Simplified Enrichment of Plasma Membrane Proteins from Arabidopsis thaliana Seedlings Using Differential Centrifugation and Brij-58 Treatment

Chloroplast envelope K+/H+ antiporters are involved in cytosol pH regulation

Variations in light intensity induce cytosol pH changes in photosynthetic tissues, providing a possible signal to adjust a variety of biochemical, physiological and developmental processes to the energy status of the cells. It was shown that these pH changes are partially due to the transport of protons in or out of the thylakoid lumen. However, the ion transporters in the chloroplast that transmit these pH changes to the cytosol are not known. KEA1 and KEA2 are K+/H+ antiporters in the chloroplast inner envelope that adjust stromal pH in light-to-dark transitions. We previously determined that stromal pH is higher in kea1kea2 mutant cells. In this study, we now show that KEA1 and KEA2 are required to attenuate cytosol pH variations upon sudden light intensity changes in leaf mesophyll cells, showing they are important components of the light-modulated pH signalling module. The kea1kea2 mutant mesophyll cells also have a considerably less negative membrane potential. Membrane potential is dependent on the activity of the plasma membrane proton ATPase and is regulated by secondary ion transporters, mainly potassium channels in the plasma membrane. We did not find significant differences in the activity of the plasma membrane proton pump but found a strongly increased membrane permeability to protons, especially potassium, of the double mutant plasma membranes. Our results indicate that chloroplast envelope K+/H+ antiporters not only affect chloroplast pH but also have a strong impact on cellular ion homeostasis and energization of the plasma membrane.

The EXO70 inhibitor Endosidin2 alters plasma membrane protein composition in Arabidopsis roots

To sustain normal growth and allow rapid responses to environmental cues, plants alter the plasma membrane protein composition under different conditions presumably by regulation of delivery, stability, and internalization. Exocytosis is a conserved cellular process that delivers proteins and lipids to the plasma membrane or extracellular space in eukaryotes. The octameric exocyst complex contributes to exocytosis by tethering secretory vesicles to the correct site for membrane fusion; however, whether the exocyst complex acts universally for all secretory vesicle cargo or just for specialized subsets used during polarized growth and trafficking is currently unknown. In addition to its role in exocytosis, the exocyst complex is also known to participate in membrane recycling and autophagy. Using a previously identified small molecule inhibitor of the plant exocyst complex subunit EXO70A1, Endosidin2 (ES2), combined with a plasma membrane enrichment method and quantitative proteomic analysis, we examined the composition of plasma membrane proteins in the root of Arabidopsis seedlings, after inhibition of the ES2-targetted exocyst complex, and verified our findings by live imaging of GFP-tagged plasma membrane proteins in root epidermal cells. The abundance of 145 plasma membrane proteins was significantly reduced following short-term ES2 treatments and these likely represent candidate cargo proteins of exocyst-mediated trafficking. Gene Ontology analysis showed that these proteins play diverse functions in cell growth, cell wall biosynthesis, hormone signaling, stress response, membrane transport, and nutrient uptake. Additionally, we quantified the effect of ES2 on the spatial distribution of EXO70A1 with live-cell imaging. Our results indicate that the plant exocyst complex mediates constitutive dynamic transport of subsets of plasma membrane proteins during normal root growth.

Quantitative phosphoproteomic analysis of chitin-triggered immune responses in the plasma membrane of Arabidopsis

Plant diseases seriously damage crop production, and most plant diseases are caused by fungi. Fungal cell walls contain chitin, a highly conserved component that is widely recognised by plants as a PAMP (pathogen-associated molecular pattern) to induce defence responses. The molecular mechanisms that function downstream of chitin-triggered intracellular phosphorylation remain largely unknown. In this study, we performed quantitative phosphoproteomics analysis to study protein phosphorylation changes in the plasma membrane after chitin treatment in Arabidopsis thaliana L. seedlings. Proteins with altered phosphorylation status after chitin treatment participated in biological processes ranging from signalling, localisation, and transport, to biogenesis, processing, and metabolism, suggesting that PAMP signalling targets multiple processes to coordinate the immune response. These results provide important insights into the molecular mechanism of chitin-induced plant immunity.

