Introduction

For purposes of brief introduction, the general characteristics of the CMV virion can be summarized as follows. Typical of the herpesvirus group, the virion of HCMV is approximately 230 nm in diameter and is composed of a nucleocapsid, surrounded by

W. Gibson

Department of Pharmacology Molecular Sciences, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA

[email protected]

T. E. Shenk and M. F. Stinski (eds.), Human Cytomegalovirus. 187

Current Topics in Microbiology and Immunology 325. © Springer-Verlag Berlin Heidelberg 2008

a less structured tegument layer, and bounded by a trilaminate membrane envelope (Fig. 1a). The HCMV genome is composed of a linear, double-stranded DNA molecule (236 kbp in wild type virus), the largest among the human herpesviruses, and over 50% larger than that of herpes simplex virus type 1 (HSV-1) (see the chapters by E. Murphy and T. Shenk, and G.S. Pari, this volume). The capsid is isosahedral and about the same diameter as that of HSV (~110 nm, depending on preparation). Accommodating a larger DNA in a similar diameter capsid may be achieved by eliminating the maturational protease (pUL80a) from the interior of CMV capsids (Chan et al. 2002; Loveland et al. 2007). The capsid is composed of four integral protein species (for HCMV, pUL46, pUL80.5, pUL85, pUL104) that are organized into 162 capsomeres (150 hexamers plus 12 pentamers) and 320 triplexes located between the capsomeres. By analogy with HSV-1, one of the pentamer positions is

Fig. 1 Particles in cytoplasm of CMV-infected cells. Shown here are electron micrographs of a virion with DNA, capsid, tegument, and envelope indicated by arrows, b virion within small vesicle or tubule indicated by thinner arrow, c tegumented C-capsid, with coarse fibrillar material especially evident on right-hand side, budding into a vesicle or tubule (arrow) to become virion, and d tegumented B-capsid budding into large tubule or vesicle (top arrow) to become NIEP; showing thickening of vesicle membrane where apposed to particle (bottom arrow)

Fig. 1 Particles in cytoplasm of CMV-infected cells. Shown here are electron micrographs of a virion with DNA, capsid, tegument, and envelope indicated by arrows, b virion within small vesicle or tubule indicated by thinner arrow, c tegumented C-capsid, with coarse fibrillar material especially evident on right-hand side, budding into a vesicle or tubule (arrow) to become virion, and d tegumented B-capsid budding into large tubule or vesicle (top arrow) to become NIEP; showing thickening of vesicle membrane where apposed to particle (bottom arrow)

shared with or occupied by a portal complex through which DNA enters and leaves the capsid. The tegument region is approximately 50 nm thick and includes seven relatively abundant virus-encoded protein species (see the chapter by R. Kalejta, this volume), at least five of which are phosphorylated. The virion envelope is estimated to be 10 nm thick and contains at least ten abundant protein species. Both the tegument and envelope contain additional less abundant virus-encoded and host-cell proteins, as well as phospholipids, polyamines, and small RNAs.

It is worth noting that determinations of particle composition ultimately depend on the nature of the starting material, which can be influenced by its source and method of preparation. Comparisons of virus particles from different origins, recovered by different methods, and analyzed by different and increasingly sensitive procedures, are focusing even more attention on the challenge of establishing which constituents are integral and present in all particles.

In addition to virions, five other types of virus particles have been recovered from CMV-infected cells and characterized. Three are intracellular and nonenvel-oped. A- and B-capsids are from nuclei prepared by treating infected cells with NP-40 and have counterparts among the other herpesviruses. A-capsids are shells composed of the four integral protein species and have the simplest structure. B-capsids contain all of the A-capsid proteins and several additional internal scaffolding species (UL80 proteins, Fig. 2) . C-capsids are recovered from the cytoplasmic fraction of infected cells treated with NP-40 (Gibson and Roizman 1972; Gibson 1981) and are composed of the DNA genome within an A-capsid shell having some tightly adherent tegument proteins (e.g., pUL32, pUL47, pUL48). The other

Fig. 2 Nested organization of HCMV UL80 genes and proteins. Shown here is a schematic representation of the nested UL80 genes (top, DNA helix; arrows indicate separate promoters for each); their 3' co-terminal mRNAs (dots indicate AUG codons starting translation); and the in-frame, carboxy-co-terminal proteins translated from each mRNA (size, name, and abbreviation indicated). Shading illustrates portions of each protein shared by the others and the location of the linker and tail portions of the scaffolding domain; asterisk indicates location of amino-conserved domain. (Adapted from Gibson 2006)

Fig. 2 Nested organization of HCMV UL80 genes and proteins. Shown here is a schematic representation of the nested UL80 genes (top, DNA helix; arrows indicate separate promoters for each); their 3' co-terminal mRNAs (dots indicate AUG codons starting translation); and the in-frame, carboxy-co-terminal proteins translated from each mRNA (size, name, and abbreviation indicated). Shading illustrates portions of each protein shared by the others and the location of the linker and tail portions of the scaffolding domain; asterisk indicates location of amino-conserved domain. (Adapted from Gibson 2006)

two types of particles are enveloped and recovered from the culture medium of CMV-infected cells; neither contains DNA. Noninfectious enveloped particles (NIEPs) are enveloped B-capsids that closely resemble virions in structure and composition, but retain the internal B-capsid scaffolding proteins. Dense bodies (DBs), which differ from NIEPs and virions by their larger and more heterogeneous size (~250-600 nm) and by the absence of all nucleocapsid constituents, are solid spheroidal aggregates of a single predominant tegument protein species (i.e., pUL83), surrounded by an envelope so far undistinguished from that of the virion. More detailed descriptions of these particles are presented in earlier reports and reviews (Irmiere and Gibson 1983, 1985; Gibson and Irmiere 1984; Gibson 1996).

Was this article helpful?

0 0

Post a comment