In vitro-assembled alphavirus core-like particles maintain a structure similar to that of nucleocapsid cores in mature virus

In vitro-assembled core-like particles produced from alphavirus capsid protein and nucleic acid were studied by cryoelectron microscopy. These particles were found to have a diameter of 420 Angstrom with 240 copies of the capsid protein arranged in a T=4 icosahedral surface lattice, similar to the nucleocapsid core in mature virions. However, when the particles were subjected to gentle purification procedures, they were damaged, preventing generation of reliable structural information. Similarly, purified nucleocapsid cores isolated from virus-infected cells or from mature virus particles were also of poor quality. This suggested that in the absence of membrane and glycoproteins, nucleocapsid core particles are fragile, lacking accurate icosahedral symmetry.