Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 187

In many virus particles, the genome is associated with specialized nucleic acid-binding proteins, such as the nucleocapsid proteins of (−) strand RNA viruses and (+) strand retroviruses, or the core proteins of adenoviruses. An important function of such proteins is to condense and protect viral genomes. Consequently, they do not recognize specific nucleic acid sequences but rather bind nonspecifically to RNA or DNA genomes. This mode of binding is exemplified by the structure of the vesicular stomatitis virus N protein, in which 9 nucleotides of RNA are tightly but nonspecifically bound in a cavity formed between the two domains of each N protein molecule (Fig. 4.7). These protein-RNA interactions both sequester the RNA genome and organize it into a helical structure. Formation of helical ribonucleoproteins by two-domain RNA-binding proteins is a packaging mechanism common among (−) strand RNA viruses in the order Mononegavirales: the N proteins of representatives of other families in the order exhibit the same two-lobed structure and mode of RNA binding (Fig. 4.21).

Electron microscopy of cores released from adenovirus particles and cryo-EM of virus particles have suggested that the internal nucleoprotein is also arranged in some regular fashion. However, how the viral DNA genome is organized and condensed by the core proteins is not known: the nucleoprotein was not observed in the high-resolution structures of adenovirus particles described previously, and the structures of core proteins have not been determined. The fundamental DNA packaging unit is a multimer of protein VII, which appears as beads on a string of adenoviral DNA when other core proteins are removed. Protein VII binds tightly to and condenses double-stranded DNA in vitro, consistent with a packaging function, but has been reported to be dispensable for assembly of virus particles (Chapter 13). Protein VII and the other core proteins are basic, as would be expected for proteins that bind to a negatively charged DNA molecule without sequence specificity.