A Structural Study of CESA1 Catalytic Domain of Arabidopsis Cellulose Synthesis Complex: Evidence for CESA Trimers

A cellulose synthesis complex with a “rosette” shape is responsible for synthesis of cellulose chains and their assembly into microfibrils within the cell walls of land plants and their charophyte algal progenitors. The number of cellulose synthase proteins in this large multisubunit transmembrane protein complex and the number of cellulose chains in a microfibril have been debated for many years. This work reports a low resolution structure of the catalytic domain of CESA1 from Arabidopsis (Arabidopsis thaliana; AtCESA1CatD) determined by small-angle scattering techniques and provides the first experimental evidence for the self-assembly of CESA into a stable trimer in solution. The catalytic domain was overexpressed in Escherichia coli, and using a two-step procedure, it was possible to isolate monomeric and trimeric forms of AtCESA1CatD. The conformation of monomeric and trimeric AtCESA1CatD proteins were studied using small-angle neutron scattering and small-angle x-ray scattering. A series of AtCESA1CatD trimer computational models were compared with the small-angle x-ray scattering trimer profile to explore the possible arrangement of the monomers in the trimers. Several candidate trimers were identified with monomers oriented such that the newly synthesized cellulose chains project toward the cell membrane. In these models, the class-specific region is found at the periphery of the complex, and the plant-conserved region forms the base of the trimer. This study strongly supports the “hexamer of trimers” model for the rosette cellulose synthesis complex that synthesizes an 18-chain cellulose microfibril as its fundamental product.Cellulose, the most abundant biopolymer on Earth, is composed of linear chains of β-1,4 linked d-Glc monomers with repeating structural units of the disaccharide cellobiose. Numerous cellulose polymers cocrystallize to form microfibrils, which provide mechanical strength and rigidity to plants. Its natural abundance makes it an attractive target for many industrial applications, including paper and pulping, construction, and textile manufacture. More recently, cellulose has been used for production of biofuels, such as ethanol (Ragauskas et al., 2006; Langan et al., 2014), and in the form of nanocellulose as a component in advanced composite materials (Reddy et al., 2013; Habibi, 2014). Cellulose microfibrils are synthesized by a large membrane-bound protein complex. In the land plants and charophycean algae, the cellulose synthesis complex (CSC) has a “rosette” shape (Mueller et al., 1976; Mueller and Brown, 1980b; Kimura et al., 1999), and the entire CSC has reported diameters between 24 to 30 nm (Lerouxel et al., 2006). This structural information was revealed by freeze-fracture transmission electron microscopy, showing six lobes in a hexagonal arrangement at the point where the transmembrane helices of multiple cellulose synthase proteins (CESAs) cross the plasma membrane. Recently KORRIGAN, a protein with cellulase activity, has also been implicated as an integral component of the CSC (Vain et al., 2014).Vascular plants produce several different CESA isoforms. For example, Arabidopsis (Arabidopsis thaliana) has 10 different isoforms with 64% to 98% sequence identity (Holland et al., 2000; Richmond, 2000; McFarlane et al., 2014). The different CESA isoforms play specific roles in cellulose synthesis during plant development. In Arabidopsis, CESA1, CESA3, and CESA6 are required for primary cell wall synthesis, while CESA4, CESA7, and CESA8 are required for secondary cell wall synthesis (Gardiner et al., 2003; Taylor et al., 2003; Persson et al., 2007). CESA2, CESA5, and CESA9 play roles in tissue-specific processes and are partially redundant with CESA6, whereas CESA10 is closely related to AtCESA1 but evidently has a minor role in plant development (Somerville, 2006). The absolute number of CESA proteins present in a CSC remains a subject of much speculation, largely because the stoichiometry of the cellulose microfibril remains unresolved (Cosgrove, 2014). The traditional representation of the microfibril has 36 cellulose chains, and based on this, one would expect that each lobe of the rosette CSC contains six CESA proteins responsible for the synthesis of six glucan chains for a total of 36 CESA proteins per rosette CSC (Herth, 1983; Perrin, 2001; Doblin et al., 2002). However, recent studies using different analytical techniques combined with computation report 18 to 24 cellulose chains per microfibril (Fernandes et al., 2011; Thomas et al., 2013; Oehme et al., 2015). A study of cellulose from mung bean (Vigna radiata) primary cell walls, using x-ray diffraction, solid-state NMR, and computational analysis, supports an 18-chain model for a cellulose microfibril (Newman et al., 2013). This implies that the CSC is composed of fewer than 36 CESA proteins or that not all of the proteins in a CSC are simultaneously active. Further, it has been recently reported that the stoichiometry of CESAs 1, 3, and 6 and CESAs 4, 7, and 8 