Amino Acids Regulate mTORC1 by an Obligate Two-step Mechanism.

The mechanistic target of rapamycin complex 1 (mTORC1) coordinates cell growth with its nutritional, hormonal, energy, and stress status. Amino acids are critical regulators of mTORC1 that permit other inputs to mTORC1 activity. However, the roles of individual amino acids and their interactions in mTORC1 activation are not well understood. Here we demonstrate that activation of mTORC1 by amino acids includes two discrete and separable steps: priming and activation. Sensitizing mTORC1 activation by priming amino acids is a prerequisite for subsequent stimulation of mTORC1 by activating amino acids. Priming is achieved by a group of amino acids that includes l-asparagine, l-glutamine, l-threonine, l-arginine, l-glycine, l-proline, l-serine, l-alanine, and l-glutamic acid. The group of activating amino acids is dominated by l-leucine but also includes l-methionine, l-isoleucine, and l-valine. l-Cysteine predominantly inhibits priming but not the activating step. Priming and activating steps differ in their requirements for amino acid concentration and duration of treatment. Priming and activating amino acids use mechanisms that are distinct both from each other and from growth factor signaling. Neither step requires intact tuberous sclerosis complex of proteins to activate mTORC1. Concerted action of priming and activating amino acids is required to localize mTORC1 to lysosomes and achieve its activation.

SCAR mediates light-induced root elongation in Arabidopsis through photoreceptors and proteasomes.

The ARP2/3 complex, a highly conserved nucleator of F-actin, and its activator, the SCAR complex, are essential for growth in plants and animals. In this article, we present a pathway through which roots of Arabidopsis thaliana directly perceive light to promote their elongation. The ARP2/3-SCAR complex and the maintenance of longitudinally aligned F-actin arrays are crucial components of this pathway. The involvement of the ARP2/3-SCAR complex in light-regulated root growth is supported by our finding that mutants of the SCAR complex subunit BRK1/HSPC300, or other individual subunits of the ARP2/3-SCAR complex, showed a dramatic inhibition of root elongation in the light, which mirrored reduced growth of wild-type roots in the dark. SCAR1 degradation in dark-grown wild-type roots by constitutive photomorphogenic 1 (COP1) E3 ligase and 26S proteasome accompanied the loss of longitudinal F-actin and reduced root growth. Light perceived by the root photoreceptors, cryptochrome and phytochrome, suppressed COP1-mediated SCAR1 degradation. Taken together, our data provide a biochemical explanation for light-induced promotion of root elongation by the ARP2/3-SCAR complex.

Fluorescence Imaging of the Cytoskeleton in Plant Roots.

During the past two decades the use of live cytoskeletal probes has increased dramatically due to the introduction of the green fluorescent protein. However, to make full use of these live cell reporters it is necessary to implement simple methods to maintain plant specimens in optimal growing conditions during imaging. To image the cytoskeleton in living Arabidopsis roots, we rely on a system involving coverslips coated with nutrient supplemented agar where the seeds are directly germinated. This coverslip system can be conveniently transferred to the stage of a confocal microscope with minimal disturbance to the growth of the seedling. For roots with a larger diameter such as Medicago truncatula, seeds are first germinated in moist paper, grown vertically in between plastic trays, and roots mounted on glass slides for confocal imaging. Parallel with our live cell imaging approaches, we routinely process fixed plant material via indirect immunofluorescence. For these methods we typically use non-embedded vibratome-sectioned and whole mount permeabilized root tissue. The clearly defined developmental regions of the root provide us with an elegant system to further understand the cytoskeletal basis of plant development.

Differential Regulation of ERK1/2 and mTORC1 Through T1R1/T1R3 in MIN6 Cells.

