beta-Glucoside Activators of Mung Bean UDP-Glucose: beta-Glucan Synthase : II. Comparison of Effects of an Endogenous beta-Linked Glucolipid with Synthetic n-Alkyl beta-d-Monoglucopyranosides

A molecular description of cellulose biosynthesis

Cellulose is the most abundant biopolymer on Earth, and certain organisms from bacteria to plants and animals synthesize cellulose as an extracellular polymer for various biological functions. Humans have used cellulose for millennia as a material and an energy source, and the advent of a lignocellulosic fuel industry will elevate it to the primary carbon source for the burgeoning renewable energy sector. Despite the biological and societal importance of cellulose, the molecular mechanism by which it is synthesized is now only beginning to emerge. On the basis of recent advances in structural and molecular biology on bacterial cellulose synthases, we review emerging concepts of how the enzymes polymerize glucose molecules, how the nascent polymer is transported across the plasma membrane, and how bacterial cellulose biosynthesis is regulated during biofilm formation. Additionally, we review evolutionary commonalities and differences between cellulose synthases that modulate the nature of the cellulose product formed.

Sitosterol-beta-glucoside as primer for cellulose synthesis in plants

Cellulose synthesis in plants requires beta-1,4-glucan chain initiation, elongation, and termination. The process of chain elongation is likely to be distinct from the process of chain initiation. We demonstrate that a CesA glucosyltransferase initiates glucan polymerization by using sitosterol-beta-glucoside (SG) as primer. Cotton fiber membranes synthesize sitosterol-cellodextrins (SCDs) from SG and uridine 5'-diphosphate-glucose (UDP-Glc) under conditions that also favor cellulose synthesis. The cellulase encoded by the Korrigan (Kor) gene, required for cellulose synthesis in plants, may function to cleave SG from the growing polymer chain.

[beta]-Glucan Synthesis in the Cotton Fiber (II. Regulation and Kinetic Properties of [beta]-Glucan Synthases

The regulation and kinetic properties of cellulose synthase as well as [beta]-1,3-glucan synthase have been studied. The cellulose was detected using acetic/nitric acid insolubility as an indicator of cellulose (this product contained only [beta]-1,4-linked glucans; K. Okuda, L. Li, K. Kudlicka, S. Kuga, R.M. Brown, Jr. [1993] Plant Physiol 101: 1131-1142). These studies reveal that (a) [beta]-1,3-glucan synthesis is enhanced up to 31-fold by cellobiose with a Ka of 1.16 mM; (b) cellulose synthesis is increased 12-fold by a combination of cellobiose (Ka = 3.26 mM) and cyclic-3[prime]:5[prime]-GMP (Ka = 100 [mu]M); (c) the common components in the reaction mixture required by both enzymes are cellobiose, calcium, and digitonin; (d) cellulose synthase has an essential requirement for magnesium (Ka = 0.89 mM); (e) cellulose synthase also requires a low concentration of calcium (Ka = 90 [mu]M); (f) the optimal pH for cellulose synthase (7.6-8.0) is slightly higher than that for [beta]-1,3-glucan synthase (7.2-7.6); (g) the Km for UGP-Glc for cotton (Gossypium hirsutum) cellulose synthase is 0.40 mM; (h) the Km for UDP-Glc for for [beta]-1,3-glucan synthase is 0.43 mM.

Stimulation of Callose Synthesis in Vivo Correlates with Changes in Intracellular Distribution of the Callose Synthase Activator [beta]-Furfuryl-[beta]-Glucoside

[beta]-Furfuryl-[beta]-glucoside (FG) has been shown to be a specific endogenous activator of higher plant callose synthase (P. Ohana, D.P. Delmer, G. Volman, J.C. Steffens, D.E. Matthews, M. Benziman [1992] Plant Physiol 98: 708-715). Because glycosides such as FG are usually sequestered in vacuoles, we have proposed that activation of callose synthesis in vivo may involve a change in the compartmentation of FG and Ca2+, resulting in a synergistic activation of callose synthase. The use of suspension-cultured barley (Hordeum bulbosum L.) cells provides evidence that FG is largely sequestered in the vacuole. Furthermore, conditions that lead to induction of callose synthesis in vivo correspondingly lead to elevation of the cytoplasmic concentration of FG. These conditions include the lowering of cytoplasmic pH or elevation of cytoplasmic Ca2+. Oligogalacturonide elicitors have also been reported to cause similar changes in cytoplasmic pH and Ca2+ concentration (Y. Mathieu, A. Kurkdjian, H. Xia, J. Guern, A. Koller, M.D. Spiro, M. O'Neill, P. Albersheim, A. Darvill [1991] The Plant Journal 1: 333-343), and such an elicitor also causes an elevation in cytoplasmic FG coupled with stimulation of callose synthesis. These results support the concept that a relative redistribution of FG between cytoplasm and vacuole may be one of the components of the signal transduction pathway for elicitation of callose synthase in vivo.

