Membrane remodelling plays an important role in cellular tasks such as endocytosis, vesiculation and protein sorting, and in the biogenesis of organelles such as the endoplasmic reticulum or the Golgi apparatus. It is well established that the remodelling process is aided by specialized proteins that can sense as well as create membrane curvature, and trigger tubulation when added to synthetic liposomes. Because the energy needed for such large-scale changes in membrane geometry significantly exceeds the binding energy between individual proteins and between protein and membrane, cooperative action is essential. It has recently been suggested that curvature-mediated attractive interactions could aid cooperation and complement the effects of specific binding events on membrane remodelling. But it is difficult to experimentally isolate curvature-mediated interactions from direct attractions between proteins. Moreover, approximate theories predict repulsion between isotropically curving proteins. Here we use coarse-grained membrane simulations to show that curvature-inducing model proteins adsorbed on lipid bilayer membranes can experience attractive interactions that arise purely as a result of membrane curvature. We find that once a minimal local bending is realized, the effect robustly drives protein cluster formation and subsequent transformation into vesicles with radii that correlate with the local curvature imprint. Owing to its universal nature, curvature-mediated attraction can operate even between proteins lacking any specific interactions, such as newly synthesized and still immature membrane proteins in the endoplasmic reticulum.

Vesicles formed in water by synthetic macro-amphiphiles have attracted much attention as nanocontainers having properties that extend the physical and chemical limits of liposomes. We sought to develop ABA block copolymeric amphiphiles that self-assemble into unilamellar vesicles that can be further oxidatively destabilized. We selected poly(ethylene glycol) (PEG) as the hydrophilic A blocks, owing to its resistance to protein adsorption and low toxicity. As hydrophobic B blocks, we selected poly(propylene sulphide) (PPS), owing to its extreme hydrophobicity, its low glass-transition temperature, and most importantly its oxidative conversion from a hydrophobe to a hydrophile, poly(propylene sulphoxide) and ultimately poly(propylene sulphone). This is the first example of the use of oxidative conversions to destabilize such carriers. This new class of oxidation-responsive polymeric vesicles may find applications as nanocontainers in drug delivery, biosensing and biodetection.

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In this paper, we present the application of four different in situ analytical techniques to monitor the solvent-mediated polymorphic transformation of L-glutamic acid. Focused beam reflectance measurement (FBRM) and particle vision and measurement (PVM) have been used to track the chord length and morphology of the crystals over the course of the transformation. The polymorphic forms present have been monitored using Raman spectroscopy, while attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy has been used to measure the liquid-phase concentration profile. The combination of the different in situ data was used to identify the fundamental phenomena of nucleation and growth that govern the process. Moreover, the measurement data were combined with a mathematical model based on population balance equations and the fundamental equations describing the kinetics of nucleation and growth of both polymorphs. This combination allowed for the estimation of the characteristic nucleation and growth rates of the two polymorphic forms, while the dissolution process of the metastable polymorph was estimated using a Sherwood correlation. Finally, the experimental results obtained with different initial conditions and their simulation allowed for the validation of the population balance model and for a deeper understanding of the transformation process.

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The 1:1 complex of the mutant Antp(C39––S) homeodomain with a 14 bp DNA fragment corresponding to the BS2 binding site was studied by nuclear magnetic resonance (NMR) spectroscopy in aqueous solution. The complex has a molecular weight of 17,800 and its lifetime is long compared with the NMR chemical shift time scale. Investigations of the three‐dimensional structure were based on the use of the fully 15N‐labelled protein, two‐dimensional homonuclear proton NOESY with 15N(omega 2) half‐filter, and heteronuclear three‐dimensional NMR experiments. Based on nearly complete sequence‐specific resonance assignments, both the protein and the DNA were found to have similar conformations in the free form and in the complex. A sufficient number of intermolecular 1H‐1H Overhauser effects (NOE) could be identified to enable a unique docking of the protein on the DNA, which was achieved with the use of an ellipsoid algorithm. In the complex there are intermolecular NOEs between the elongated second helix in the helix‐turn‐helix motif of the homeodomain and the major groove of the DNA. Additional NOE contacts with the DNA involve the polypeptide loop immediately preceding the helix‐turn‐helix segment, and Arg5. This latter contact is of special interest, both because Arg5 reaches into the minor groove and because in the free Antp(C39––S) homeodomain no defined spatial structure could be found for the apparently flexible N‐terminal segment comprising residues 0‐6.

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Fab-7 deletions in the bithorax complex have a novel gain-of-function phenotype, typically transforming parasegment 11 (PSll) into PS12 identity. Genetic analysis indicates that removal of the Fab-7 element results in the fusion of the lab-6 (PSll) and iab-7 (PS12) cis-regulatory domains into a single regulatory domain that inappropriately regulates Abdominal-B in PSll. This has led to the hypothesis that Fab-7 is a chromatin domain boundary that normally functions to ensure the autonomous activity of the iab-6 and iab-7 cis-regulatory domains. We use several different enhancer blocking assays to demonstrate that Fab-7 has the insulating properties expected of a domain boundary. We define a minimal fragment of Fab-7 sufficient for enhancer blocking, and demonstrate that it is completely distinct from an adjacent Polycomb-dependent silencer. We compare Fab-7 to the su ( [Key Words: Bithorax complex; Fab-7; chromatin domain boundary; insulator; suppressor of Hairy-wing] Received September 19, 1996; revised version accepted November 1, 1996.Body patterning in many organisms involves the development of segments whose unique identities are specified by homeotic genes. The precise expression patterns of the homeotic genes are crucial for generating a normal body plan, and the misregulation of these genes can result in dramatic transformations of one body segment into another. In Drosophila, the identity of parasegments in the posterior of the fly is controlled by the three homeotic genes of the bithorax complex (BX-C), Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B} (Lewis 1978;Duncan 1987). The parasegmentspecific expression patterns of these three genes are generated by a complicated cis-regulatory region that spans a DNA segment of 300 kb. This cis-regulatory region is organized in a series of nine parasegment-specific regulatory elements or domains, abx /bx, bxd/pbx, iab-2, iab-3, iab-4, iab-5, lab-6, iab-7, and iab-8 (Lewis 1978;Karch et al. 1985;Celniker et al. 1990;Sanchez-Herrero 1991). Each of these domains directs the expression of one of the three BX-C homeotic genes in a specific parasegment. For example, the lab-5, lab-6, and lab-7 cisregulatory domains direct Abd-B expression in parasegmerits (PS) 10, 11, and 12, respectively (Karch et al. 1985). ~Corresponding author. 2present address: