How does a virus shell assemble around its DNA, step by step?
SV40 capsid proteins first kink and loop the DNA within minutes, then slowly lock together into stronger intermediates until a complete, force-resistant shell forms.
Source
Revealing in real-time a multistep assembly mechanism for SV40 virus-like particles
Study at a glance
- Design
- Animal / in-vitro — In vitro single-molecule stretching of DNA tethers incubated with truncated or wild-type SV40 VP1 pentamers, with worm-like-chain fits and AFM imaging
- N
- Counts vary by measurement: e.g. persistence length of bare DNA from 19 tethers and with truncated VP1 from 23 tethers; VLP heights from 112 particles in AFM
- Population
- Purified SV40 VP1 capsid-protein pentamers and double-stranded DNA templates (pKYB1, lambda DNA)
- Outcome
- Effective persistence length, stretch modulus, contour length, rupture forces and step sizes of DNA-protein complexes over time
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The researchers tethered single DNA molecules between beads and surfaces and stretched them with optical tweezers and acoustic force spectroscopy while SV40 capsid-protein pentamers bound. They first used truncated pentamers that can grab DNA but cannot bond to each other, then used wild-type pentamers that can form full virus-like particles. Force-extension curves were fitted with an extensible worm-like chain model, and atomic force microscopy confirmed what structures formed.
What they found
Truncated pentamers cut the DNA's effective persistence length from about 50 nm to about 8 nm within minutes, which the authors attribute to kinking, and created loops that ruptured in roughly 40 nm steps. These pentamers could only compact DNA when the pulling force was below about 1 pN. With wild-type pentamers, rupture steps grew to about 100 nm and rupture forces rose from about 12.0 pN to 19 pN over 90 minutes, and the most compacted tethers shortened by an amount close to the SV40 genome length, often showing no ruptures at all, consistent with a finished capsid.
The limits
What it doesn't show
The kinking interpretation of the drop in persistence length is inferred from fits, not imaged directly. Intermediates are grouped by contour length rather than followed individually from start to finish, so the proposed sequence is a reconstruction. The experiments used purified VP1 without VP2/VP3 and a plasmid template, not infection in cells, so the pathway in real infections may differ. Identification of fully formed capsids on tethers rests on indirect lines of evidence (length change, stability, timing).
Key terms
- Persistence length
- The length over which a polymer stays roughly straight; a lower value means a more flexible or kinked chain.
- Worm-like chain model
- A model of a semi-flexible polymer that predicts how its extension grows with applied force.
- Optical tweezers
- A focused laser beam that traps a microscopic bead and measures piconewton forces applied to a molecule attached to it.
- Contour length
- The full length of a polymer if it were pulled perfectly straight; compaction reduces the apparent contour length.
- Virus-like particle (VLP)
- A capsid shell assembled from viral proteins that looks like a virus but lacks the full viral genome.
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Quiz yourself
What did a drop in DNA persistence length from about 50 nm to about 8 nm indicate?
Common questions
Why use a truncated protein first?
Without its C-terminal arms the pentamer can still bind DNA but cannot bond to other pentamers, so the DNA-binding step can be studied on its own.
Why does a pulling force stop compaction?
Forming a loop requires the DNA to bend back on itself, and tension makes that energetically costly; above about 1 pN compaction stopped.
How do they know a complete capsid formed on the tether?
The DNA shortened by about the length of the SV40 genome, only one capsid fits on the template, the timing matched AFM assembly, and many such tethers showed no ruptures under force.
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