in the primary and secondary cell walls, respectively, is 1:1:1 (Gonneau et al., 2014; Hill et al., 2014). Together, these reports suggest a rosette CSC composed of 18 CESA proteins with three CESAs per lobe as the most likely composition of a rosette CSC to account for an 18-chain cellulose microfibril (Newman et al., 2013; Gonneau et al., 2014; Hill et al., 2014). In addition, it should also be noted that 24 CESA proteins in a rosette CSC with four proteins per lobe is incompatible with a 1:1:1 CESA stoichiometry.Numerous efforts to isolate active CESA proteins directly from plants or by recombinant expression have not been successful, preventing a detailed structural analysis of CESA proteins or the mechanism of plant cellulose synthesis. In contrast, the recently reported crystal structure of cellulose synthase from Rhodoobacter sphaeroides (Morgan et al., 2013) clearly showed that only a single cellulose synthase polypeptide is required for glucan polymerization and also identified the conserved sequence motifs responsible for catalysis. In addition, based on the presence of an 18-residue glucan chain in the protein tunnel, a mechanism for cellulose synthesis and translocation across a cytoplasmic membrane was proposed that also addressed how the alternate d-Glc molecules are inverted during polymer synthesis (Morgan et al., 2013; Omadjela et al., 2013). However, this structure cannot provide insight into the formation of microfibrils from the cellulose chains synthesized by single polypeptides of CESA.The CESA proteins of land plants and their charophycean algal relatives are multidomain single polypeptide chains of approximately 1000 amino acids. They are predicted to have eight transmembrane helices and to have their N- and C-terminal regions facing the cytoplasm (Pear et al., 1996). Although they share sequence similarity with the bacterial counterpart, they also have unique structural features not found in the bacterial enzymes. The N-terminal domain contains a Zn-binding site that may play a role in oligomerization of CESA proteins (Kurek et al., 2002). The putative cytosolic domain, which is flanked by a two-helix N-terminal transmembrane domain and a six-helix C-terminal transmembrane domain (McFarlane et al., 2014; Slabaugh et al., 2014), has D, D, D, QxxRW motifs that are conserved substrate binding and catalytic residues in the glycosyltransferase-2 superfamily (Nagahashi et al., 1995; Pear et al., 1996; Saxena and Brown, 1997; Yoshida et al., 2000). This domain also has a plant-conserved region (P-CR) and a class-specific region (CSR) that are only found in CESAs that form rosette CSCs. Although the roles of these regions are unknown, they are proposed to be involved in regulatory functions, such as interactions with other proteins and oligomerization to form the rosette shape. In the Arabidopsis CESAs, the sequence identity within the P-CR regions is greater than 80%, while in CSR regions, it is only about 40%. A recent computational model of the cytosolic domain of cotton (Gossypium hirsutum) CESA1 provides the first detailed structural model of the catalytic domain of CESA (Sethaphong et al., 2013). This model structure aligns well with the crystal structure of the bacterial cellulose synthase, indicating that a common mechanism exists for cellulose synthesis in bacteria and plants and that CESAs within rosette CSCs contain a single active synthetic site. In addition, this model made it possible to test possible configurations for the assembly of CESA monomers into a functional rosette CSC (Newman et al., 2013; Sethaphong et al., 2013).Our understanding of the mechanism of cellulose biosynthesis in plants at the molecular level is hampered by the lack of an atomic level CESA model. To gain deeper insight into the structure and role of the catalytic domain of CESA in rosette formation, we carried out a structural characterization of the cytosolic domain of Arabidopsis CESA1, a protein that is essential for cellulose synthesis in the primary cell wall (Arioli et al., 1998). The recombinant protein was purified from Escherichia coli in a two-step process that allowed us to obtain low-resolution structural information about the monomeric and trimeric forms of the recombinant protein using small-angle scattering (SAS) techniques. This study provides the first experimental evidence to support the self-assembly of CESAs into a stable trimer complex, revealing the possible role of the catalytic domain in the formation of the rosette CSC. Comparison of the size of the catalytic domain trimer with dimensions of rosette CSCs obtained from TEM studies strongly supports the “hexamer of trimers” model for rosette CSCs. Computational analysis of the scattering data suggested configurations for how the monomers, including the plant-specific P-CR and CSR domains, may be arranged in the trimeric lobes of the rosette CSC. Knowledge of how CESA proteins assemble in the CSC will enable approaches for rational genetic manipulation of plant cell wall synthesis, which offers enormous opportunities to improve feedstocks for the production of sustainable fuels and chemicals.     

