Ultrastructure and development during meiosis and the tetrad period of sporogenesis in the leptosporangiate fern Alsophila setosa (Cyatheaceae) compared with corresponding stages in Psilotum nudum (Psilotaceae)

The pre-meiotic, meiotic and tetrad stages of development in microsporangia of Alsophila setosa were studied with particular emphasis on the early establishment of patterning in the microspore wall and the subsequent development of the sporoderm. The data obtained were compared with corresponding ontogenetic stages of Psilotum nudum. Tapetal behaviour was also examined. During the tetrad period, only one layer, a thin undulating sheet, appeared alongside the plasma membrane of the tetraspores, and this was evidently formed on a pre-patterned structure – a fibrillar layer, corresponding to a kind of primexine matrix. The early free microspores had a wavy plasma membrane with a parallel, sinusoidal, thin initial sporoderm layer. The proximal apertural fold was observed to be an extended outgrowth of this initial spore envelope. Sporoderm ontogeny during the tetrad period in Alsophila and Psilotum show some common points, but also fundamental differences, mainly in the relative timing of events: in Alsophila the end of the tetrad period is the starting point for exospore development, whereas in Psilotum the exospore is already complete at this stage. Considerable differences were also observed in the tapetum of the two species.     

Trypsin In Lecithin Based w/o Microemulsions. Fluorescence and Enzyme Activity Studies

Lecithin based microemulsions were used as model systems for enzymic studies. The phase behavior of the system: purified soya bean lecithin/propan-1-ol/isooctane/water was examined. It was found that the ability of the system to solubilize water was strongly affected by the lecithin and alcohol concentrations. Trypsin was entrapped in lecithin microemulsion systems of different composition and tested for proteolytic activity on the hydrolysis of lysine-p-nitroanilide (LNA). The kinetic constants were determined and in most cases the ratio k_cat/Km was higher than that observed in aqueous solution. The optimum enzyme activity was found at pH 9 for the system formulated with 5% w/w lecithin in isooctane, while increasing w_o, where w_o = [H₂o]/[Lecithin], the enzyme activity followed a bell-shaped pattern with a maximum at w_o= 20. The stability of trypsin in microemulsions was higher in the low water containing systems. Using the fluorescence quenching technique it was found that the system compartmentalization depended on the water content and the presence of the enzyme. Time-resolved luminescence decay studies were carried out to clarify the effect of the water content and the presence of the enzyme molecules on the micro-emulsion structure. The analysis of the luminescence data was done with a “percolation” model of stretched exponential. A dramatic variation of the water/oil interface occurred above the percolation threshold, while the addition of the enzyme induced a more restricted microenvironment.     

New trends in cryoenzymology : II. - Aqueous solutions of enzymes in apolar solvents.

In order to set up new procedures to investigate enzyme systems at subzero temperatures in pure aqueous media, we used micromicellar solutions which are homogeneous, optically transparent and of low viscosity in that range of temperatures. The preparation and the main properties of such solutions are described along with the behavior of enzyme-substrate intermediates. A critical discussion of results permits to examine advantages as well as limitations of this very promising procedure.     

Bicelle size modulates the rate of bacteriorhodopsin folding

The conformational equilibria of integral membrane proteins have proven extremely difficult to characterize within native lipid bilayers. To circumvent technical issues, investigations of the structure and stability of α‐helical membrane proteins are often carried out in mixed micelle or bicelle solvents that mimic the membrane and facilitate measurements of reversible folding. Under these conditions, the energetics of membrane protein folding are typically proportional to the mole fraction of an anionic detergent in the micelle. However, investigations of the folding and unfolding of bacteriorhodopsin (bR) surprisingly revealed that the folding rate is also highly sensitive to the bulk molar concentration of lipids and detergents. We show here that this rate enhancement coincides with changes in bicelle size and suggest this effect arises through restriction of the conformational search space during folding. In conjunction with previous mutagenic studies, these results provide additional evidence that a topological search limits the rate of bR folding. Furthermore, this finding provides insights into the manner by which micellar and bicellar environments influence the conformational stability of polytopic membrane proteins.     

Tuning micelles of a bioactive heptapeptide biosurfactant via extrinsically induced conformational transition of surfactin assembly

We have studied the effects of extrinsic environmental conditions on the conformation of surfactin, a heptapeptide biosurfactant from Bacillus subtilis, in aqueous solutions. It has been made clear that temperature, pH, Ca²⁺ ions and the synthetic nonionic surfactant hepta-ethylene glycol (C₁₂E₇) affect the conformation of surfactin in aqueous solutions. The β-sheet formation reached a maximum at 40°C both in presence and absence of (C₁₂E₇) and the nonionic surfactant enhances the β-sheet formation even at 25°C. Ca²⁺ induced the formation of a-helices and caused this transition at 0.3 mm with surfactin monomers or at 0.5 mm with surfactin micelles, but above these transition concentrations of Ca²⁺ β-sheets were observed. In micellar solution the β-sheet structure was stabilized at pH values below 7 or upon addition of Ca²⁺ in concentrations above 0.5 mm . Our results indicated that the bioactive conformation of surfactin is most likely the β-sheets when the molecules are assembled in micelles. The β-sheet structure in micelles could be retained by tuning the micelles. Surfactin micelles could be tuned in the bioactive conformation by manipulating pH, temperature, Ca²⁺ or (C₁₂E₇) concentrations in surfactin solutions. Our results strongly indicated that Ca²⁺ and other molecules (such as C₁₂E₇) may function as directing templates in the assembly and conformation of surfactin in micelles. Thus, we suggest environmental manipulation and template-aided micellation (TAM) as a new approach for preparing predesigned micelles, microemulsions or micro-spheres for specific application purposes. © 1998 European Peptide Society and John Wiley & Sons, Ltd.     

