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 Angstrom 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 BN are identical at medium resolution; they have a diameter of 710 Angstrom 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 Angstrom, giving the capsid an overall diameter of 790 Angstrom. This contrasts with VP2 of BTV, which projects outwards 95 Angstrom to give the capsid a diameter of 900 Angstrom. 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. (C) 1997 Academic Press.