Structure of the baculovirus nucleocapsid

A low-resolution structural model of the nucleocapsid of Spodoptera litura granulosis virus, a member of the Baculoviridae family, has been determined using contrast variation methods in both electron microscopy and low-angle X-ray solution scattering. The cylindrical portion of the characteristic capsid surface is composed of a 12 start helix system of monomer subunits, giving rise to a relatively open stacked ring structure running parallel to the cylinder axis and repeating approximately every third ring. Structural proteins which appear distinct from those constituting the cylinder comprise the stacked ring-like assemblies which form caps at both ends of the nucleocapsid. Within the nucleocapsid the double-stranded closed loop of DNA associates heterogeneously with a highly basic polypeptide to form a cylindrical core. The basic protein is thought to participate in condensation and protection of the genome both prior to and during encapsidation.