Cellulose synthase ‘class specific regions’ are intrinsically disordered and functionally undifferentiated

Cellulose synthases(CESAs) are glycosyltransferases that catalyze formation of cellulose microfibrils in plant cell walls.Seed plant CESA isoforms cluster in six phylogenetic clades, whose non-interchangeable members play distinct roles within cellulose synthesis complexes(CSCs).A ‘class specific region’(CSR), with higher sequence similarity within versus between functional CESA classes, has been suggested to contribute to specific activities or interactions of different isoforms.We investigated CESA isoform specificity in the moss,Physcomitrella patens(Hedw.) B.S.G.to gain evolutionary insights into CESA structure/function relationships.Like seed plants, P.patens has oligomeric rosette-type CSCs, but the Pp CESAs diverged independently and form a separate CESA clade.We showed that P.patens has two functionally distinct CESAs classes, based on the ability to complement the gametophore-negative phenotype of a ppcesa5 knockout line.Thus, non-interchangeable CESA classes evolved separately in mosses and seed plants.However, testing of chimeric moss CESA genes for complementation demonstrated that functional classspecificity is not determined by the CSR.Sequence analysis and computational modeling showed that the CSR is intrinsically disordered and contains predicted molecular recognition features, consistent with a possible role in CESA oligomerization and explaining the evolution of class-specific sequences without selection for class-specific function.     

Annexins V and XII insert into bilayers at mildly acidic pH and form ion channels

The functional hallmark of annexins is the ability to bind to the surface of phospholipid membranes in a reversible, Ca(2+)-dependent manner. We now report that human annexin V and hydra annexin XII reversibly bound to phospholipid vesicles in the absence of Ca(2+) at low pH; half-maximal vesicle association occurred at pH 5.3 and 5. 8, respectively. The following biochemical data support the hypothesis that these annexins insert into bilayers at mildly acidic pH. First, a photoactivatable reagent (3-trifluoromethyl)-3-(m-[(125)I]iodophenyl)diazirine) which selectively labels proteins exposed to the hydrophobic domain of bilayers reacted with these annexins at pH 5.0 and below but not at neutral pH. Second, in a Triton X-114 partitioning assay, annexins V and XII act as integral membrane proteins at low pH and as hydrophilic proteins at neutral pH; in the presence of phospholipids half-maximal partitioning into detergent occurred at pH approximately 5.0. Finally, annexin V or XII formed single channels in phospholipid bilayers at low pH but not at neutral pH. A model is discussed in which the concentrations of H(+) and Ca(2+) regulate the reversible conversion of three forms of annexins-soluble, peripheral membrane, and transmembrane.     

Annexins V and XII alter the properties of planar lipid bilayers seen by conductance probes

Annexins are proteins that bind lipids in the presence of calcium. Though multiple functions have been proposed for annexins, there is no general agreement on what annexins do or how they do it. We have used the well-studied conductance probes nonactin, alamethicin, and tetraphenylborate to investigate how annexins alter the functional properties of planar lipid bilayers. We found that annexin XII reduces the nonactin-induced conductance to approximately 30% of its original value. Both negative lipid and approximately 30 microM Ca(2+) are required for the conductance reduction. The mutant annexin XIIs, E105K and E105K/K68A, do not reduce the nonactin conductance even though both bind to the membrane just as wild-type does. Thus, subtle changes in the interaction of annexins with the membrane seem to be important. Annexin V also reduces nonactin conductance in nearly the same manner as annexin XII. Pronase in the absence of annexin had no effect on the nonactin conductance. But when added to the side of the bilayer opposite that to which annexin was added, pronase increased the nonactin-induced conductance toward its pre-annexin value. Annexins also dramatically alter the conductance induced by a radically different probe, alamethicin. When added to the same side of the bilayer as alamethicin, annexin has virtually no effect, but when added trans to the alamethicin, annexin dramatically reduces the asymmetry of the I-V curve and greatly slows the kinetics of one branch of the curve without altering those of the other. Annexin also reduces the rate at which the hydrophobic anion, tetraphenylborate, crosses the bilayer. These results suggest that annexin greatly reduces the ability of small molecules to cross the membrane without altering the surface potential and that at least some fraction of the active annexin is accessible to pronase digestion from the opposite side of the membrane.     

