412U5 - TMV: Tobacco Mosaic Virus Viral self-assembly: The...

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TMV: Tobacco Mosaic Virus • Viral self-assembly: The Constituents of a virus assemble into a complete, functional (infectious) virus particle or virion simply by the specific physical interactions among them (fit and complimentary interactions).
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TMV • The chromosome is a ssRNA of ~6400 nucleotides with a protein coat of 2130 identical protein subunits. • The chromosome is coiled around a central axis with 3 residues of the RNA bound to each protein subunit.
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The 3 bases bind by hydrophobic and London interactions with HC side chains of one α helix of the coat subunit. The 3 phosphates form electrostatic, or ionic bonds (- ...... +) with arg side chains (R=(CH 2 ) 3 NHC(NH 2 )=NH 2 + ) on the subunit below. These interactions cause the RNA to wind between the layers of the coat protein in a helical pattern. • TMV
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TMV • The RNA is between the protein layers, not in the helix cavity. • The virus seems to be in the form of a solid rod, but it is a thick- walled hollow tube.
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A “protohelix” of ~2 1/3 turns consisting of ~39 coat subunits forms by subunit association. In the presence of TMV RNA, the protohelices form intact virions within ~ 10 min, but they interact very slowly with other ssRNA. A virion is a complete, assembled, infectious virus particle.
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At pH = 5, the protohelices can associate with each other to at first form a nicked helix that slowly converts to form a rod that is like the viral coat helix.
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TMV Assembly Assembly begins with interaction of a protohelix with the “packaging signal”, a hairpin that has a loop at the end; the loop is in an 18 residue sequence in which there is a G at every third position and no C. Most of the Gs are not bpd to Cs. This packaging signal is required for, and accounts for, the rapid, specific packaging of TMV RNA. The RNA-protein interactions cause the RNA to wind between the layers of the protohelix.
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• The base of every third residue of the RNA is bound to the coat protein in the unusual syn conformation of the glycosidic bond. • G residues most easily adopt the syn conformation; the ability of the G at every third position to do this accounts for much of the effectiveness of the packaging signal.
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TMV Assembly Assembly proceeds with addition of protohelices to the “top” (loop end of RNA). Each protohelix associates with the previously bound one, with the 5’ end of the RNA being “pulled up” through the hollow core of the growing rod and winding between the layers of the protohelix because of its strong hydrophobic and ionic interactions with the coat protein. The 3’ end of the RNA projects a constant length out the “bottom” end.
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Evidence for this model of the assembly process: a. Electron micrographs show, projecting from one end of the assembling virus particle, one constant-length RNA “tail” (the 3’ end) and one “tail” (the 5’ end) that gets shorter as the growing viral rod gets longer.
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Evidence for this model of the assembly process:
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This note was uploaded on 04/22/2011 for the course BIOCHEM 412 taught by Professor Thomas during the Spring '11 term at Tennessee Martin.

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412U5 - TMV: Tobacco Mosaic Virus Viral self-assembly: The...

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