Core-controlled polymorphism in virus-like particles

Abstract

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...

Controlling viral capsid assembly with templating

“…Experiments show that capsid assembly around cores competes with spontaneous assembly at subunit concentrations well above the threshold concentration for empty capsid assembly (critical subunit concentration, CSC) [33], and that assembly occurs in the presence of cores below the CSC [68]. These results point to the ability of nanoparticles to act as heterogeneous nucleation agents.…”

“…The parameter g c can be related to the experimentally controlled functionalized surface charge density. Protein adsorption and assembly kinetics can be monitored with time resolved light scattering [68], and packaging efficiencies can be determined from TEM micrographs [33]. Successful validation of model predictions would provide strong evidence for the control of assembly mechanisms by a templating component, which prior works suggest is important for understanding viral assembly in vivo [58,67].…”

“…Core curvature that is inconsistent with the lowest free energy subunit-subunit bonding configurations will introduce frustration and thus may limit the robustness of assembly. Experimental observations that solid cores [33] and nucleic acid cores [86] with different sizes promote assembly of different capsid morphologies demonstrate that frustration is an important consideration in biological and nanostructured assembly processes. Comparison of our current model with experimental results will identify frustration and future work will explicitly address geometrical frustration.…”

“…The lowest energy states in the model correspond to separate 'capsids', which consist of multiples of 60 monomers in a shell with icosahedral symmetry. In this work we model experiments in which BMV capsid proteins assemble around 6 nm nanoparticles, for which only T1 capsid geometries are observed [33]. Because the basic assembly unit of BMV is a dimer [72][73][74], our model subunit represents a protein dimer.…”

“…Our models are motivated by recent experiments in which Brome mosaic virus (BMV) capsid proteins dynamically encapsidate functionalized inorganic nanoparticle cores, creating unique biological and synthetic composite structures called virus-like particles (VLPs) [33][34][35][36]. By combining the unparalleled self-assembly and targeting capabilities of viruses with the functionalizability of nanoparticles, VLPs show promise as imaging agents [36][37][38][39], diagnostic and therapeutic vectors [40][41][42], and as subunits or templates for synthesis of advanced nanomaterials [43][44][45][46].…”