A

Uncleaved protomer

Pentamer

Cleaved pentamer

Procapsid

Procapsid

NCVPIa ASSemb'y (NCVPta), C'eaVage. (VPO,t,3)5 Assembl* . [(VP0,1,3)511E

Virion

Provirion

Virion

Provirion

Cleavage + RNA

Fig. 3-8 Overview of the assembly of poliovirus, a picomavirus The capsomer precursor protein (NCVPIa) aggregates to form pentamers; eacli of the five NCVPIa molecules is then cleaved by viral protease into VPO, VPl, and VP3 Twelve such pentamers aggregate to form a procapsid A final proteolytic event, which cleaves each VPO molecule into VP2 and VP4, is required for RNA incorporation The mature virion is a dodecahedron with 6(1 capsomeis, each of which is made up of one molecule each of VPl, VP2, VP3, and VP4 X-ray crystallography shows (hat the assembling units have extensions that reach across adjacent units to form second and third nearest neighbor relationships

Plasma membrane

Plasma membrane

Fig. 3-9 Maturation of enveloped viruses (A) Viruses whose virion contains a matrix protein (and some viruses which do not) bud through a patch of the plasma membrane in which glycoprotein peplomers have accumulated over the patch of matrix protein (B) Most enveloped viruses without a matrix protein bud inlo cytoplasmic vesicles jiough endoplasmic reticulum (RER) or Golgi], then pass through the cytoplasm m smooth-walled vesicles and are released by cxocytosis fB, Modified from K V Holmes, m "Fields Virology" (B N Pields, D M Kmpe, R M Chanock, M S HirschJ L Mclnick, I' P Monath, and B Roi/,man, eds ), 2nd Pd , p 847 Raven, New York, 1990 |

Fig. 3-9 Maturation of enveloped viruses (A) Viruses whose virion contains a matrix protein (and some viruses which do not) bud through a patch of the plasma membrane in which glycoprotein peplomers have accumulated over the patch of matrix protein (B) Most enveloped viruses without a matrix protein bud inlo cytoplasmic vesicles jiough endoplasmic reticulum (RER) or Golgi], then pass through the cytoplasm m smooth-walled vesicles and are released by cxocytosis fB, Modified from K V Holmes, m "Fields Virology" (B N Pields, D M Kmpe, R M Chanock, M S HirschJ L Mclnick, I' P Monath, and B Roi/,man, eds ), 2nd Pd , p 847 Raven, New York, 1990 |

cellular proteins from that patch of membrane (Fig. 3-9A). The monomeric cleaved viral glycoprotein molecules associate into oligomers to form the typical rod-shaped or club-shaped peplomer with a hydrophilic domain projecting from the external surface of the membrane, a hydrophobic transmembrane anchor domain, and a short hydrophilic domain projecting slightly into the cytoplasm. In the case of icosahedral viruses (e g., togaviruses) each protein molecule of the nucleocapsid binds directly to the cytoplasmic domain of the membrane glycoprotein oligomer, thus molding the envelope around the nucleocapsid. In the more usual case of viruses with helical nucleocapsids, it is the matrix protein which attaches to the cytoplasmic domain of the glycoprotein peplomer; in turn the nucleocapsid protein recognizes the matrix protein and this presumably initiates budding. Release of each enveloped virion does not breach the integrity of the plasma membrane; hence, thousands of virus particles can be shed over a period of several hours or days without significant cell damage (Fig 3-10). Many but not all viruses thai bud from the plasma membrane are noncytopathogenic and may be associated with persistent infections.

Epithelial cells display polarity, having an "apical" surface facing the outside world which is separated by a tight junction from the "basolateral" surface. These surfaces are chemically and physiologically distinct. Viruses that are shed to the exterior, for example, influenza virus, tend to bud from the apical surface, whereas others, such as C-type retroviruses, bud through the basolateral membrane (see Chapter 5).

Fig. 3-10 Virions of lentiviruses budding from the plasma membrane (bars, 100 nm) (A) Transmission electron micrograph of feline immunodeficiency vims huddmg through the plasma membrane of a leline T lymphocyte Retroviruses have no malrix protein, the arrows indicate accumulai ions of patches of viral glycoprotein peplomers in the plasma membrane (B) Scanning electron micrograph of a human immunodeficiency vnus budding from the plasma membrane of a human T lymphocyte, from a culture of H9 human T lymphocytes infected in vitro |A, From S C C Iriend, C J Birch, I1 M Lording, J A. Marshall, and M J Studdeit,/tirsf Vet / 67,237 (199(1), 13, courtesy Dr C S Goldsmith J

Fig. 3-10 Virions of lentiviruses budding from the plasma membrane (bars, 100 nm) (A) Transmission electron micrograph of feline immunodeficiency vims huddmg through the plasma membrane of a leline T lymphocyte Retroviruses have no malrix protein, the arrows indicate accumulai ions of patches of viral glycoprotein peplomers in the plasma membrane (B) Scanning electron micrograph of a human immunodeficiency vnus budding from the plasma membrane of a human T lymphocyte, from a culture of H9 human T lymphocytes infected in vitro |A, From S C C Iriend, C J Birch, I1 M Lording, J A. Marshall, and M J Studdeit,/tirsf Vet / 67,237 (199(1), 13, courtesy Dr C S Goldsmith J

Exociftosis

Flaviviruses, corona vi ruses, and bunyaviruses mature by budding through membranes of the Golgi complex or rough endoplasmic reticulum; vesicles containing the virus then migrate to the plasma membrane with which they fuse, thereby releasing the virions by exon/losis (Fig. 3-9B) Uniquely, the envelope of the herpesviruses is acquired by budding through the inner lamella of the nuclear membrane, the enveloped virions then pass directly from the space between the two lamellae of the nuclear membrane to the exterior of the cell via the cisternae of the endoplasmic reticulum.

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