Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 277

RNA polymerases of multiple (+) and (–) strand RNA viruses have been reported to form dimers and higher-order oligomers (Fig. 6.14). There is evidence that such arrangements may increase the stability and catalytic activity of these enzymes. In many cases, deletions of amino acids that prevent oligomerization also inhibit or cause complete loss of enzyme activity.

The first poliovirus 3D^pol structure revealed that the polymerase molecules interacted in a head-to-tail manner and formed fibers; subsequently the protein was shown to form a lattice. The head-to-tail fibers were formed by an interface comprising parts of the thumb of one polymerase and the back of the palm of another. Amino acid changes in the back of the thumb that disrupt this interface impaired replication. Repetition of this interaction in a head-to-tail fashion results in long fibers of polymerase molecules 50 Å in diameter. The presence of a second interface, formed by N-terminal polypeptide segments, may lead to a network of polymerase fibers. These interacting N-terminal polypeptide segments may originate from different polymerase molecules and are required for enzyme activity. Intermolecular cross-linking has been observed between cysteines engineered at Ala29 and Ile441 of poliovirus 3D^pol, and disruption of these interactions led to reduced infectivity. Polymerase-containing oligomeric structures resembling those seen with purified 3D^pol were observed on the surface of vesicles isolated from poliovirus-infected cells. Because picornavirus RNA synthesis occurs on membranous vesicles, the concept of a catalytic lattice is attractive mechanistically.

Figure 6.13 Functional N- and C-terminal extensions of RNA polymerases. The smallest known RdRP is encoded in picornavirus genomes and consists of a core catalytic unit made of thumb (green), fingers (red), and palm (yellow) domains. An N-terminal extension of the flavivirus dengue virus RNA polymerase (tan) has methyltransferase activity. The rhabdovirus RNA polymerase has both N- (blue) and C-terminal (light gray) extensions; the latter contain the capping and methyltransferase domains.

Figure 6.14 Oligomerization of RNA-dependent RNA polymerases. Ribbon diagrams of dimers of murine norovirus RdRP (PDB ID: 3QID), tetramers of influenza virus RdRP (PDB ID: 3J9B), and cryo-electron microscopy density data showing tubular arrangement of sheets of poliovirus 3D^pol (EMD ID: 2270).