Optical trapping and manipulation
How does a virus shell assemble around its DNA, step by step?
Open access · cc by · source: Europe PMC
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.
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
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Key findings
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.
Methodology
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.
Limitations
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).
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
Tweezers turn molecular assembly into measurable forces and lengths.
Optical tweezers can measure piconewton-scale biomolecular events: pulling DNA during SV40 capsid assembly showed loop-rupture steps growing from about 40 nm to about 100 nm and rupture forces rising from about 12 to 19 pN over 90 minutes.
Evidence for the claim as stated.
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