This study concerns the self-assembly of virus-like particles (VLPs) composed of an icosahedral virus protein coat encapsulating a functionalized spherical nanoparticle core. The recent development of efficient methods for VLP self-assembly has opened the way to structural studies. Using electron microscopy with image reconstruction, the structures of several VLPs obtained from brome mosaic virus capsid proteins and gold nanoparticles were elucidated. Varying the gold core diameter provides control over the capsid structure. The number of subunits required for a complete capsid increases with the core diameter. The packaging efficiency is a function of the number of capsid protein subunits per gold nanoparticle. VLPs of varying diameters were found to resemble to three classes of viral particles found in cells (T ‫؍‬ 1, 2, and 3). As a consequence of their regularity, VLPs form three-dimensional crystals under the same conditions as the wild-type virus. The crystals represent a form of metallodielectric material that exhibits optical properties influenced by multipolar plasmonic coupling. metamaterials ͉ protein cage ͉ self-assembly ͉ surface plasmon ͉ virus assembly E ngineered virus capsids and protein cage structures have shown increasing promise as therapeutic and diagnostic vectors (1-6), imaging agents (7-10), and as templates and microreactors for advanced nanomaterials synthesis (11)(12)(13)(14)(15)(16)(17). Here, we address the rules for the formation of symmetric protein cages consisting of viral capsid subunits formed over a functionalized inorganic nanoparticle core, called virus-like particles (VLPs).VLPs provide an example of how biomimetic self-organization can combine the natural characteristics of virus capsids with the exquisite physical properties of nanoparticles (18)(19)(20). Interactions between the artificial cargo and the protein carrier affects both the self-assembly and the stability of the resulting structure, yet very little is known about them. Progress toward the basic development and the practical use of VLPs requires an understanding of how relevant parameters contribute to complex formation.Symmetric VLPs may provide a technology for therapeutic or diagnostic agent delivery that is improved over amorphous shell nanoparticles that are already known to be efficient in similar applications (21). The advantage of a regular surface protein motif is that the binding domains are functionally identical by virtue of their equivalent environment. It has been shown in several situations that receptor-mediated targeting can be achieved even when using amorphous coatings (22). However, the principle challenges for nanoparticle delivery currently include: limited life-time in body fluids, nanoparticle transduction across the cellular membrane, avoidance of the exocytotic pathways, and target specificity. To optimize their infectivity, viruses have evolved to overcome these challenges. We still must learn how to apply virus strategies to targeted delivery. A simple question is central to this o...

Toll-like receptors (TLRs) mediate responses to pathogenassociated molecules as part of the vertebrate innate immune response to infection. Receptor dimerization is coupled to downstream signal transduction by the recruitment of a postreceptor complex containing the adaptor protein MyD88 and the IRAK protein kinases. In this work, we show that the death domains of human MyD88 and IRAK-4 assemble into closed complexes having unusual stoichiometries of 7:4 and 8:4, the Myddosome. Formation of the Myddosome is likely to be a key event for TLR4 signaling in vivo as we show here that pathway activation requires that the receptors cluster into lipid rafts. Taken together, these findings indicate that TLR activation causes the formation of a highly oligomeric signaling platform analogous to the death-inducing signaling complex of the Fas receptor pathway.

Self-assembling icosahedral protein cages have potencially useful physical and chemical characteristics for a variety of nanotechnology applications, ranging from therapeutic or diagnostic vectors to building blocks for hierarchical materials. For application-specific functional control of protein cage assemblies, a deeper understanding of the interaction between the protein cage and its payload is necessary. Protein-cage encapsulated nanoparticles, with their well-defined surface chemistry, allow for systematic control over key parameters of encapsulation such as the surface charge, hydrophobicity, and size. Independent control over these variables allows experimental testing of different assembly mechanism models. Previous studies done with Brome mosaic virus capsids and negatively charged gold nanoparticles indicated that the result of the selfassembly process depends on the diameter of the particle. However, in these experiments, the surface-ligand density was maintained at saturation levels, while the total charge and the radius of curvature remained coupled variables, making the interpretation of the observed dependence on the core size difficult. The current work furnishes evidence of a critical surface charge density for assembly through an analysis aimed at decoupling the surface charge and the core size.

Incorporation of CdSe/ZnS semiconductor quantum dots (QDs) into viral particles provides a new paradigm for the design of intracellular microscopic probes and vectors. Several strategies for the incorporation of QDs into viral capsids were explored; those functionalized with poly(ethylene glycol) (PEG) can be self-assembled into viral particles with minimal release of photoreaction products and enhanced stability against prolonged irradiation.

Features attributed to ferric iron in remotely sensed spectral data of Mars and the magnetic nature of Martian soil at the Viking landing sites are consistent with the occurrence of hematite (o•-Fe203) as both superparamagnetic (nanocrystalline) hematite (sp-Hm) and larger-diameter hematite (bulk-Hm) particles. These hematite particles most likely occur in pigmentary form, that is, as particles dispersed throughout the volume of a relatively spectrally neutral (silicate?) material. Likely physical forms of this pigmented volume include rocks, dust and soil particles, and coatings (weathering rinds) thereon. Accommodation of Martian data by hematite is a result of differences in optical and magnetic properties of sp-Hm and bulk-Hm particles. Optical, magnetic. and Mossbauer properties of sp-Hm particles dispersed within particles of high-area silica gel are reported in this study and compared to the corresponding properties of bulk-Hm powders. Samples were prepared by calcining (---550øC) powders of high-area silica gel that had been impregnated with ferric nitrate solutions. The samples are classified according to type of Mossbauer spectrum observed at 293 K. (1) Type S + D samples, which by Mossbauer granulometry contain hematite particles both larger and smaller than 10(2) nm, are characterized by a hematite sextet plus superparamagnetic doublet. (Uncertainties are given in parentheses and refer to the final digit(s).) (2) Type D samples, which contain hematite particles smaller than 10(2) nm, are characterized by only a superparamagnetic doublet and so contain only sp-Hm. The presence of larger particles in type S + D samples is consistent with X ray diffraction data; the diffraction patterns of type S + D samples are characterized by a few, broad hematite lines, and type D samples have no lines because the particles are too small to coherently scatter X rays. Measurements of internal field strengths (Hint) at 22 K for both type S + D and type D samples show that Hin t is not constant but decreases with decreasing particle diameter from 54.0 T for bulk-Hm to 46.6 T for 5.4-nm sp-Hm. This dependence implies that phase identifications based solely on comparisons to bulk values of Hin t are equivocal when superparamagnetic particles are present. Sp-Hm (< 10-nm diameter) is much more magnetic than bulk-Hm; the saturation magnetization at 293 K for type D samples is 7(2) A m2/kg as compared to 0-0.5 A m2/kg for bulk-Hm. Optical properties of type S + D samples are similar to those of bulk-Hm; in particular, a well-defined band minimum is present near 860 nm. Optical properties of type D samples, with only sp-Hm at 293 K, are significantly different in that a step-shaped feature instead of a well-defined band is centered near 860 nm. The transition from well-defined band to step-shaped feature occurs at a hematite particle diameter of -• 10 nm. The position of the UV-visible absorption edge and the absorption strength at 860 nm depend on the number density of sp-Hm particles, the Fe20 3 concentration, and the physio...