Sucrose synthase localizes to cellulose synthesis sites in tracheary elements

The synthesis of crystalline cellulose microfibrils in plants is a highly coordinated process that occurs at the interface of the cortex, plasma membrane, and cell wall. There is evidence that cellulose biogenesis is facilitated by the interaction of several proteins, but the details are just beginning to be understood. In particular, sucrose synthase, microtubules, and actin have been proposed to possibly associate with cellulose synthases (microfibril terminal complexes) in the plasma membrane. Differentiating tracheary elements of Zinnia elegans L. were used as a model system to determine the localization of sucrose synthase and actin in relation to the plasma membrane and its underlying microtubules during the deposition of patterned, cellulose-rich secondary walls. Cortical actin occurs with similar density both between and under secondary wall thickenings. In contrast, sucrose synthase is highly enriched near the plasma membrane and the microtubules under the secondary wall thickenings. Both actin and sucrose synthase lie closer to the plasma membrane than the microtubules. These results show that the preferential localization of sucrose synthase at sites of high-rate cellulose synthesis can be generalized beyond cotton fibers, and they establish a spatial context for further work on a multi-protein complex that may facilitate secondary wall cellulose synthesis.

Hysteresis and the dynamic phase transition in thin ferromagnetic films

Hysteresis and the nonequilibrium dynamic phase transition in thin magnetic films subject to an oscillatory external field have been studied by Monte Carlo simulation. The model under investigation is a classical Heisenberg spin system with a bilinear exchange anisotropy Lambda in a planar thin film geometry with competing surface fields. The film exhibits a nonequilibrium phase transition between dynamically ordered and dynamically disordered phases characterized by a critical temperature T(cd), whose location is determined by the amplitude H0 and frequency omega of the applied oscillatory field. In the presence of competing surface fields the critical temperature of the ferromagnetic-paramagnetic transition for the film is suppressed from the bulk system value T(c) to the interface localization-delocalization temperature T(ci). The simulations show that in general T(cd)<T(ci) for the model film. The profile of the time-dependent layer magnetization across the film shows that the dynamically ordered and dynamically disordered phases coexist within the film for T<T(cd). In the presence of competing surface fields, the dynamically ordered phase is localized at one surface of the film.

Adherens junctions and beta-catenin-mediated cell signalling in a non-metazoan organism

Mechanical forces between cells have a principal role in the organization of animal tissues. Adherens junctions are an important component of these tissues, connecting cells through their actin cytoskeleton and allowing the assembly of tensile structures. At least one adherens junction protein, beta-catenin, also acts as a signalling molecule, directly regulating gene expression. To date, adherens junctions have only been detected in metazoa, and therefore we looked for them outside the animal kingdom to examine their evolutionary origins. The non-metazoan Dictyostelium discoideum forms a multicellular, differentiated structure. Here we describe the discovery of actin-associated intercellular junctions in Dictyostelium. We have isolated a gene encoding a beta-catenin homologue, aardvark, which is a component of the junctional complex, and, independently, is required for cell signalling. Our discovery of adherens junctions outside the animal kingdom shows that the dual role of beta-catenin in cell-cell adhesion and cell signalling evolved before the origins of metazoa.

The cellulose synthase gene of Dictyostelium

Cellulose is a major component of the extracellular matrices formed during development of the social amoeba, Dictyostelium discoideum. We isolated insertional mutants that failed to accumulate cellulose and had no cellulose synthase activity at any stage of development. Development proceeded normally in the null mutants up to the beginning of stalk formation, at which point the culminating structures collapsed onto themselves, then proceeded to attempt culmination again. No spores or stalk cells were ever made in the mutants, with all cells eventually lysing. The predicted product of the disrupted gene (dcsA) showed significant similarity to the catalytic subunit of cellulose synthases found in bacteria. Enzyme activity and normal development were recovered in strains transformed with a construct expressing the intact dcsA gene. Growing amoebae carrying the construct accumulated the protein product of dcsA, but did not make cellulose until they had developed for at least 10 hr. These studies show directly that the product of dcsA is necessary, but not sufficient, for synthesis of cellulose.

Surface-induced ordering in thin uniaxial liquid crystal films

The interface localization transition in thin uniaxial liquid crystal films with competing surface fields has been studied using Metropolis Monte Carlo simulations. The model is constructed from a lattice of continuously orientable interacting spins, and the Hamiltonian contains both bilinear and biquadratic contributions. The biquadratic contribution to the Hamiltonian is familiar from the Lebwohl-Lasher model, and accounts for the particle anisotropy in a liquid crystal. The head-tail asymmetry of the molecules in a uniaxial liquid crystal is taken into account through a bilinear contribution familiar from the classical ferromagnetic Heisenberg model with exchange anisotropy Lambda. The critical temperature T(c), characterizing the interface localization transition within the uniaxial liquid crystal film, depends strongly on the relative magnitudes of the bilinear and biquadratic interactions between the spins. For systems dominated by the biquadratic interaction, T(c) is found to be close to the bulk critical temperature of the system. But as the biquadratic interaction strength is reduced, T(c) departs markedly from the bulk critical temperature of the system.

Cellulose microfibrils, cell motility, and plasma membrane protein organization change in parallel during culmination in Dictyostelium discoideum

Prestalk cells of Dictyostelium discoideum contribute cellulose to two distinct structures, the stalk tube and the stalk cell wall, during culmination. This paper demonstrates by freeze fracture electron microscopy that two distinct types of intramembrane particle aggregates, which can be characterized as cellulose microfibril terminal complexes, occur in the plasma membranes of cells synthesizing these different forms of cellulose. The same terminal complexes were observed in situ in developing culminants and in vitro in monolayer cells induced to synthesize the two types of cellulose. We propose that cessation of cell motility is associated with a change in packing and intramembrane mobility of the particle aggregates, which cause a change in the nature of the cellulose synthesized. The terminal complexes are compared to those described in other organisms and related to the previous hypothesis of two modes of cellulose synthesis in Dictyostelium.