How do non-enveloped virions, such as those of the icosahedral picornaviruses, adenoviruses, and papovaviruses, enter cells?
Poliovirus (and rhinoviruses, which cause about 50% of “colds”) have an icosahedral structure which has been highly characterized. As we have seen, the icosahedral virion capsid consists of 60 copies each of three proteins designated VP1, VP2, and VP3, arranged in a way that forms both hexameric and pentameric vertices. The twelve pentameric vertices are composed of five copies each of VP1. A depression, or “canyon”, in the structure is the region that provides the specific binding site for attachment to cells. After attachment, receptor-mediated endocytosis begins, and this leads to the RNA genome of the virus getting into the cytoplasm. The detailed mechanism of how this RNA entry step occurs is not known, but is thought to involve viral protein conformational change that opens a channel through the membrane of the endocytic vesicle. For example, there is a 2006 article by Tuthill et al. in the Journal of Virology titled “Characterization of Early Steps in the Poliovirus Infection Pr
