Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 179

The mature capsid of the tailed, double-stranded DNA bacteriophage HK97 is a T = 7 structure built from hexamers and pentamers of a single viral protein, Gp5. The first hints of the remarkable and unprecedented mechanism of stabilization of this particle came from biochemical experiments, which showed the following:

 A previously unknown covalent protein-protein linkage forms in the final reaction in the assembly of the capsid: the side chain of a lysine in every Gp5 subunit forms a covalent isopeptide bond with an asparagine in an adjacent subunit. Consequently, all subunits are joined covalently to each other.

 This reaction is autocatalytic, depending only on Gp5 subunits organized in a particular conformational state: the capsid is enzyme, substrate, and product.

 HK97 mature particles are extraordinarily stable and cannot be disassembled into individual subunits by boiling in strong ionic detergent.

It was therefore proposed that the cross-linking also interlinks the subunits from adjacent structural units to catenate rings of hexamers and pentamers. The determination of the structure of the HK97 capsid to 3.6-Å resolution by X-ray crystallography has confirmed the formation of such capsid “chain mail” (figure, panel A), akin to that widely used in armor (B) until the development of the crossbow. The HK97 capsid is the first example of a protein catenane (an interlocked ring). This unique structure has been shown to increase the stability of the virus particle, and it may be necessary as the capsid shell is very thin. The delivery of the DNA genome to host cells via the tail of the particle obviates the need for capsid disassembly.

Chain mail in the bacteriophage HK97 capsid. (A) The exterior of the HK97 capsid is shown at the top, with structural units of the Gp5 protein in gray. The segments of subunits that are cross-linked into rings are colored the same, to illustrate the formation of catenated rings of subunits. The cross-linking is shown in the more detailed view below, down a quasithreefold axis with three pairs of cross-linked subunits. The isopeptide bonds are shown in yellow. The cross-linked monomers (shown in blue) loop over a second pair of covalently joined subunits (green), which in turn cross over a third pair (magenta). Adapted from Wikoff WR et al. 2000. Science 289:2129–2133, with permission. Courtesy of J. Johnson, The Scripps Research Institute. (B) Chain mail armor and schematic illustration of the rings that form the chain mail.

 Duda RL. 1998. Protein chainmail: catenated protein in viral capsids. Cell 94:55–60.

 Wikoff WR, Liljas L, Duda RL, Tsuruta H, Hendrix RW, Johnson JE. 2000. Topologically linked protein rings in the bacteriophage HK97 capsid. Science 289:2129–2133.