The Capsid Proteins of a Large, Icosahedral dsDNA Virus

Abstract

SummaryChilo iridescent virus (CIV) is a large (~1850 Å diameter) insect virus with an icosahedral, T=147 capsid, a dsDNA genome, and an internal lipid membrane. The structure of CIV was determined to 13 Å resolution by means of cryo-electron microscopy (cryoEM) and three-dimensional image reconstruction. A homology model of P50, the CIV major capsid protein (MCP), was built based on its amino acid sequence and the structure of the homologous Paramecium bursaria chlorella virus 1 (PBCV-1) Vp54 MCP. This model was fitted into the cryoEM density for each of the 25 trimeric CIV capsomers per icosahedral asymmetric unit. A difference map, in which the fitted CIV MCP capsomers were subtracted from the CIV cryoEM reconstruction, showed that there are at least three different types of minor capsid proteins associated with the capsomers outside the lipid membrane. "Finger" proteins are situated at many, but not all, of the spaces between three adjacent capsomers within each trisymmetron, and "zip" proteins are situated between sets of three adjacent capsomers at the boundary between neighboring trisymmetrons and pentasymmetrons. Based on the results of segmentation and density correlations, there are at least eight finger proteins, and three dimeric and two monomeric zip proteins in one asymmetric unit of the CIV capsid. These minor proteins appear to stabilize the virus by acting as intercapsomer cross-links. One transmembrane "anchor" protein per icosahedral asymmetric unit, which extends from beneath one of the capsomers in the pentasymmetron to the internal leaflet of the lipid membrane, may provide additional stabilization for the capsid. These results are consistent with the observations for other large, icosahedral dsDNA viruses that also utilize minor capsid proteins for stabilization and determining their assembly.

Structure of faustovirus, a large dsDNA virus

“…3 and 4). Although it is impossible to segment individual proteins in a 15 Å resolution map, the presence of cement proteins has been observed in the higher resolution structures of adenovirus (21), PRD-1 (22), PBCV-1 (23), and CIV (16). Additional densities similar to the cement proteins observed in these viruses are present in faustovirus and presumably serve to maintain the stability of the outer shell.…”

“…PBCV-1 has one fiber associated with one specific capsomer per trisymmetron that is probably important for the initial attachment of the virus to host cells (23,29). CIV has one fiber associated with each capsomer, decorating the whole capsid, but the function of these fibers is currently unknown (16).…”

Comparative Pathology of Ranaviruses and Diagnostic Techniques

“…During the procedure, multiple photos are taken and reconstructed to yield a 3D image of the virus. Researchers have been using cryo-EM to evaluate the role of various surface proteins in viral assembly and replication (see Yan et al 2009 ;Whitley et al 2010 ;Tran et al 2011 ). It is possible that these fi ndings may provide insight into treatment protocols or vaccine development.…”

Genomic and Proteomic Analysis of Invertebrate Iridovirus Type 9

“…The genomes are encapsidated within an icosahedral shell ranging between 120 and 180 nm in diameter and comprised predominantly of a 50-kDa major capsid protein (MCP). The invertebrate iridoviruses (IIV), studied by cryo-electron microscopy, have 2-nm-diameter surface fibrils (23,42); for invertebrate iridovirus type 6 (IIV-6), these fibrils extend from the 3-fold rotational axis of the 1,460 hexameric capsids found in the virus particle (43). IV are divided into 5 genera (Table 1), with members of three genera infecting poikilothermic vertebrates and members of the Iridovirus and Chloriridovirus genera infecting invertebrates.…”