Self Assembly

Virions self-assemble within the infected cell. In most cases, assembly appears to begin with the interaction of one

FIGURE 2.20 Morphology of orthopox and parapox virions. (A, B) Diagrams of orthopox and parapox virions. At the far left and right are shown the surfaces of the particles as they are isolated from infected cells, with the outer tubules or the outer filament shown in turquoise. The inner parts of each diagram show the enveloped particle in cross section illustrating the core membrane, the lateral bodies, and the nucleoprotein. [Adapted from Fenner and Nakano (1988).] (C) Purified vaccinia virus negatively stained with phosphotungstate. Magnification is 120,000. [From Dalton and Haguenau (1973, p. 111).] (D) Thin section of a particle in an agglutinated clump of vaccinia. L, lateral bodies; C, core. The membrane is completely coated with antibody. [From Dalton and Haguenau (1973, p. 116).] (E) Outer surface of nonenveloped parapox virus with a single long filament wound around the particle. [From Murphy et al. (1995, p. 79).]

FIGURE 2.20 Morphology of orthopox and parapox virions. (A, B) Diagrams of orthopox and parapox virions. At the far left and right are shown the surfaces of the particles as they are isolated from infected cells, with the outer tubules or the outer filament shown in turquoise. The inner parts of each diagram show the enveloped particle in cross section illustrating the core membrane, the lateral bodies, and the nucleoprotein. [Adapted from Fenner and Nakano (1988).] (C) Purified vaccinia virus negatively stained with phosphotungstate. Magnification is 120,000. [From Dalton and Haguenau (1973, p. 111).] (D) Thin section of a particle in an agglutinated clump of vaccinia. L, lateral bodies; C, core. The membrane is completely coated with antibody. [From Dalton and Haguenau (1973, p. 116).] (E) Outer surface of nonenveloped parapox virus with a single long filament wound around the particle. [From Murphy et al. (1995, p. 79).]

or more of the structural proteins with an encapsidation signal in the viral genome, which ensures that viral genomes are preferentially packaged. After initiation, encapsidation continues by recruitment of additional structural protein molecules until the complete helix or icosahe-dral structure has been assembled. Thus, packaging of the viral genome is coincident with assembly of the virion, or of the nucleocapsid in the case of enveloped viruses. The requirement for a packaging signal may not be absolute. In many viruses that contain an encapsidation signal, RNAs or DNAs lacking such a signal may be encapsidated but with lower efficiency. For some viruses, there is no evidence for an encapsidation signal.

Assembly of the TMV rod (Fig. 2.2) has been well studied. Several coat protein molecules, perhaps in the form of a disk, bind to a specific nucleation site within TMV RNA to initiate encapsidation. Once the nucleation event occurs, additional protein subunits are recruited into the structure and assembly proceeds in both directions until the RNA is completely encapsidated. The length of the virion is thus determined by the size of the RNA.

The assembly of the icosahedral turnip crinkle virion has also been well studied. Assembly of this T=3 structure is initiated by formation of a stable complex that consists of six capsid protein molecules bound to a specific encapsida-tion signal in the viral RNA. Additional capsid protein dimers are then recruited into the complex until the structure is complete.

It is probable that most other viruses assemble in a manner similar to these two well-studied examples. At least some viruses deviate from this model, however, and assemble an empty particle into which the viral genome is later recruited. It is also known that many viruses will assemble empty particles if the structural proteins are expressed in large amounts in the absence of viral genomes, even if assembly is normally coincident with encapsidation of the viral genome in infected cells.

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