Tobacco Mosaic Virus (TMV)is a positive-sense single stranded RNA virus that infects plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns, such as "mosaic"-like mottling and discoloration on the leaves (hence the name). TMV was the first virus to ever be discovered. It is composed of 2130 identical coat protein subunits following the
right-handed helix of an accompanying RNA strand to produce a 300 nm hollow cylinder with an
outer diameter of 18 nm and a 4 nm wide central channel. This here is a protein-nucleic acid interactions in the virus.
Caspar Carboxylates
Two clusters of acidic amino acids are thought to be important for the disassembly of the helical structure of TMV. When the virus enters cells, the higher pH and lower calcium levels increase the repulsion of neighboring carboxylates, destabilizing the capsid. This concept was originally proposed by Caspar and subsequently examined by site-directed mutagenesis [1]. Several carboxylates were identified including Glu50 and ASP77, which contact each other in the helical stack by hydrogen bond (look closely at the two subunits that overlap in this lockwasher complex), and a cluster of four acidic amino acids, GLU95, GLU97, GLU106 and GLU109, near the center of the ring.
Th repeating basic unit consists of 49 subunits which cover a section of 69 Å of height in three turns of the helix (= a rise of 23 Å per turn). A single strand of RNA is placed between the protein subunits at a distance of 40 Å from the outer diameter. The figure depicts one basic unit. Protein and RNA are shown in spacefill mode, RNA in white, the protein changing from red to blue along the helix. The uppermoust subunit is marked light brown. Each protein subunit covers three nucleotides of the RNA.
Repeating Subunits of Helical Tobacco Mosaic Virus
Publication Abstract from PubMed
The structure of tobacco mosaic virus (TMV) has been determined by fiber diffraction methods at 2.9 A resolution, and refined by restrained least-squares to an R-factor of 0.096. Protein-nucleic acid interactions are clearly visible. The final model contains all of the non-hydrogen atoms of the RNA and the protein, 71 water molecules, and two calcium-binding sites. Viral disassembly is driven by electrostatic repulsions between the charges in two carboxyl-carboxylate pairs and a phosphate-carboxylate pair. The phosphate-carboxylate pair and at least one of the carboxyl-carboxylate pairs appear to be calcium-binding sites. Nucleotide specificity, enabling TMV to recognize its own RNA by a repeating pattern of guanine residues, is provided by two guanine-specific hydrogen bonds in one of the three base-binding sites.
Visualization of protein-nucleic acid interactions in a virus. Refined structure of intact tobacco mosaic virus at 2.9 A resolution by X-ray fiber diffraction., Namba K, Pattanayek R, Stubbs G, J Mol Biol. 1989 Jul 20;208(2):307-25. PMID:2769760
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