Carbon partitioning to cellulose synthesis

This article discusses the importance and implications of regulating carbon partitioning to cellulose synthesis, the characteristics of cells that serve as major sinks for cellulose deposition, and enzymes that participate in the conversion of supplied carbon to cellulose. Cotton fibers, which deposit almost pure cellulose into their secondary cell walls, are referred to as a primary model system. For sucrose synthase, we discuss its proposed role in channeling UDP-Glc to cellulose synthase during secondary wall deposition, its gene family, its manipulation in transgenic plants, and mechanisms that may regulate its association with sites of polysaccharide synthesis. For cellulose synthase, we discuss the organization of the gene family and how protein diversity could relate to control of carbon partitioning to cellulose synthesis. Other enzymes emphasized include UDP-Glc pyrophosphorylase and sucrose phosphate synthase. New data are included on phosphorylation of cotton fiber sucrose synthase, possible regulation by Ca²⁺ of sucrose synthase localization, electron microscopic immunolocalization of sucrose synthase in cotton fibers, and phylogenetic relationships between cellulose synthase proteins, including three new ones identified in differentiating tracheary elements of Zinnia elegans. We develop a model for metabolism related to cellulose synthesis that implicates the changing intracellular localization of sucrose synthase as a molecular switch between survival metabolism and growth and/or differentiation processes involving cellulose synthesis. Abbreviations: CesA, cellulose synthase; Csl, cellulose-like synthase (genes); DCB, dichlobenil; DPA, days after anthesis; SPS, sucrose phosphate synthase; SuSy, sucrose synthase; P-SuSy, particulate SuSy; S-SuSy, soluble SuSy     

Geminivirus-mediated gene silencing from Cotton leaf crumple virus is enhanced by low temperature in cotton

A silencing vector for cotton (Gossypium hirsutum) was developed from the geminivirus Cotton leaf crumple virus (CLCrV). The CLCrV coat protein gene was replaced by up to 500 bp of DNA homologous to one of two endogenous genes, the magnesium chelatase subunit I gene (ChlI) or the phytoene desaturase gene (PDS). Cotyledons of cotton cultivar 'Deltapine 5415' bombarded with the modified viral vectors manifested chlorosis due to silencing of either ChlI or PDS in approximately 70% of inoculated plants after 2 to 3 weeks. Use of the green fluorescence protein gene showed that replication of viral DNA was restricted to vascular tissue and that the viral vector could transmit to leaves, roots, and the ovule integument from which fibers originate. Temperature had profound effects on vector DNA accumulation and the spread of endogenous gene silencing. Consistent with reports that silencing against viruses increases at higher temperatures, plants grown at a 30 degrees C/26 degrees C day/night cycle had a greater than 10-fold reduction in viral DNA accumulation compared to plants grown at 22 degrees C/18 degrees C. However, endogenous gene silencing decreased at 30 degrees C/26 degrees C. There was an approximately 7 d delay in the onset of gene silencing at 22 degrees C/18 degrees C, but silencing was extensive and persisted throughout the life of the plant. The extent of silencing in new growth could be increased or decreased by changing temperature regimes at various times following the onset of silencing. Our experiments establish the use of the CLCrV silencing vector to study gene function in cotton and show that temperature can have a major impact on the extent of geminivirus-induced gene silencing.     

Biotransformation of nitrobenzene by bacteria containing toluene degradative pathways.

