Generalized Structural Polymorphism in Self-Assembled Viral Particles

The protein shells, called capsids, of nearly all spherical viruses adopt icosahedral symmetry; however, self-assembly of such empty structures often occurs with multiple misassembly steps resulting in the formation of aberrant structures. Using simple models that represent the coat proteins preassembled in the two different predetermined species that are common motifs of viral capsids (i.e., pentameric and hexameric capsomers), we perform molecular dynamics simulations of the spontaneous self-assembly of viral capsids of different sizes containing T = 1,3,4,7,9,12,13,16, and 19 proteins in their icosahedral repeating unit. We observe, in addition to icosahedral capsids, a variety of nonicosahedral yet highly ordered and enclosed capsules. Such structural polymorphism is demonstrated to be an inherent property of the coat proteins, independent of the capsid complexity and the elementary kinetic mechanisms. Moreover, there exist two distinctive classes of polymorphic structures: aberrant capsules that are larger than their respective icosahedral capsids, in T = 1-7 systems; and capsules that are smaller than their respective icosahedral capsids when T = 7-19. Different kinetic mechanisms responsible for self-assembly of those classes of aberrant structures are deciphered, providing insights into the control of the self-assembly of icosahedral capsids.