The study of the conformation and interaction of two tachykinin peptides in membrane mimicking systems by NMR spectroscopy and pulsed field gradient diffusion

Pulsed‐field gradient diffusion has been used to study the binding of two tachykinin peptides, [Tyr⁸]‐substance P (SP) and [Tyr⁰]‐neurokinin A (NKA) to two membrane‐mimicking micelles, dodecylphosphocholine, and sodium dodecylsulfate. The structure of these peptides bound to the micelles have also been studied by using two‐dimensional nmr and restrained simulated annealing calculations. No major difference in the structures of each peptide in the two micellar media was found. The difference between the micelle‐bound structure of [Tyr⁸]SP and that of SP was also minor. The longer helical conformation on the C‐terminus for [Tyr⁰]NKA was observed, compared with that for NKA. The relationship between the difference in the biological potencies of [Tyr⁸]SP and SP and the differences in their structure, especially the interaction of the side chains of the two aromatic residues, and the difference in their binding affinities to membrane was discussed. In addition, differences between the result of restrained molecular dynamics simulations of [Tyr⁸]SP in the presence of an explicit micelle and the present results were observed and discussed. © 1999 John Wiley & Sons, Inc. Biopoly 50: 555–568, 1999     

Extraction of lysozyme and ribonuclease-a using reverse micelles: Limits to protein solubilization

Experiments are reported here on the equilibrium partitioning of lysozyme and ribonuclease-a between aqueous and reversed micellar phases comprised of an anionic surfactant, sodium di-2-ethylhexyl sulfosuccinate (AOT), in isooctane. A distinct maximum, [P](rm,max) was found for the quantity of a given protein that can be solubilized in the reverse micelle phase by the phase-transfer method. This upper limit depended upon the size of the protein, the surfactant concentration, and the aqueous phase ionic strength, and was determined by complex formation between protein and surfactant molecules to form an insoluble interfacial precipitate at high values of [P](rm). In this work, it was found to be possible to dissociate the protein-surfactant complex and recover the precipitated protein. The kinetics of protein-surfactant complex formation depended upon the nature and concentration of the solubilized protein and on the surfactant concentration. Calculations of micellar occupancy and the relative surface areas of protein molecules and surfactant head-groups suggested that it was the exposure of the solubilized protein to the bulk organic solvent which promoted protein-surfactant complex formation as [P](rm) --> [P](rm,max). In the light of the experimental results and calculations described above, a mechanistic model is proposed to account for the observed phenomena. This is based upon the competing effects of increasing the solubilized protein concentration and the corresponding increase in the rate of protein-surfactant complex formation. The dynamic nature of the reverse micelles is inherent in the model, explaining the formation of the interfacial precipitate with time and its dependence on the internal phase volume of the micellar phase. Experiments on the co-partitioning of water and measurement ofthe AOT concentration in both phases verified the loss of protein, water, and surfactant from the organic phase at high values of [P](rm). (c) 1995 John Wiley & Sons Inc.     

Solution structure of human saposin C in a detergent environment

Saposin C is a lysosomal, membrane-binding protein that acts as an activator for the hydrolysis of glucosylceramide by the enzyme glucocerebrosidase. We used high-resolution NMR to determine the three-dimensional solution structure of saposin C in the presence of the detergent sodium dodecyl sulfate (SDS). This structure provides the first representation of membrane bound saposin C at the atomic level. In the presence of SDS, the protein adopts an open conformation with an exposed hydrophobic pocket. In contrast, the previously reported NMR structure of saposin C in the absence of SDS is compact and contains a hydrophobic core that is not exposed to the solvent. NMR data indicate that the SDS molecules interact with the hydrophobic pocket. The structure of saposin C in the presence of SDS is very similar to a monomer in the saposin B homodimer structure. Their comparison reveals possible similarity in the type of protein/lipid interaction as well as structural components differentiating their quaternary structures and functional specificity.     

Minimal model of self-replicating nanocells: a physically embodied information-free scenario

The building of minimal self-reproducing systems with a physical embodiment (generically called protocells) is a great challenge, with implications for both theory and applied sciences. Although the classical view of a living protocell assumes that it includes information-carrying molecules as an essential ingredient, a dividing cell-like structure can be built from a metabolism-container coupled system only. An example of such a system, modelled with dissipative particle dynamics, is presented here. This article demonstrates how a simple coupling between a precursor molecule and surfactant molecules forming micelles can experience a growth-division cycle in a predictable manner, and analyses the influence of crucial parameters on this replication cycle. Implications of these results for origins of cellular life and living technology are outlined.     

Conformational analysis of bioactive peptides in reverse micelles as mimics of cell membrane environments

Summary Conformational preferences of secretin as a model peptide have been analyzed by CD and IR spectroscopy in reverse micelles of AOT/isooctane/water and compared to those in aqueous TFE, in SDS micelles and in DMPG vesicles. Among the systems examined, reverse micelles and phospholipid vesicles displayed almost identical conformational equilibria. Very high lipid-to-peptide ratios can be obtained in reverse micelles with full retention of optical transparency, even at millimolar peptide concentrations, thus indicating this system to be an interesting mimic of cell membrane environments for spectroscopic analysis of bioactive peptide conformations.Abbreviations TFE trifluoroethanol - SDS sodium dodecyl sulfate - DMPG dimyristoylphosphatidylglycerol - AOT bis(2-ethylhexyl)sulfosuccinate - CMC critical micellar concentration - VIP vasoactive intestinal peptide