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.

Fig. 1.

Incorporation efficiency. (A) Relative incorporation efficiency as a function of CP/Au. A threshold greater than ≈100 protein subunits per gold nanoparticle is required for assembly. Lines are sigmoidal fits to data. (B) Maximum incorporation efficiency was reached for 12-nm cores. All data in A and B are from TEM. (C) VLP average diameter increases uniformly with the core diameter (CP/Au = 270:1) by DLS and TEM. However, size histograms show that different populations of particles coexist. The square dots represent the most frequent VLP diameters (local histogram maxima).

Fig. 2.

Typical spreads of R3BMV (A), PEG-Au (B), and VLP₁₂ (C) as used for single-particle image reconstruction. PEG-Au lacks any finer structural detail, and the density modulations with native BMV and the 12 nm VLPs suggest distinct protein subunit architecture.

Fig. 3.

Negative-stain electron micrographs, Fourier transforms (inserts), and corresponding Fourier projection maps. (A) R3BMV 2D crystal. The lattice constant is 26 nm (one unit cell is drawn), and the arrangement of the densities suggests a T = 3 structure. (B) VLP₁₂ 2D lattice. The lattice constant is 25 nm.

Fig. 4.

Three-dimensional reconstructions of R3BMV and VLP using negative stain data. (A) T = 1, 2, and 3 models of BMV capsids. The T = 1 and pseudoT = 2 structures were obtained from the VIPER database (54) and ref. . The T = 3 structure is the reconstructed image of R3BMV in this work. (Scale bar, 210 Å.) (B) VLP₆ is characterized by the absence of electron density at the threefold symmetry axes. Its structure and diameter bring it close to a T = 1 capsid. (C) The VLP₉ structure is reminiscent of a pseudoT = 2. The presence of electron density at the threefold axes distinguishes it from the VLP₆ structure. (D) The VLP₁₂ shape resembles more to the spherical shape of R3BMV although it still lacks clear evidence of hexameric capsomers. Concentric layering is a characteristic of all VLPs.

Fig. 5.

VLP crystal properties. (A) AC-mode AFM image of the face of a R3BMV crystal immersed in its mother liquor. Transmission optical images of R3BMV (B) and VLP₁₂ crystals (C). (D) Optical spectrum of a 3D VLP crystal is distinct from the single resonance characteristic of dilute VLP₁₂ solutions.