C terminus of infectious bursal disease virus major capsid protein VP2 is involved in definition of the T number for capsid assembly

Infectious bursal disease virus (IBDV), a member of the Birnaviridae family , is a double-stranded RNA virus. The IBDV capsid is formed by two major st ructural proteins, VP2 and VP3, which assemble to form a T=13 markedly nons pherical capsid. During viral infection, VP2 is initially synthesized as a precursor, called VPX, whose C end is proteolytically processed to the matu re form during capsid assembly. We have computed three-dimensional maps of IBDV capsid and virus-like particles built up by NT2 alone by using electro n cryomicroscopy and image-processing techniques. The IBDV single-shelled c apsid is characterized by the presence of 260 protruding trimers on the out er surface. Five classes of trimers can be distinguished according to their different local environments. When VP2 is expressed alone in insect cells, dodecahedral particles form spontaneously; these may be assembled into lar ger, fragile icosahedral capsids built up by 12 dodecahedral capsids. Each dodecahedral capsid is an empty T=1 shell composed of 20 trimeric clusters of NT2. Structural comparison between IBDV capsids and capsids consisting o f VP2 alone allowed the determination of the major capsid protein locations and the interactions between them. Whereas NT2 forms the outer protruding trimers, VP3 is found as trimers on the inner surface and may be responsibl e for stabilizing functions. Since elimination of the C-terminal region of VPX is correlated with the assembly of T=1 capsids, this domain might be in volved (either alone or in cooperation with VP3) in the induction of differ ent conformations of VP2 during capsid morphogenesis.