Swelling of brome mosaic virus as studied by intensity fluctuation spectroscopy

Hydrophobic Cargo Encapsulation into Virus Protein Cages by Self-Assembly in an Aprotic Organic Solvent

While extensive studies of virus capsid assembly in environments mimicking in vivo conditions have led to an understanding of the thermodynamic driving forces at work, applying this knowledge to virus assembly in other solvents than aqueous buffers has not been attempted yet. In this study, Brome mosaic virus (BMV) capsid proteins were shown to preserve their self-assembly abilities in an aprotic polar solvent, dimethyl sulfoxide (DMSO). This facilitated protein cage encapsulation of nanoparticles and dye molecules that favor organic solvents, such as β-NaYF4-based upconversion nanoparticles and BODIPY dye. Assembly was found to be robust relative to a surprisingly broad range of DMSO concentrations. Cargos with poor initial stability in aqueous solutions were readily encapsulated at high DMSO concentrations and then transferred to aqueous solvents, where they remained stable and preserved their function for months.

Virus-mimicking optical nanomaterials: near infrared absorption and fluorescence characteristics and physical stability in biological environments

The use of viruses as platforms for the development of optical imaging materials has received increasing attention in recent years. We have engineered a hybrid nanomaterial composed of the capsid proteins of genome-depleted plant-infecting Brome mosaic virus that encapsulates the near-infrared (NIR) dye indocyanine green. Herein, we investigate the NIR absorption and fluorescence characteristics of these nanomaterials in biological environments consisting of cell culture media with and without serum proteins. Our results demonstrate that the NIR absorption and fluorescence emission of the constructs are enhanced in the presence of serum proteins. The constructs remain physically stable and maintain their NIR absorption and fluorescence properties for at least 79 days. The presence of serum proteins also reduces the aggregation of the constructs. These findings have relevance for the further development of optical imaging and phototherapeutic methods on the basis of such virus-mimicking nanomaterials as well as the expected optical and physical characteristics of these nanomaterials in vivo.

Photothermal imaging and measurement of protein shell stoichiometry of single HIV-1 Gag virus-like nanoparticles

Virus life stages often constitute a complex chain of events, difficult to track in vivo and in real-time. Challenges are associated with spatial and time limitations of current probes: most viruses are smaller than the diffraction limit of optical microscopes while the entire time scale of virus dynamics spans over 8 orders of magnitude. Thus, virus processes such as entry, disassembly, and egress have generally remained poorly understood. Here we discuss photothermal heterodyne imaging (PHI) as a possible alternative to fluorescence microscopy in the study of single virus-like nanoparticle (VNP) dynamics, with relevance in particular to virus uncoating. Being based on optical absorption rather than emission, PHI could potentially surpass some of the current limitations associated with fluorescent labels. As proof-of-principle, single VNPs self-assembled from 60 nm DNA-functionalized gold nanoparticles (DNA-Au NPs) encapsulated in a Gag protein shell of the human immunodeficiency virus (HIV-1) were imaged, and their photothermal response was compared with DNA-Au NPs. For the first time, the protein stoichiometry of a single virus-like particle was estimated by a method other than electron microscopy.

The kinetics of swelling of southern bean mosaic virus: a study using photon correlation spectroscopy

Southern bean mosaic virus swells upon removal of Ca2+ at pH 8.25. Virions do not seem to aggregate significantly; the z-average hydrodynamic diameter increases from 29.9 nm to 44.0 nm. Swelling is virtually complete within 3 min, and swollen virions have a z-average hydrodynamic diameter similar to that of virions swollen by dialysis overnight.

Conformational changes preceding decapsidation of bromegrass mosaic virus under hydrostatic pressure: a small-angle neutron scattering study

The stability of bromegrass mosaic virus (BMV) and empty shells reassembled in vitro from purified BMV coat protein was investigated under hydrostatic pressure, using solution small-angle neutron scattering. This technique allowed us to monitor directly the dissociation of the particles, and to detect conformational changes preceding dissociation. Significant dissociation rates were observed only if virions swelled upon increase of pressure, and pressure effects became irreversible at very high-pressure in such conditions. At pH 5.0, in buffers containing 0.5 M NaCl and 5 mM MgCl(2), BMV remained compact (radius 12.9 nm), dissociation was limited to approximately 10 % at 200 MPa, and pressure effects were totally reversible. At pH 5.9, BMV particles were slightly swollen under normal pressure and swelling increased with pressure. The dissociation was reversible to 90 % for pressures up to 160 MPa, where its rate reached 28 %, but became totally irreversible at 200 MPa. Pressure-induced swelling and dissociation increased further at pH 7.3, but were essentially irreversible. The presence of (2)H(2)O in the buffer strongly stabilized BMV against pressure effects at pH 5.9, but not at pH 7.3. Furthermore, the reversible changes of the scattered intensity observed at pH 5.0 and 5.9 provide evidence that pressure could induce the release of coat protein subunits, or small aggregates of these subunits from the virions, and that the dissociated components reassociated again upon return to low pressure. Empty shells were stable at pH 5.0, at pressures up to 260 MPa. They became ill-shaped at high-pressure, however, and precipitated slowly after return to normal conditions, providing the first example of a pressure-induced conformational drift in an assembled system.

