Studies on the Nucleocapsid Structure of a Group A Arbovirus

When Sindbis virus (273S) was treated with sodium desoxycholate, a nonhemagglutinating 136S particle was liberated from the virion, representing the viral nucleocapsid (core). Electron microscopically it appeared as a spherical particle 35 nm in diameter, showing ringlike morphological units 12 to 14 nm in diameter on its surface. When the one- and two-sided images of core particles were correlated, their structure could be demonstrated to have the T = 3 arrangement of 32 hexamer-pentamer morphological units within a symmetrical surface lattice. The core contained a further spherical structure (12 to 16 nm in diameter) which was designated as the central core component. Two proteins were found associated with the core, a third viral protein belonged to the hemagglutinating surface structures. The significance of these findings for virus classification is discussed.