Retrograde signaling in plants: A critical review focusing on the GUN pathway and beyond

Plastids communicate their developmental and physiological status to the nucleus via retrograde signaling, allowing nuclear gene expression to be adjusted appropriately. Signaling during plastid biogenesis and responses of mature chloroplasts to environmental changes are designated "biogenic" and "operational" controls, respectively. A prominent example of the investigation of biogenic signaling is the screen for gun (genomes uncoupled) mutants. Although the first five gun mutants were identified 30 years ago, the functions of GUN proteins in retrograde signaling remain controversial, and that of GUN1 is hotly disputed. Here, we provide background information and critically discuss recently proposed concepts that address GUN-related signaling and some novel gun mutants. Moreover, considering heme as a candidate in retrograde signaling, we revisit the spatial organization of heme biosynthesis and export from plastids. Although this review focuses on GUN pathways, we also highlight recent progress in the identification and elucidation of chloroplast-derived signals that regulate the acclimation response in green algae and plants. Here, stress-induced accumulation of unfolded/misassembled chloroplast proteins evokes a chloroplast-specific unfolded protein response, which leads to changes in the expression levels of nucleus-encoded chaperones and proteases to restore plastid protein homeostasis. We also address the importance of chloroplast-derived signals for activation of flavonoid biosynthesis leading to production of anthocyanins during stress acclimation through sucrose non-fermenting 1-related protein kinase 1. Finally, a framework for identification and quantification of intercompartmental signaling cascades at the proteomic and metabolomic levels is provided, and we discuss future directions of dissection of organelle-nucleus communication.

Characterization of plasma membrane proteins in stylosanthes leaves and roots using simplified enrichment method with a nonionic detergent

Plant plasma membranes (PMs) play an important role in maintaining the stability of the intracellular environment and exchanging information with the external environment. Therefore, deciphering dynamics of PM proteome provides crucial information for elucidating cellular regulation in response to diverse stimuli. In the study, we developed a simplified method for enriching PM proteins in leaf and root tissues of a tropical forage Stylosanthes by combining differential centrifugation and Brij-58 treatment. Both immunoblot analysis and mass spectrometry demonstrated that the representation and abundance of PM proteins were increased in the enrichment fraction, and the contamination of other organellar proteins was decreased. A total of 426 and 388 proteins were predicted to be PM proteins in leaves and roots, respectively. Functional analysis classified these PM proteins into six major categories (transporter, enzyme, receptor, membrane structure protein, vesicular trafficking and chaperone), and orthologs of many PM proteins regulating the responses to abiotic and biotic stresses have been detected. In addition, the sequence analysis, subcellular localization and gene expression analyses of a newly identified receptor-like kinase, SgRKL1, has been performed. Together, these results show that the simplified PM enrichment method can be successfully applied to different plant tissue types and to study the dynamics of PM proteome of Stylosanthes in response to multiple stresses.

Lipid-mediated activation of plasma membrane-localized deubiquitylating enzymes modulate endosomal trafficking

The abundance of plasma membrane-resident receptors and transporters has to be tightly regulated by ubiquitin-mediated endosomal degradation for the proper coordination of environmental stimuli and intracellular signaling. Arabidopsis OVARIAN TUMOR PROTEASE (OTU) 11 and OTU12 are plasma membrane-localized deubiquitylating enzymes (DUBs) that bind to phospholipids through a polybasic motif in the OTU domain. Here we show that the DUB activity of OTU11 and OTU12 towards K63-linked ubiquitin is stimulated by binding to lipid membranes containing anionic lipids. In addition, we show that the DUB activity of OTU11 against K6- and K11-linkages is also stimulated by anionic lipids, and that OTU11 and OTU12 can modulate the endosomal degradation of a model cargo and the auxin efflux transporter PIN2-GFP in vivo. Our results suggest that the catalytic activity of OTU11 and OTU12 is tightly connected to their ability to bind membranes and that OTU11 and OTU12 are involved in the fine-tuning of plasma membrane proteins in Arabidopsis.

