinase pathway in Chlamydomonas. We used quantitative phosphoproteomics to investigate the effects of inhibiting Chlamydomonas TOR kinase on dynamic protein phosphorylation. Wild-type and AZD-insensitive Chlamydomonas strains were treated with TOR-specific chemical inhibitors (rapamycin, AZD8055 and Torin1), after which differentially affected phosphosites were identified. Our quantitative phosphoproteomic dataset comprised 2547 unique phosphosites from 1432 different proteins. Inhibition of TOR kinase caused significant quantitative changes in phosphorylation at 258 phosphosites, from 219 unique phosphopeptides. Our results include Chlamydomonas homologs of TOR signaling-related proteins, including a site on RPS6 with a decrease in phosphorylation. Additionally, phosphosites on proteins involved in translation and carotenoid biosynthesis were identified. Follow-up experiments guided by these phosphoproteomic findings in lycopene beta/epsilon cyclase showed that carotenoid levels are affected by TORC1 inhibition and carotenoid production is under TOR control in algae.

Heterotrimeric G protein complexes are molecular switches relaying extracellular signals sensed by G protein-coupled receptors (GPCRs) to downstream targets in the cytoplasm, which effect cellular responses. In the plant heterotrimeric GTPase cycle, GTP hydrolysis, rather than nucleotide exchange, is the rate-limiting reaction and is accelerated by a receptor-like regulator of G signaling (RGS) protein. We hypothesized that posttranslational modification of the Gα subunit in the G-protein complex regulates the RGSdependent GTPase cycle. Our structural analyses identified an invariant phosphorylated tyrosine residue (Tyr-166 in the Arabidopsis Gα subunit AtGPA1) located in the intramolecular domain interface where nucleotide binding and hydrolysis occurs. We also identified a receptor-like kinase that phosphorylates AtGPA1 in a Tyr-166-dependent manner. Discrete molecular dynamics simulations predicted that phosphorylated Tyr-166 forms a salt bridge in this interface and potentially affects the RGS protein-accelerated GTPase cycle. Using a Tyr-166 phosphomimetic substitution, we found that the cognate RGS protein binds more tightly to the GDP-bound Gα substrate, consequently reducing its ability to accelerate GTPase activity. In conclusion, we propose that phosphorylation of Tyr-166 in AtGPA1 changes the binding pattern with AtRGS1 and thereby attenuates the steady-state rate of the GTPase cycle. We coin this newly identified mechanism "substrate phosphoswitching." __________________________________ http://www.jbc.org/cgi

The identification of dynamic protein phosphorylation events is critical for understanding kinase/phosphatase-regulated signaling pathways. To date, protein phosphorylation and kinase expression have been examined independently in photosynthetic organisms. Here we present a method to study the global kinome and phosphoproteome in tandem in a model photosynthetic organism, the alga Chlamydomonas reinhardtii (Chlamydomonas), using mass spectrometry-based label-free proteomics. A dual enrichment strategy targets intact protein kinases via capture on immobilized multiplexed inhibitor beads with subsequent proteolytic digestion of unbound proteins and peptide-based phosphorylation enrichment. To increase depth of coverage, both data-dependent and data-independent (via SWATH, Sequential Windowed Acquisition of All Theoretical Fragment Ion Mass Spectra) mass spectrometric acquisitions were performed to obtain a more than 50% increase in coverage of the enriched Chlamydomonas kinome over coverage found with no enrichment. The quantitative phosphoproteomic dataset yielded 2250 phosphopeptides and 1314 localized phosphosites with excellent reproducibility across biological replicates (90% of quantified sites with coefficient of variation below 11%). This approach enables simultaneous investigation of kinases and phosphorylation events at the global level to facilitate understanding of kinase networks and their influence in cell signaling events.