Digitonin Solubilization and Protease Stimulation of β Glucan Synthetases of Saprolegnia

Rice matrix metalloproteinase OsMMP1 plays pleiotropic roles in plant development and symplastic-apoplastic transport by modulating cellulose and callose depositions

Matrix metalloproteinases (MMPs) are well-known proteolytic enzymes in animal systems and play roles in tissue differentiation, growth, and defence. Although a few plant MMPs have been reported, their exact functions in development and growth remain elusive. In this study, we characterized the promoter and coding sequence of OsMMP1, one of the putative MMP genes in rice (Oryza sativa). The OsMMP1 catalytic domain is structurally similar to human MMPs with respect to cofactor orientation as predicted by homology modeling. Bacterially expressed recombinant OsMMP1 showed protease activity with bovine serum albumin and gelatin as substrates. Analyses of transcript accumulation and promoter-reporter gene expression revealed that OsMMP1 is spatio-temporally expressed in vegetative and reproductive parts of plants. The plasma membrane-localized OsMMP1 protease affected plant development upon heterologous expression in tobacco and endogenous gene silencing in rice. Transgenic tobacco plants expressing OsMMP1 showed enhanced deposition of cellulose and callose, leading to impairment of symplastic and apoplastic translocations. Moreover, transgenic tobacco tissues exhibited tolerance to oxidative stress-inducing agent by confining the area of tissue death owing to callose lining. Collectively, these findings demonstrate the involvement of a plant MMP in growth, organ differentiation, and development in relation to cell wall modification.

β-1-4-and β-1-3-glucan synthases are associated with the plasma membrane of the fungus Saprolegnia

Cytoplasmic membranes from mycelium or protoplasts of Saprolegnia monoica (a cellulosic cell-wall fungus) were separated by continuous sucrose-density-gradient centrifugation. Glucan synthases assayed at low (micromolar uridine 5'-diphosphate (UDP) glucose for β-1-4-glucan synthase) and high (millimolar UDP glucose for β-1-3-glucan synthase) substrate concentrations were associated with membranes exhibiting vanadate-sensitive, oligomycin-insensitive ATPase and equilibrating at density 1.16 g cm(-3). Synthase activities were also bound to membranes of lower density (1.10 and 1.145 g cm(-3)). Plasma membranes were stabilized by coating protoplasts with concanavalin A. After lysis of the protoplasts, plasma membranes recovered by low centrifugal forces were isolated in continuous isopycinic gradients. Both synthase activities peaked with [(3)H]concanavalin A and Na-vanadate ATPase indicating that the synthetases are located at the plasma membrane. Treatments of intact protoplasts with cold glutaraldehyde or proteases before disruption lead to a diminution of glucan-synthase activities indicating that at least part of the enzymes of plasma membrane face the outside of the cell.

Nucleotide effects on glucan-synthesis activities of particulate enzymes from Saprolegnia

Membrane-bound β-1-3- and β-1-4-glucan synthetases of Saprolegnia are affected in vitro by the presence of nucleotides. Both enzymatic activities are inhibited by uridine nucleotides. Guanosine 5'-triphosphate and ATP reduce β-1-3-glucan synthesis but stimulate β-1-4-glucan production; they also increase V max without effect on the K m for uridine 5'-diphosphate glucose. The stimulation by ATP could be the result of an activation or stabilization of the enzymes and might have implications for cell-wall construction during hyphal growth.

β-1-3- and β-1-4-glucan synthesis by membrane fractions from the fungus Saprolegnia

The β-glucan synthetase activity of the fungus Saprolegnia monoica was assayed by supplying UDP-glucose to membrane fractions of mycelial homogenate. The analysis of glucan products by hydrolysis with various β-glucanases and by chromatography show that both β-1-3- and β-1-4-linkages are formed at high substrate concentrations. In the absence of MgCl2, β-1-3-linked glucans are mainly produced. By increasing MgCl2 concentrations the total synthesis activity and β-1-3-linkages production are reduced. At low substrate concentrations in the presence of MgCl2, β-1-4-linked glucans are the only polysaccharide synthesized. Electron microscopy of radioactive products, synthesized by original membrane fractions or by membrane fractions isolated from continuous sucrose density gradients, shows microfibrils when the assays are conducted at high substrate concentrations in the absence of MgCl2.