Glucan cellulose biosynthesis

PF had been proposed as the terminal complex (23) and associated pores were reported on the outer membrane EF (24). Due to their proximity to the site of cellulose ribbon extrusion from the cell surface, these structures were assumed to be responsible for cellulose synthesis. A model was advanced in which cellulose synthase was localized on the outer membrane, which invoked adhesion sites between the outer and plasma membranes as a mechanism to explain the transfer of uridine-diphosphoryl-glucose (UDPG) from the cytoplasm to the cellulose synthases (25,26). However, when the outer and plasma membranes of Acetobacter were isolated separately by density-gradient centrifugation, the cellulose synthase activity was localized only in the plasma membrane fraction (27). Therefore, the linear structures observed on the Acetobacter outer membrane, while they may be associated in some manner with cellulose biosynthesis, are probably not the cellulose synthase terminal complexes. Since no ultrastructural evidence for adhesion sites between the outer and plasma membranes has been presented, a thorough investigation of the mechanism of / (1-4) glucan chain translocation from the cytoplasmic membrane to the outer membrane in Acetobacter xylinvm is now in order. 

A characteristic feature of the SuSy isoforms is a conserved phosphorylated serine residue near the N-terminus [8-10]. In-vivo studies have demonstrated that phosphorylation and dephosphorylation direct the distribution of SuSy isoforms in the plant cell [10-12]. The soluble phosphorylated SuSy interacts with the actin cytoskeleton in the cytoplasm [13], and the dephosphorylated SuSy isoforms are targeted to the cell membrane to form complexes with other enzymes, e.g., glucan synthase, catalyzing cellulose biosynthesis from sucrose [4, 10, 14]. In this respect, recent studies on the dephosphorylated enzymes by cloning and expression of SMS genes in E. coli have shown differences in some biochemical features when compared to the enzymes isolated from the corresponding plant material. Recom-... 

Dichlobenil was reported to be more than 100 times more inhibitory than coumarin toward cellulose biosynthesis in two further cell wall synthesizing systems. In cotton fibers growing from in vitro cultured ovules, dichlobenil at a concentration of 10 fiM caused 80% inhibition of cellulose biosynthesis within 2-3 h the inhibition was reversed by removal of the herbicide.Micromolar concentrations were effective within 10 min of incubation, indicating that cellulose biosynthesis is a likely primary target for this herbicide. The action of dichlobenil is more specific for the )8-(1 — 4)-glucan synthesis of cellulose rather than noncellulosic )8-(l — 3)-glucan synthesis, which is less sensitive. In tobacco cells, specific inhibition of cellulose biosynthesis was apparent at 0.5-5.0/iM dichlobenil however, no inhibition of in u/tro j3-glucan synthesis in particulate membrane... 

Another problem for biosynthesis concerns the fact that certain other, native plant-polysaccharides, most notably the xyloglucans found in higher-plant cell-walls,6 contain backbone structures of (1— 4)-/3-D-glucan, and it is necessary to consider whether separate enzymes synthesize these structures, and, if so, whether these enzymes could be confused with enzymes specifically involved in the synthesis of cellulose. 

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