Identification of regions of brome mosaic virus coat protein chemically cross-linked in situ to viral RNA

RNA-protein cross-links were introduced into brome mosaic virus in situ by using the heterobifunctional agent p-azidophenylglyoxal. An improved RNA isolation method, without phenol extraction, was used to isolate RNA cross-linked with protein. RNA of the covalently linked complex was acid-digested and the oligonucleotides still attached to protein were 5'-end-labelled with 32P. The complexes were digested with trypsin and the tryptic peptides were purified by reversed-phase high-performance liquid chromatography. Amino acid analyses of cross-linked tryptic peptides revealed that out of the total 188 amino acids of brome mosaic virus coat protein only the 80 N-terminal amino acids are involved in the interaction with viral RNA. These results are discussed in connection with a predicted secondary structure of the coat protein. Both alpha helix (for amino acids 11-19) and other structures (between amino acids 20 and 80) are implicated in the coat protein-viral RNA interactions.

Structure of the Top a-t component of alfalfa mosaic virus. A non-icosahedral virion

Neutron-scattering in combination with quasi-elastic light-scattering and electron microscopy was used to derive a model for the capsid structure of the Top a-t component of alfalfa mosaic virus (AMV-Ta-t). In the electron microscope, AMV-Ta-t appears as an irregular ellipsoidal particle with apparent dimensions 275 (+/- 31) A X 225 (+/- 22) A. Assuming that the particles are monodisperse, model calculations show that the neutron-scattering data are best explained by an oblate ellipsoidal shape for the virion with external dimensions 284 A X 284 A X 216 A. Based on this result, and in combination with the known composition of the virion, it is suggested that the capsid structure could be based on a deltahedron with 52 pointgroup symmetry and comprising 120 subunits. Such a model would imply a greater deviation from equivalent subunit interactions than normally necessary in icosahedral capsids. The neutron and photon correlation data, however, do not allow us to rule out the possibility that Top a-t is a slightly polydisperse preparation of irregular prolate shapes with mean dimensions 312 A X 232 A X 232 A. Both possibilities support the concept of alfalfa mosaic virus coat protein being capable of a wide range of intersubunit interactions, this flexibility resulting in considerable polymorphism in capsid structures.

Self-assembly of brome mosaic virus protein into capsids. Initial and final states of aggregation

The pH and ionic strength dependence of the states of aggregation of brome mosaic virus protein has been investigated by small angle neutron scattering, quasielastic light-scattering, analytical centrifugation and electron microscopy. At pH above neutrality, protein oligomers are found in dynamical equilibrium, comprising monomers, dimers and aggregates of higher molecular weight. By lowering the pH, capsids assemble spontaneously with dimensions in solution which depend on ionic strength. If formed by dialysis, they contain 180 monomers, but are 30 A larger in diameter than the native virus. If formed by pH-jump, they contain less monomers: the deficiency decreases with decreasing the final pH and the initial protein concentration. Upon dehydration for electron microscopy, capsids contract by 10%.

Functional acetylcholine receptor from Torpedo marmorata in planar membranes

Planar bilayer membranes containing functional acetylcholine receptor were formed from vesicles of Torpedo marmorata electric organ without extracting the acetylcholine receptor from its native environment. Native vesicles were transformed into monolayers which subsequently were apposed into planar bilayers. In the absence of agonists the membrane conductance was similar to that of lipid bilayers. Addition of carbamoylcholine or succinylcholine caused increased membrane conductance and this could be competitively inhibited by d-tubocurarine and suppressed by alpha-bungarotoxin. The amplitude of the conductance response was proportional to the number of alpha-bungarotoxin binding sites in the bilayers. Asymmetric membranes could be formed with the ligand binding sites on only one membrane surface. Desensitization of acetylcholine receptor was evident from equilibrium and kinetic data of the carbamoylcholine-activated conductance. Carbamoylcholine-induced membrane permeability was about 7 times higher for K+ and Na+ ions than for Cl-. At low levels of conductance, single-channel fluctuations of 20-25 pS in conductance and 1.3-msec lifetime were resolved in physiological saline containing carbamoylcholine. The ratio of observed channels to alpha-bungarotoxin sites present showed that a significant fraction of acetylcholine receptor in the membrane was functional. The quantitative aspects of the cation channel, the desensitization, and the ligand binding properties were in close agreement with established values. This transformation of natural acetylcholine receptor vesicles to planar bilayers conserves the essential properties of the in vivo receptor.

Exchange and interactions between lipid layers at the surface of a liposome solution

Lipid organization and lipid transport processes occurring at the air-water interface of a liposome (lipid vesicle) solution are studied by conventional surface pressure-area measurements and interpreted by an adequate theory. At the interface of a dioleoyl phosphatidylcholine vesicle solution, used for demonstration, a well defined two layer structure selfassembles: vesicles disintegrate at the interface forming a surface-adsorbed lipid monolayer, which prevents further disintegration beyond about 1 dyne/cm surface pressure. A layer of vesicles now assembles in close association with the monolayer. This layer is in vesicle diffusion exchange with the solution and in lipid exchange with the monolayer. The lipid exchange occurs exclusively between the monolayer and the outer lipid layer of the vesicles; it is absent between outer and inner vesicle layers. Equilibration of the lipid density in the monolayer with that in the vesicle outer layer provides a coherent and quantitative explanation of the observed hysteresis effects and equilibrium states. The correspondence between monolayer and vesicle outer layer is traced down to equilibrium constants and rate constants and their dependences on surface pressure, vesicle size and concentration. Other alternate realizations of surface structure and exchange, including induced lipid flip-flop within vesicles or vesicle monolayer adhesion or fusion are potential applications of the proposed analysis.