The MAPKs ERK1/2 respond to nutrients and other insulin secretagogues in pancreatic ß-cells and mediate nutrient-dependent insulin gene transcription. Nutrients also stimulate the mechanistic target of rapamycin complex 1 (mTORC1) to regulate protein synthesis. We showed previously that activation of both ERK1/2 and mTORC1 in the MIN6 pancreatic ß-cell-derived line by extracellular amino acids (AAs) is at least in part mediated by the heterodimeric T1R1/T1R3, a G protein-coupled receptor. We show here that AAs differentially activate these two signaling pathways in MIN6 cells. Pretreatment with pertussis toxin did not prevent the activation of either ERK1/2 or mTORC1 by AAs, indicating that G(I) is not central to either pathway. Although glucagon-like peptide 1, an agonist for a G(s-)coupled receptor, activated ERK1/2 well and mTORC1 to a small extent, AAs had no effect on cytosolic cAMP accumulation. Ca(2+) entry is required for ERK1/2 activation by AAs but is dispensable for AA activation of mTORC1. Pretreatment with UBO-QIC, a selective G(q) inhibitor, reduced the activation of ERK1/2 but had little effect on the activation of mTORC1 by AAs, suggesting a differential requirement for G(q). Inhibition of G(12/13) by the overexpression of the regulator of G protein signaling domain of p115 ρ-guanine nucleotide exchange factor had no effect on mTORC1 activation by AAs, suggesting that these G proteins are also not involved. We conclude that AAs regulate ERK1/2 and mTORC1 through distinct signaling pathways.

Fluorescent protein-based reporters of the actin cytoskeleton in living plant cells: fluorophore variant, actin binding domain, and promoter considerations.

Genetically encoded filamentous actin (F-actin) reporters designed based on fluorescent protein fusions to F-actin binding domains of actin regulatory proteins have emerged as powerful tools to decipher the role of the actin cytoskeleton in plant growth and development. However, these probes could interfere with the function of endogenous actin binding proteins and in turn impact actin organization and plant growth. We therefore surveyed F-actin labeling and compared organ growth in Arabidopsis thaliana lines expressing a variety of F-actin markers. Here we show that the variant of fluorescent protein, type of actin binding domain, and the promoter that drives reporter expression can influence the quality of F-actin labeling particularly in stable plant lines. For example, older red fluorescent protein (RFP)-based probes such as DsRed2 and mOrange induced more aberrant labeling compared to the newer RFP-based, mCherry, GFP, and GFP-derived fluorophores such as YFP and CFP. Moreover, qualitative and quantitative analyses revealed differences in F-actin organization in seedlings expressing Talin- and Lifeact-based reporters in some cell types compared to the fimbrin actin binding domain 2 (ABD2)-based reporters. Finally, the use of the ubiquitin10 (UBQ10) promoter to drive expression of the GFP-ABD2-GFP probe minimized loss of fluorescence and growth defects observed in the 35S-driven version. Taken together, this study shows that care must be taken in the interpretation of data derived from stable expression of certain F-actin reporters and that using alternative promoters such as UBQ10 can overcome some of the pitfalls that accompany the use of in vivo F-actin probes in plants. © 2014 Wiley Periodicals, Inc.

Sample preparation for fluorescence imaging of the cytoskeleton in fixed and living plant roots.

During the past decade the use of live cytoskeletal probes has increased dramatically due to the introduction of the green fluorescent protein. However, to make full use of these live cell reporters it is necessary to implement simple methods to maintain plant specimens in optimal growing conditions during imaging. To image the cytoskeleton in living Arabidopsis root cells, we rely on a system involving coverslips coated with nutrient supplemented agar where the seeds are directly germinated. This coverslip system can be conveniently transferred to the stage of a confocal microscope with minimal disturbance to the growth of the seedling. Parallel to our live cell imaging approaches, we routinely process fixed plant material via indirect immunofluorescence. For these methods we typically use nonembedded vibratome-sectioned and whole mount permeabilized root tissue. The clearly defined developmental regions of the root provide us with an elegant system to further understand the cytoskeletal basis of plant development.