DYNAMIN-RELATED PROTEIN DRP1A functions with DRP2B in plant growth, flg22-immune responses, and endocytosis

Ligand-induced endocytosis of the immune receptor FLAGELLIN SENSING2 (FLS2) is critical for maintaining its proper abundance in the plasma membrane (PM) to initiate and subsequently down regulate cellular immune responses to bacterial flagellin or flg22-peptide. The molecular components governing PM abundance of FLS2, however, remain mostly unknown. Here, we identified Arabidopsis (Arabidopsis thaliana) DYNAMIN-RELATED PROTEIN1A (DRP1A), a member of a plant-specific family of large dynamin GTPases, as a critical contributor to ligand-induced endocytosis of FLS2 and its physiological roles in flg22-signaling and immunity against Pseudomonas syringae pv. tomato DC3000 bacteria in leaves. Notably, drp1a single mutants displayed similar flg22-defects as those previously reported for mutants in another dynamin-related protein, DRP2B, that was previously shown to colocalize with DRP1A. Our study also uncovered synergistic roles of DRP1A and DRP2B in plant growth and development as drp1a drp2b double mutants exhibited severely stunted roots and cotyledons, as well as defective cell shape, cytokinesis, and seedling lethality. Furthermore, drp1a drp2b double mutants hyperaccumulated FLS2 in the PM prior to flg22-treatment and exhibited a block in ligand-induced endocytosis of FLS2, indicating combinatorial roles for DRP1A and DRP1B in governing PM abundance of FLS2. However, the increased steady-state PM accumulation of FLS2 in drp1a drp2b double mutants did not result in increased flg22 responses. We propose that DRP1A and DRP2B are important for the regulation of PM-associated levels of FLS2 necessary to attain signaling competency to initiate distinct flg22 responses, potentially through modulating the lipid environment in defined PM domains.

Selective Enrichment Coupled with Proteomics to Identify S-Acylated Plasma Membrane Proteins in Arabidopsis

Protein S-acylation, predominately in the form of palmitoylation, is a reversible lipid post-translational modification on cysteines that plays important roles in protein localization, trafficking, activity, and complex assembly. The functions and regulatory mechanisms of S-acylation have been extensively studied in mammals owing to remarkable development of high-resolution proteomics and the discovery of the S-acylation-related enzymes. However, the advancement of S-acylation studies in plants lags behind that in mammals, mainly due to the lack of knowledge about proteins responsible for this process, such as protein acyltransferases and their substrates. In this article, a set of systematic protocols to study global S-acylation in Arabidopsis seedlings is described. The procedures are presented in detail, including preparation of Arabidopsis seedlings, enrichment of plasma membrane (PM) proteins, ensuing enrichment of S-acylated proteins/peptides based on the acyl-biotin exchange method, and large-scale identification of S-acylated proteins/peptides via mass spectrometry. This approach enables researchers to study S-acylation of PM proteins in plants in a systematic and straightforward way. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Preparation of Arabidopsis seedling materials Basic Protocol 2: Isolation and enrichment of plasma membrane proteins Support Protocol 1: Determination of protein concentration using BCA assay Basic Protocol 3: Enrichment of S-acylated proteins by acyl-biotin exchange method Support Protocol 2: Protein precipitation by methanol/chloroform method Basic Protocol 4: Trypsin digestion and proteomic analysis Alternate Protocol: Pre-resin digestion and peptide-level enrichment.

Mass Spectrometry Untangles Plant Membrane Protein Signaling Networks

Plasma membranes (PMs) act as primary cellular checkpoints for sensing signals and controlling solute transport. Membrane proteins communicate with intracellular processes through protein interaction networks. Deciphering these signaling networks provides crucial information for elucidating in vivo cellular regulation. Large-scale proteomics enables system-wide characterization of the membrane proteome, identification of ligand-receptor pairs, and elucidation of signals originating at membranes. In this review we assess recent progress in the development of mass spectrometry (MS)-based proteomic pipelines for determining membrane signaling pathways. We focus in particular on current techniques for the analysis of membrane protein phosphorylation and interaction, and how these proteins may be connected to downstream changes in gene expression, metabolism, and physiology.