beta-Furfuryl-beta-Glucoside: An Endogenous Activator of Higher Plant UDP-Glucose:(1-3)-beta-Glucan Synthase : Biological Activity, Distribution, and in Vitro Synthesis

In a recent paper (P Ohana, DP Delmer, JC Steffens, DE Matthews, R Mayer, M Benziman [1991] J Biol Chem 266: 13472-13475), we described the purification and structural characterization of beta-furfuryl-beta-glucoside (FG), an endogenous activator of plant UDP-glucose:(1-->3)-beta-glucan (callose) synthase. In the present report, we provide evidence that FG specifically stimulates callose synthase. The effects of FG on the kinetic properties of callose synthase were studied, and we ascertained that FG, or at least a very similar compound, is present in other plant systems. Chemically synthesized alpha-furfuryl-beta-glucoside also stimulates callose synthase, exhibiting a slightly higher K(a) of 80 micromolar, compared with 50 micromolar for FG. In addition, we have identified and partially characterized an enzyme that catalyzes the synthesis of FG using beta-furfuryl alcohol and UDP-glucose as substrates. A model for the regulation of callose synthesis in vivo, involving changes in intracellular compartmentation of FG and Ca(2+), is proposed.

Cellulose biosynthesis and function in bacteria

The current model of cellulose biogenesis in plants, as well as bacteria, holds that the membranous cellulose synthase complex polymerizes glucose moieties from UDP-Glc into beta-1,4-glucan chains which give rise to rigid crystalline fibrils upon extrusion at the outer surface of the cell. The distinct arrangement and degree of association of the polymerizing enzyme units presumably govern extracellular chain assembly in addition to the pattern and width of cellulose fibril deposition. Most evident for Acetobacter xylinum, polymerization and assembly appear to be tightly coupled. To date, only bacteria have been effectively studied at the biochemical and genetic levels. In A. xylinum, the cellulose synthase, composed of at least two structurally similar but functionally distinct subunits, is subject to a multicomponent regulatory system. Regulation is based on the novel nucleotide cyclic diguanylic acid, a positive allosteric effector, and the regulatory enzymes maintaining its intracellular turnover: diguanylate cyclase and Ca2(+)-sensitive bis-(3',5')-cyclic diguanylic acid (c-di-GMP) phosphodiesterase. Four genes have been isolated from A. xylinum which constitute the operon for cellulose synthesis. The second gene encodes the catalytic subunit of cellulose synthase; the functions of the other three gene products are still unknown. Exclusively an extracellular product, bacterial cellulose appears to fulfill diverse biological roles within the natural habitat, conferring mechanical, chemical, and physiological protection in A. xylinum and Sarcina ventriculi or facilitating cell adhesion during symbiotic or infectious interactions in Rhizobium and Agrobacterium species. A. xylinum is proving to be most amenable for industrial purposes, allowing the unique features of bacterial cellulose to be exploited for novel product applications.

Direct Photolabeling with [P]UDP-Glucose for Identification of a Subunit of Cotton Fiber Callose Synthase

We have identified a 52 kilodalton polypeptide as being a likely candidate for the catalytic subunit of the UDP-glucose: (1-->3)-beta-glucan (callose) synthase of developing fibers of Gossypium hirsutum (cotton). Such a polypeptide migrates coincident with callose synthase during glycerol gradient centrifugation in the presence of EDTA, and can be directly photolabeled with the radioactive substrate, alpha-[(32)P]UDP-glucose. Interaction with the labeled probe requires Ca(2+), a specific activator of callose synthase which is known to lower the K(m) of higher plant callose synthases for the substrate UDP-glucose. Using this probe and several other related ones, several other proteins which interact with UDP-glucose were also identified, but none satisfied all of the above criteria for being components of the callose synthase.

beta-Glucoside Activators of Mung Bean UDP-Glucose: beta-Glucan Synthase : I. Identification of an Endogenous beta-Linked Glucolipid Activator

Heat-stable activators of membranous beta-glucan synthase have been isolated from the supernatant fraction of crude mung bean (Vigna radiata) extracts by DEAE-cellulose and silica-gel chromatography. One of the activators has been partially purified and characterized on the basis of susceptibility to various enzymes and by analysis of the products formed upon total acid hydrolysis, alkaline-methanolysis, and beta-glucosidase digestion. This activator has the characteristics of a 1,2-dioleoyl diglyceride containing beta-linked glucose residue(s) at the C-3 position. When expressed per mole of glucosyl residues, the maximal K(a) value of the activator is estimated to be 25 micromolar. Both the intact glucosyl and fatty acid moiety are essential to the stimulatory effect of the activator.