Plasma membrane-associated SCAR complex subunits promote cortical F-actin accumulation and normal growth characteristics in Arabidopsis roots.

The ARP2/3 complex, a highly conserved nucleator of F-actin polymerization, and its activator, the SCAR complex, have been shown to play important roles in leaf epidermal cell morphogenesis in Arabidopsis. However, the intracellular site(s) and function(s) of SCAR and ARP2/3 complex-dependent actin polymerization in plant cells remain unclear. We demonstrate that putative SCAR complex subunits BRK1 and SCAR1 are localized to the plasma membrane at sites of cell growth and wall deposition in expanding cells of leaves and roots. BRK1 localization is SCAR-dependent, providing further evidence of an association between these proteins in vivo. Consistent with plasma membrane localization of SCAR complex subunits, cortical F-actin accumulation in root tip cells is reduced in brk1 mutants. Moreover, mutations disrupting the SCAR or ARP2/3 complex reduce the growth rate of roots and their ability to penetrate semi-solid medium, suggesting reduced rigidity. Cell walls of mutant roots exhibit abnormal structure and composition at intercellular junctions where BRK1 and SCAR1 are enriched in the adjacent plasma membrane. Taken together, our results suggest that SCAR and ARP2/3 complex-dependent actin polymerization promotes processes at the plasma membrane that are important for normal growth and wall assembly.

BRICK1/HSPC300 functions with SCAR and the ARP2/3 complex to regulate epidermal cell shape in Arabidopsis.

The Arp2/3 complex, a highly conserved nucleator of F-actin polymerization, is essential for a variety of eukaryotic cellular processes, including epidermal cell morphogenesis in Arabidopsis thaliana. Efficient nucleation of actin filaments by the Arp2/3 complex requires the presence of an activator such as a member of the Scar/WAVE family. In mammalian cells, a multiprotein complex consisting of WAVE, PIR121/Sra-1, Nap1, Abi-2 and HSPC300 mediates responsiveness of WAVE to upstream regulators such as Rac. Essential roles in WAVE complex assembly or function have been demonstrated for PIR121/Sra-1, Nap1 and Abi-2, but the significance of HSPC300 in this complex is unclear. Plant homologs of all mammalian WAVE complex components have been identified, including HSPC300, the mammalian homolog of maize BRICK1 (BRK1). We show that, like mutations disrupting the Arabidopsis homologs of PIR121/Sra-1, Nap1 and Scar/WAVE, mutations in the Arabidopsis BRK1 gene result in trichome and pavement cell morphology defects (and associated alterations in the F-actin cytoskeleton of expanding cells) similar to those caused by mutations disrupting the ARP2/3 complex itself. Analysis of double mutants provides genetic evidence that BRK1 functions in a pathway with the ARP2/3 complex. BRK1 is required for accumulation of SCAR1 protein in vivo, potentially explaining the apparently essential role of BRK1 in ARP2/3 complex function.

IRREGULAR TRICHOME BRANCH1 in Arabidopsis encodes a plant homolog of the actin-related protein2/3 complex activator Scar/WAVE that regulates actin and microtubule organization.

The dynamic actin cytoskeleton is important for a myriad of cellular functions, including intracellular transport, cell division, and cell shape. An important regulator of actin polymerization is the actin-related protein2/3 (Arp2/3) complex, which nucleates the polymerization of new actin filaments. In animals, Scar/WAVE family members activate Arp2/3 complex-dependent actin nucleation through interactions with Abi1, Nap1, PIR121, and HSCP300. Mutations in the Arabidopsis thaliana genes encoding homologs of Arp2/3 complex subunits PIR121 and NAP1 all show distorted trichomes as well as additional epidermal cell expansion defects, suggesting that a Scar/WAVE homolog functions in association with PIR121 and NAP1 to activate the Arp2/3 complex in Arabidopsis. In a screen for trichome branching defects, we isolated a mutant that showed irregularities in trichome branch positioning and expansion. We named this gene IRREGULAR TRICHOME BRANCH1 (ITB1). Positional cloning of the ITB1 gene showed that it encodes SCAR2, an Arabidopsis protein related to Scar/WAVE. Here, we show that itb1 mutants display cell expansion defects similar to those reported for the distorted class of trichome mutants, including disruption of actin and microtubule organization. In addition, we show that the scar homology domain (SHD) of ITB1/SCAR2 is necessary and sufficient for in vitro binding to Arabidopsis BRK1, the plant homolog of HSPC300. Overexpression of the SHD in transgenic plants causes a dominant negative phenotype. Our results extend the evidence that the Scar/WAVE pathway of Arp2/3 complex regulation exists in plants and plays an important role in regulating cell expansion.

