Size control and polymorphism mechanisms in capsid assembly

Many pathogenic viruses require a specific structure to be infectious, and their proteins form that structure in vivo with remarkable fidelity. The same proteins, however, assemble into capsids with different sizes and morphologies, still with high selectivity, to accommodate nucleic acids [1], inorganic nanoparticles [2], and polyanions [3, 4] with different sizes. For example, Fig. 1 shows electron microscopy images of brome mosaic virus (BMV) proteins assembled into icosahedral morphologies with different numbers of capsid proteins around charge-functionalized nanoparticles with commensurate sizes. What features of viral components enable assembly that is so selective and yet also highly adaptable?

Figure 1 

Figure 1: Electron microscopy images of brome mosaic virus (BMV) capsid proteins assembled around charge-functionalized nanoparticles with diameters: (B) 6 nm, (C) 9 nm, and (D) 12 nm. In the top panel (A), images of empty capsids are shown with T=1 (60 proteins), pseudo T=2 (120 proteins), and T=3 (180 proteins) morphologies. Figure taken from Ref. [2].

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