The capsids of helical viruses must assemble around the genome. This process is relatively well studied in tobacco mosaic virus (TMV) of plants. As noted previously, the basic process appears
Fig. 6.5 Expression of a varicella-zoster virus protein following transfection of a cell with the viral gene under the control of a promoter that is active in the uninfected cell. (a) The basic process. The cell membrane is treated with agents that allow it to readily take up large aggregates of protein and nucleic acids by phagocytosis. The transfecting DNA is caused to form aggregates with the use of calcium phosphate (Ca3(PO4)2), and then mixed with cells that have been appropriately treated. While most of the DNA taken up by the cell is degraded, some gets to the nucleus by nonspecific cellular transport of macromolecules, and this DNA can be transcribed and any genes within it expressed as proteins. (b) An actual experiment. Cells were made permeable and then transfected with DNA containing the varicella-zoster virus glycoprotein L gene. The protein encoded in this gene was expressed following its transcription into mRNA (see Chapter 13). Cells were treated with fluorescent antibody reactive with the glycoprotein at (clockwise from the top left) 0, 12, and 24 hours after infection. The expression of the glycoprotein in the cytoplasm is clearly evident from the green fluorescence. (See Chapter 12 for a description of the method.) (Photographs courtesy of C. Grose, University of Iowa.)
to be similar for all helical viruses. This similarity depends on the fact that single- or double-stranded RNA (or DNA, for that matter) can readily form a helical structure when associated with the proper type of protein.
The assembly of the helical capsid and RNA genome of TMV is shown in Fig. 6.6. Cap-somers self-assemble to form disks, and the disks formed by the capsomers initially interact with a specific sequence in the genome called pac (for packaging signal). Interaction with the RNA itself converts the disk into a "lock washer" conformation, and subsequent capsomer assemblies then thread onto the growing helical array to form the complete capsid. Note that, for TMV, the RNA forms the equivalent of a "screw," which penetrates the disk assembly of capsomers. This penetration allows translocation to a helical arrangement that grows by continued association with the genomic RNA.
Fig. 6.6 Assembly of the helical tobacco mosaic virus. Steps in the preassembly of the capsomer disk, insertion of viral RNA, and the translational "screwlike" helix assembly process with sequential addition of more capsomers are shown. (Adapted from Dimmock NJ, Primrose SB. Introduction to modern virology, 4th ed. Boston: Blackwell Science, 1994.)
Individual capsomers associate to form disc
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