Endogenous Nod-factor-like signal molecules promote early somatic embryo development in Norway spruce.

Embryogenic cultures of Norway spruce (Picea abies) are composed of pro-embryogenic masses (PEMs) and somatic embryos of various developmental stages. Auxin is important for PEM formation and proliferation. In this report we show that depletion of auxin blocks PEM development and causes large-scale cell death. Extracts of the media conditioned by embryogenic cultures stimulate development of PEM aggregates in auxin-deficient cultures. Partial characterization of the conditioning factor shows that it is a lipophilic, low-molecular-weight molecule, which is sensitive to chitinase and contains GlcNAc residues. On the basis of this information, we propose that the factor is a lipophilic chitin oligosaccharide (LCO). The amount of LCO correlates to the developmental stages of PEMs and embryos, with the highest level in the media conditioned by developmentally blocked cultures. LCO is not present in nonembryogenic cultures. Cell death, induced by withdrawal of auxin, is suppressed by extra supply of endogenous LCO or Nod factor from Rhizobium sp. NGR234. The effect can be mimicked by a chitotetraose or chitinase from Streptomyces griseus. Taken together, our data suggest that endogenous LCO acts as a signal molecule stimulating PEM and early embryo development in Norway spruce.

Activation of Arp2/3 complex-dependent actin polymerization by plant proteins distantly related to Scar/WAVE.

The Arp2/3 complex, a highly conserved nucleator of F-actin polymerization, plays a key role in the regulation of actin dynamics eukaryotic cells. In animal cells and yeasts, Wiskott-Aldrich Syndrome protein (WASP)/suppressor of cAMP receptor (Scar)/WASP family verprolin homologous (WAVE) family proteins activate the Arp2/3 complex in response to localized cues. Like other eukaryotes, plants have an Arp2/3 complex, which has recently been shown to play an important role in F-actin organization and cell morphogenesis. However, no activators of the Arp2/3 complex have been identified in plants, which lack obvious homologs of WASP/Scar/WAVE family proteins. Here, we identify a family of Scar/WAVE-related plant Arp2/3 activators. Like Scar/WAVE proteins, four proteins identified in Arabidopsis thaliana (AtSCAR1 to AtSCAR4) and one in maize (ZmSCAR1) have a C-terminal WASP homology 2 (WH2)/acidic (WA)-verprolin homology/cofilin homology/acidic (VCA)-like domain, which we show can activate the bovine Arp2/3 complex. At their N termini, AtSCAR1 to ATSCAR4, along with a fifth protein lacking a VCA/WA-like domain at its C terminus (At4g18600), are related to the N-terminal Scar homology domains of Scar/WAVE family proteins. Analysis of gene expression patterns suggests functional redundancy among members of the AtSCAR family. Full-length AtSCAR1 and ATSCAR3 proteins and their Scar homology domains bind in vitro to AtBRICK 1 (AtBRK1), the Arabidopsis homolog of HSPC300, a WAVE-binding protein recently identified as a component of a complex implicated in the regulation of Scar/WAVE activity. Thus, AtSCAR proteins are likely to function in association with AtBRK1, and perhaps other Arabidopsis homologs of WAVE complex components, to regulate activation of the Arp2,3 complex in vivo.