Three-dimensional structures of a native simian and reassortant rotavirus have been determined by electron cryomicroscopy and computer image processing. The structural features of the native virus confirm that the hemagglutinin spike is a dimer of VP4, substantiated by in vivo radiolabeling studies. Exchange of native VP4 with a bovine strain equivalent results in a poorly infectious reassortant. No VP4 spikes are detected in the three-dimensional reconstruction of the reassortant. The difference map between the two structures reveals a novel large globular domain of VP4 buried within the virion that interacts extensively with the intermediate shell protein, VP6. Our results suggest that assembly of VP4 precedes that of VP7, the major outer shell protein, and that VP4 may play an important role in the receptor recognition and budding process through the rough endoplasmic reticulum during virus maturation.

Trypsin enhances rotavirus infectivity by an unknown mechanism. To examine the structural basis of trypsin-enhanced infectivity in rotaviruses, SA11 4F triple-layered particles (TLPs) grown in the absence (nontrypsinized rotavirus [NTR]) or presence (trypsinized rotavirus [TR]) of trypsin were characterized to determine the structure, the protein composition, and the infectivity of the particles before and after trypsin treatment. As expected, VP4 was not cleaved in NTR particles and was cleaved into VP5‫ء‬ and VP8 ‫ء‬ in TR particles. However, surprisingly, while the VP4 spikes were clearly visible and well ordered in the electron cryomicroscopy reconstructions of TR TLPs, they were totally absent in the reconstructions of NTR TLPs. Biochemical analysis with radiolabeled particles indicated that the stoichiometry of the VP4 in NTR particles was the same as that in TR particles and that the VP8 ‫ء‬ portion of NTR, but not TR, particles is susceptible to further proteolysis by trypsin. Taken together, these structural and biochemical data show that the VP4 spikes in the NTR TLPs are icosahedrally disordered and that they are conformationally different. Structural studies on the NTR TLPs after trypsin treatment showed that spike structure could be partially recovered. Following additional trypsin treatment, infectivity was enhanced for both NTR and TR particles, but the infectivity of NTR remained 2 logs lower than that of TR particles. Increased infectivity in these particles corresponded to additional cleavages in VP5 ‫ء‬ , at amino acids 259, 583, and putatively 467, which are conserved in all P serotypes of human and animal group A rotaviruses and also corresponded with a structural change in VP7. These biochemical and structural results show that trypsin cleavage imparts order to VP4 spikes on de novo synthesized virus particles, and these ordered spikes make virus entry into cells more efficient.Rotaviruses are the leading cause of severe gastroenteritis in young children worldwide (16). Structural and biochemical analyses show that rotaviruses are large, icosahedral particles having a complex architecture consisting of three concentric capsid layers surrounding a genome of 11 segments of doublestranded RNA (41, 42). The innermost capsid layer, composed of VP2, encloses the genomic double-stranded RNA. A significant portion of the genomic RNA, particularly that in close contact with the inner surface of the VP2 layer, is icosahedrally ordered (43). Anchored to the inner surface of VP2 at the icosahedral vertices are two proteins, VP1 and VP3, involved in transcription of the genome within the intact particle. The intermediate capsid layer is composed of trimers of VP6 organized on a Tϭ13 (levo) icosahedral lattice (44). The outermost layer in the infectious virus is composed of the major capsid glycoprotein VP7 and the hemagglutinin spike protein VP4 (16). VP7 is present as 780 molecules grouped as 260 trimers at the local and strict threefold axes of a Tϭ13 left-handed icosahedral lattice (44, 51). The VP7 ...

Cell-to-cell spread of tobacco mosaic virus (TMV) is presumed to occur through plant intercellular connections, the plasmodesmata. Viral movement is an active process mediated by a specific virus-encoded P30 protein. P30 has at least two functions, to cooperatively bind single-stranded nucleic acids and to increase plasmodesmatal permeability. Here, we visualized P30 complexes with single-stranded DNA and RNA. These complexes are long, unfolded, and very thin (1.5 to 2.0 nm in diameter). Unlike TMV virions (300 x 18 nm), the complexes are compatible in size with the P30-induced increase in plasmodesmatal permeability (2.4 to 3.1 nm), making them likely candidates for the structures involved in the cell-to-cell movement of TMV. Mutational analysis using single and double deletion mutants of P30 revealed three regions potentially important for the protein function. Amino acid residues 65 to 86 possibly are required for correct folding of the active protein, and the regions between amino acid residues 112 to 185 and 185 to 268 potentially contain two independently active single-stranded nucleic acid binding domains designated binding domains A and B, respectively.

Our aquareovirus structure displays marked similarity to the mammalian reovirus intermediate subviral particles, suggesting a close evolutionary relationship. However, the noticeable distinction is that aquareovirus lacks the hemagglutinin spike observed in reovirus. The T=1 inner layer organization observed in the aquareovirus appears to be common to other members of the Reoviridae. Such organization may be of fundamental significance in the endogenous transcription of the genome in these viruses.

Summary. Structural studies on rotavirus using electron cryomicroscopy and computer image analysis have permitted visualization of each shell in the triple-layered rotavirus structure. Biochemical results have aided our interpretation of the structural organization of these layers and protein interactions seen in the three-dimensional structure, and have provided a better understanding of the structure-function relationships of the rota virus structural proteins.