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Review
, 1 (2), 142-9

Herpesvirus Capsid Assembly: Insights From Structural Analysis

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Review

Herpesvirus Capsid Assembly: Insights From Structural Analysis

Jay C Brown et al. Curr Opin Virol.

Abstract

In all herpesviruses, the capsid is icosahedral in shape, composed of 162 capsomers, and assembled in the infected cell nucleus. Once a closed capsid has formed, it is packaged with the virus DNA and transported to the cytoplasm where further morphogenetic events take place. Herpesvirus capsid populations are highly uniform in shape, and this property has made them attractive for structural analysis particularly by cryo electron microscopy followed by three-dimensional image reconstruction. Here we describe what is known about herpesvirus capsid structure and assembly with emphasis on herpes simplex virus and on the contribution of structural studies. The overall analysis has demonstrated that herpesvirus capsids are formed by a pathway resembling that established for dsDNA bacteriophage such as P22 and HK97. For example herpes capsid assembly is found to: (1) involve a scaffolding protein not present in the mature virus; (2) proceed through a fragile, spherical procapsid intermediate; and (3) result in incorporation of a portal complex at a unique capsid vertex.

Figures

Figure 1
Figure 1
Structures of the HSV-1 virion and capsid. (a) Electron micrograph showing a single HSV-1 virion in cross-section. Note the four concentric layers that constitute the virion. The virion diameter is ~210 nm. (b) CryoEM reconstruction of an HSV-1 capsid shown in surface-shaded representation viewed along a face (3-fold axis). One of the capsid faces is illustrated in color with the hexons, pentons and triplexes shown in red, orange and blue, respectively. The capsid diameter is 125 nm. (c) View of the HSV-1 capsid with a single hexon and a single major capsid protein molecule extracted. Also shown are the VP5 upper domain whose structure was determined by X-ray crystallography (1NO7; [17**]), and an example of the bacteriophage capsid protein that docks into the major capsid protein floor domain (2FT1; [47]). Alpha helix, coil and beta structure are indicated in blue, gray and yellow, respectively.
Figure 2
Figure 2
Structures of the HSV-1 portal (a) and CCSC (b). In (a), the structure of the HSV-1 portal is shown beside the portals of phages T7 and P22 [, –49]. Note that the three are similar in size and structure suggesting their function is the same. All are cylindrical in shape with 12-fold rotational symmetry and an axial channel through which virus DNA enters and exits the capsid. In (b) the CCSC is highlighted in green and shown in the full capsid (left) and at higher magnification in a single vertex (right). Note that five CCSC rods project radially outward from each capsid vertex. CCSC graphic courtesy of James Conway (University of Pittsburgh; [25]).
Figure 3
Figure 3
Drawing illustrating the pathway of herpesvirus capsid formation. The basic assembly units are small complexes of the major capsid and scaffolding proteins. In the presence of the triplexes (not illustrated), complexes of the major capsid-scaffolding proteins assemble to form angular segments of the closed, spherical procapsid (partial procapsids). A complex of the portal and scaffolding protein is incorporated at an early stage of procapsid formation. Once procapsids are closed, they become the substrate for initiation of DNA encapsidation which occurs concurrently with loss of the scaffolding protein from the procapsid interior and angularization of the capsid shell.
Figure 4
Figure 4
Structure of the HSV-1 procapsid compared to the mature capsid. CryoEM reconstructions are shown in surface rendering as viewed along the 2-fold axis of icosahedral symmetry (a–c). Note that the procapsid is spherical in overall shape while the mature capsid is angular. At higher magnification it can be seen that the hexons in the procapsid are oval in exterior view while those of the mature capsid are hexagonal (compare d and e). The procapsid is found to have openings in the floor layer which are closed in the mature capsid (arrows in f and g).

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