Inge Erk

A complex genetic locus, polyhomeotic, is required for segmental specification and epidermal development in D. melanogaster

Two mutagenic events are required to make null mutations of polyhomeotic (ph), which suggests that the locus is complex. Amorphic mutations (ph degrees) die in mid-embryogenesis and completely lack ventral thoracic and abdominal epidermal derivatives, whereas single-event mutations lead to transformations similar to those of known dominant gain of function mutants in the Antennapedia and bithorax complexes. After a chromosomal walk, the ph gene was localized using deficiencies and ph mutations that result from DNA rearrangements. Hybridization analyses show that there are two large, duplicated sequences in the ph region, and DNA lesions affecting either one of these repeats alter the function of the ph locus. We propose a model that may account for this unusual functional organization.

Visualization of the Target-Membrane-Inserted Fusion Protein of Semliki Forest Virus by Combined Electron Microscopy and Crystallography

Semliki Forest virus enters cells by receptor-mediated endocytosis. The acidic environment of the endosome triggers a membrane fusion reaction that is mediated by the E1 glycoprotein. During fusion, E1 rearranges from an E1/E2 heterodimer to a highly stable, membrane-inserted E1 homotrimer (E1HT). In this study, we analyzed E1HT by a combination of electron cryomicroscopy, electron crystallography of negatively stained 2D crystals, and fitting of the available X-ray structure of the monomeric E1 ectodomain into the resulting 3D reconstruction. The visualized E1HT reveals that the ectodomain has reoriented vertically and inserted the distal tip of domain II into the lipid bilayer. Our data allow the visualization of a viral fusion protein inserted in its target membrane and demonstrate that insertion is a cooperative process, resulting in rings composed of five to six homotrimers.

Structural polymorphism of the major capsid protein of rotavirus

Rotaviruses are important human pathogens with a triple‐layered icosahedral capsid. The major capsid protein VP6 is shown here to self‐assemble into spherical or helical particles mainly depending upon pH. Assembly is inhibited either by low pH (<3.0) or by a high concentration (>100 mM) of divalent cations (Ca2+ and Zn2+). The structures of two types of helical tubes were determined by electron cryomicroscopy and image analysis to a resolution of 2.0 and 2.5 nm. In both reconstructions, the molecular envelope of VP6 fits the atomic model determined by X‐ray crystallography remarkably well. The 3‐fold symmetry of the VP6 trimer, being incompatible with the helical symmetry, is broken at the level of the trimer contacts. One type of contact is maintained within all VP6 particles (tubes and virus), strongly suggesting that VP6 assemblies arise from different packings of a unique dimer of trimers. Our data show that the protonation state and thus the charge distribution are important switches governing the assembly of macromolecular assemblies.

Rotavirus Nonstructural Protein NSP5 Interacts with Major Core Protein VP2

ABSTRACT Rotavirus is a nonenveloped virus with a three-layered capsid. The inner layer, made of VP2, encloses the genomic RNA and two minor proteins, VP1 and VP3, with which it forms the viral core. Core assembly is coupled with RNA viral replication and takes place in definite cellular structures termed viroplasms. Replication and encapsidation mechanisms are still not fully understood, and little information is available about the intermolecular interactions that may exist among the viroplasmic proteins. NSP2 and NSP5 are two nonstructural viroplasmic proteins that have been shown to interact with each other. They have also been found to be associated with precore replication intermediates that are precursors of the viral core. In this study, we show that NSP5 interacts with VP2 in infected cells. This interaction was demonstrated with recombinant proteins expressed from baculovirus recombinants or in bacterial systems. NSP5-VP2 interaction also affects the stability of VP6 bound to VP2 assemblies. The data presented showed evidence, for the first time, of an interaction between VP2 and a nonstructural rotavirus protein. Published data and the interaction demonstrated here suggest a possible role for NSP5 as an adapter between NSP2 and the replication complex VP2-VP1-VP3 in core assembly and RNA encapsidation, modulating the role of NSP2 as a molecular motor involved in the packaging of viral mRNA.

A Zinc Ion Controls Assembly and Stability of the Major Capsid Protein of Rotavirus

ABSTRACT The recent determination of the crystal structure of VP6, the major capsid protein of rotavirus, revealed a trimer containing a central zinc ion coordinated by histidine 153 from each of the three subunits. The role of the zinc ion in the functions of VP6 was investigated by site-directed mutagenesis. The mutation of histidine 153 into a serine (H153S and H153S/S339H) did not prevent the formation of VP6 trimers. At pH <7.0, about the pK of histidine, wild-type and mutated VP6 proteins display similar properties, giving rise to identical tubular and spherical assemblies. However, at pH >7.0, histidine 153 mutant proteins did not assemble into the characteristic 45-nm-diameter tubes, in contrast to wild-type VP6. These observations showed that under conditions in which histidine residues are not charged, the properties of VP6 depended on the presence of the centrally coordinated zinc atom in the trimer. Indeed, wild-type VP6 depleted of the zinc ion by a high concentration (100 mM) of a metal-chelating agent behaved like the H153 mutant proteins. The susceptibility of wild-type VP6 to proteases is greatly increased in the absence of zinc. NH2-terminal sequencing of the proteolytic fragments showed that they all contained the β-sheet-rich VP6 head domain, which appeared to be less sensitive to protease activity than the α-helical basal domain. Finally, the mutant proteins assembled well on cores, as demonstrated by both electron microscopy and rescue of transcriptase activity. Zinc is thus not necessary for the transcription activity. All of these observations suggest that, in solution, VP6 trimers present a structural flexibility that is controlled by the presence of a zinc ion.