Nonpolar nitroaromatic compounds have been considered resistant to attack by oxygenases because of the electron withdrawing properties of the nitro group. We have investigated the ability of seven bacterial strains containing toluene degradative pathways to oxidize nitrobenzene. Cultures were induced with toluene vapor prior to incubation with nitrobenzene, and products were identified by high-performance liquid chromatography and gas chromatography-mass spectrometry. Pseudomonas cepacia G4 and a strain of Pseudomonas harboring the TOL plasmid (pTN2) did not transform nitrobenzene. Cells of Pseudomonas putida F1 and Pseudomonas sp. strain JS150 converted nitrobenzene to 3-nitrocatechol. Transformation of nitrobenzene in the presence of 18O2 indicated that the reaction in JS150 involved the incorporation of both atoms of oxygen in the 3-nitrocatechol, which suggests a dioxygenase mechanism. P. putida 39/D, a mutant strain of P. putida F1, converted nitrobenzene to a compound tentatively identified as cis-1,2-dihydroxy-3-nitrocyclohexa-3,5-diene. This compound was rapidly converted to 3-nitrocatechol by cells of strain JS150. Cultures of Pseudomonas mendocina KR-1 converted nitrobenzene to a mixture of 3- and 4-nitrophenol (10 and 63%, respectively). Pseudomonas pickettii PKO1 converted nitrobenzene to 3- and 4-nitrocatechol via 3- and 4-nitrophenol. The nitrocatechols were slowly degraded to unidentified metabolites. Nitrobenzene did not serve as an inducer for the enzymes that catalyzed its oxidation. These results indicate that the nitrobenzene ring is subject to initial attack by both mono- and dioxygenase enzymes.     

Biodegradation of 4-nitrotoluene by Pseudomonas sp. strain 4NT.

A strain of Pseudomonas spp. was isolated from nitrobenzene-contaminated soil on 4-nitrotoluene as the sole source of carbon, nitrogen, and energy. The organism also grew on 4-nitrobenzaldehyde, and 4-nitrobenzoate. 4-Nitrobenzoate and ammonia were detected in the culture fluid of glucose-grown cells after induction with 4-nitrotoluene. Washed suspensions of 4-nitrotoluene- or 4-nitrobenzoate-grown cells oxidized 4-nitrotoluene, 4-nitrobenzaldehyde, 4-nitrobenzyl alcohol, and protocatechuate. Extracts from induced cells contained 4-nitrobenzaldehyde dehydrogenase, 4-nitrobenzyl alcohol dehydrogenase, and protocatechuate 4,5-dioxygenase activities. Under anaerobic conditions, cell extracts converted 4-nitrobenzoate or 4-hydroxylaminobenzoate to protocatechuate. Conversion of 4-nitrobenzoate to protocatechuate required NADPH. These results indicate that 4-nitrotoluene was degraded by an initial oxidation of the methyl group to form 4-nitrobenzyl alcohol, which was converted to 4-nitrobenzoate via 4-nitrobenzaldehyde. The 4-nitrobenzoate was reduced to 4-hydroxylaminobenzoate, which was converted to protocatechuate. A protocatechuate 4,5-dioxygenase catalyzed meta-ring fission of the protocatechuate. The detection of 4-nitrobenzaldehyde and 4-nitrobenzyl alcohol dehydrogenase and 4-nitrotoluene oxygenase activities in 4-nitrobenzoate-grown cells suggests that 4-nitrobenzoate is an inducer of the 4-nitrotoluene degradative pathway.     

Identification and characterization of columbid annexin Icp37. Insights into the evolution of annexin I phosphorylation sites.

Annexin I (AnxI) contains phosphorylation sites in its "hinge region" that have been implicated in the regulation of cell growth and/or differentiation. A pigeon (Columba livia) isoform of this protein, annexin Icp35 (cp35), has a very similar amino acid sequence overall but an unrelated sequence that lacks phosphorylation sites in the hinge region. We now report the identification and characterization of annexin Icp37 (cp37) from pigeon. Genomic cloning and Southern blot analysis demonstrated that cp37 and cp35 were encoded by separated genes. Prolactin induced the expression of cp35 mRNA but not cp37. The amino acid sequence of cp37 was deduced from a cDNA clone and found to share 93 and 75% sequence identity with cp35 and human AnxI, respectively. The amino acid sequence of cp37 bore similarities to both AnxI and cp35 in the critical hinge region. Like AnxI, cp37 contained consensus phosphorylation sites in its amino acid sequence and was phosphorylated on tyrosine by the EGF receptor/kinase and on serine by protein kinase C in vitro. Despite the functional similarities between cp37 and AnxI, the nucleotide sequence that encoded the hinge region of cp37 was very similar to the analogous region of cp35, but different from that of AnxI. We propose that certain features shared by cp37 and AnxI are the products of convergent evolution. The fact that evolution independently selected for two annexin I-like genes (cp37 and anxI) encoding analogous phosphorylation sites is strong evidence that phosphorylation is important for the regulation of the biological activity of these proteins.