Three-dimensional reconstruction of baculovirus expressed bluetongue virus core-like particles by cryo-electron microscopy.

When the viral proteins VP3 and VP7 of bluetongue virus (BTV) are expressed simultaneously in the baculovirus system, core-like particles form spontaneously. The 3-D structure of these core-like particles, determined from cryo-electron micrographs, reveals an icosahedral structure 72.5 nm in diameter with 200 triangular spikes arranged on a T = 13,I lattice; The five spikes around each of the fivefold axes are absent. This is in contrast to the native BTV core particles which have a complete T = 13,I lattice of 260 spikes. The spikes, attributed to VP7 trimers appear as triangular columns 8.0 nm in height with distinct inner and outer domains. The inner shell of the core-like particles, or subcore-like particle, has a T = 1 lattice composed of 60 copies of VP3. The subcore-like particle is noticeably thicker around the fivefold positions. Pores in the subcore-like particle are situated near each of the local sixfold axes, below each six-membered ring of spikes. These pores could allow the passage of metabolites and RNA to and from the core for RNA transcription during infection. It is possible that the synthetic core-like particles have an incomplete complement of VP7 spikes because the ratio of VP7 to VP3 produced in the dual expression system is less than the 13:1 required for complete core-like particles. Only the VP7 spikes which have the strongest affinity for the VP3 inner core and are involved in maintaining the structural integrity of the core-like particle are incorporated. The BTV core-like particle shows greater morphological similarity to the rotavirus than to the reovirus core particle.