Characterization of oligonucleotide/lipid interactions in submicron cationic emulsions: influence of the cationic lipid structure and the presence of PEG-lipids

We have recently described how oligonucleotide (ON) stability and release from O/W cationic emulsions are governed by the lipid composition. The aim of the present paper was to investigate the properties of the ON/lipid complexes through fluorescence resonance energy transfer (FRET), size, surface tension measurements and cryomicroscopy. Starting from a typical emulsion containing stearylamine as a cationic lipid, the influence of the lipid structure (monocationic molecules bearing mono or diacyl chains, or polycations) as well as of the presence of PEGylated lipids, were studied. The presence of a positive charge on the droplet surface clearly contributed to enhance the ON interaction with lipid monolayers and to bring the ON molecules closer to the interface. Hydrophobic interactions through the acyl chains were shown to further enhance the anchorage of the ON/lipid complexes. In contrast, the incorporation of PEGylated lipids acted as a barrier against the establishment of electrostatic bindings, the polyethyleneglycol chains acting themselves as interaction sites for the ON leading to hydrophilic complexes. Similar features were observed for the polycationic lipid, and cryomicroscopy revealed the existence of bridges of various intensities between the droplets of the emulsion containing either PEG or the polycation, probably because of the configuration of the ON at the interface.

Distribution of ring-X chromosomes in the blastoderm of gynandromorphic D. melanogaster

Embryos of the D. melanogaster strain producing gynandromorphs by loss of the ring-X chromosome were treated with vinblastine to obtain blastoderms with all mitoses arrested in metaphase, and with tetracaine to improve the resolution of chromosomes. Ring-X and ringless mitoses were recorded in the major part of the blastoderm in 18 eggs. Limits between females and male areas were very irregular and some embryos had several isolated areas of one type or the other. The proportion of male nuclei varied from 80.8 to 0.4%, indicating that there must have been more than one loss of the ring-X in most of the eggs and that losses occurred as late as the ninth division. When the percentages of male nuclei were compared with theoretical values, all the observed percentages could be accounted for by two losses. In early cleavages the lost ring could be found halfway between ringless mitoses. Examination of chromosomes in the three polar nuclei showed that the ring often remained undivided in meiosis. If the resulting ringless haploid group became a female pronucleus, an XO or YO embryo was produced after fertilization. We propose a hypothesis to explain the two losses.

New Bicompartmental Structures Are Observed When Stearylamine is Mixed with Triglyceride Emulsions

Over the past 30 years, efforts have been made in conceiving colloidal systems for drug administration and targeting. A wide range of dispersed formulations have been designed including liposomes, nanoand microparticles, as well as single emulsions, and some of these systems are now even associated with marketed new drugs. Recently, lipids and especially cationic lipid-based systems have gained an increasing interest due to their ability to condense DNA and to allow efficient transfection at least in vitro and exvivo (1–3). Numerous investigations have been performed in order to characterize the supramolecular assemblies formed between nucleic acids and lipids and to find some structure–activity relationship (4). In this context, we have recently developed a positivelycharged emulsion based on lecithin, medium-chain triglycerides, poloxamer 188, and stearylamine as a delivery system for oligonucleotides. This emulsion, easy to prepare and welltolerated, was found to dramatically improve stability of oligonucleotides in the presence of serum (5,6). With the aim to characterize the morphology of this emulsion, we have performed some cryomicroscopic examinations leading to the identification of unexpected structures. This rapid communication describes these new colloidal systems consisting of two compartments with an oily core and an aqueous phase associated with the same structure. Such items, which were never described before in the literature, could be useful for drug administration when both hydrophilic and lipophilic molecules need to be administered together.

The interaction between lipid derivatives of colchicine and tubulin: consequences of the interaction of the alkaloid with lipid membranes.

Colchicine is a potent antimitotic poison which is well known to prevent microtubule assembly by binding tubulin very tightly. Colchicine also possesses anti-inflammatory properties which are not well understood yet. Here we show that colchicine tightly interacts with lipid layers. The physical and biological properties of three different lipid derivatives of colchicine are investigated parallel to those of membrane lipids in the presence of colchicine. Upon insertion in the fatty alkyl chains, colchicine rigidifies the lipid monolayers in a fluid phase and fluidifies rigid monolayers. Similarly X-ray diffraction data show that lecithin-water phases are destabilized by colchicine. In addition, an unexpectedly drastic enhancement of the photoisomerization rate of colchicine into lumicolchicine in the lipid environment is observed and further supports insertion of the alkaloid in membranes. Finally the interaction of colchicine with lipids makes the drug inaccessible to tubulin. The possible in vivo significance of these results is discussed.

Electron cryo‐microscopy of vitrified biological specimens: towards high spatial and temporal resolution

A decade after the development of electron cryo‐microscopy for vitrified specimens, its advantages and limitations are analysed. Indeed, recent work carried out by different laboratories strengthens the idea that electron cryo‐microscopy might soon be an alternative method to X‐ray crystallography and NMR techniques for determining the structure of biological assemblies with both high spatial and temporal resolutions. High pressure freezing allows vitrification of larger volumes of biological suspensions. Thick vitrified objects can be cryosectioned. Electron cryo‐microscopy of the sections gives images having a resolution better than 2 nm. Although the high resolution imaging mode under low dose conditions is not yet fully understood, microscopes are being developed to provide better and better images. Image averaging is being facilitated by the development of both crystallization and computer methods. Thus, we can expect that electron microscopy will soon become a potential technique for structural determination at atomic resolution. Finally, much effort is being devoted to improving the temporal resolution of electron cryo‐microscopy. Soon, we may be able to observe molecules during their biological activity.