Structure of Broadhaven virus by cryoelectron microscopy: correlation of structural and antigenic properties of Broadhaven virus and bluetongue virus outer capsid proteins.

The three-dimensional structure of Broadhaven virus (BRDV) has been determined to 23 A resolution by cryoelectron microscopy and image processing. As predicted from sequence homology, the BRDV structure resembles that of bluetongue virus (BTV) with the notable exception of one of the outer shell proteins. The cores of BRDV and BTV are identical at medium resolution; they have a diameter of 710 A and the VP7 trimers are arranged on a T = 13 icosahedral lattice. The outer shell proteins, VP5 of BRDV and BTV, have roughly the same molecular weight while VP4 of BRDV is only half the molecular weight of the corresponding VP2 of BTV. This size difference allows unambiguous determination of the identity of the triskelion shape as trimers of VP4 of BRDV (VP2 of BTV). The VP4 of BRDV sits on the VP7 trimers and projects outwards 40 A, giving the capsid an overall diameter of 790 A. This contrasts with VP2 of BTV, which projects outwards 95 A to give the capsid a diameter of 900 A. The difference in accessibility of the outer shell proteins of BRDV and BTV correlates with the difference in antigenic properties of these viral proteins. The shape of the BRDV VP5 indicates that it too is a trimer, thus implying that there are 360 copies of VP5 and 180 copies of VP4 per virion.