PMID:11607035

Citation	Smith, DE, Tans, SJ, Smith, SB, Grimes, S, Anderson, DL and Bustamante, C (2001) The bacteriophage straight phi29 portal motor can package DNA against a large internal force. Nature 413:748-52
Abstract	As part of the viral infection cycle, viruses must package their newly replicated genomes for delivery to other host cells. Bacteriophage straight phi29 packages its 6.6-microm long, double-stranded DNA into a 42 x 54 nm capsid by means of a portal complex that hydrolyses ATP. This process is remarkable because entropic, electrostatic and bending energies of the DNA must be overcome to package the DNA to near-crystalline density. Here we use optical tweezers to pull on single DNA molecules as they are packaged, thus demonstrating that the portal complex is a force-generating motor. This motor can work against loads of up to 57 pN on average, making it one of the strongest molecular motors reported to date. Movements of over 5 microm are observed, indicating high processivity. Pauses and slips also occur, particularly at higher forces. We establish the force-velocity relationship of the motor and find that the rate-limiting step of the motor's cycle is force dependent even at low loads. Notably, the packaging rate decreases as the prohead is filled, indicating that an internal force builds up to approximately 50 pN owing to DNA confinement. Our data suggest that this force may be available for initiating the ejection of the DNA from the capsid during infection.
Links	PubMed Online version:10.1038/35099581
Keywords	Bacillus Phages/genetics; Biomechanical Phenomena; DNA, Viral/metabolism; Microspheres; Models, Genetic; Optics and Photonics; Virus Assembly/physiology
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Annotations

Gene product	Qualifier	GO Term	Evidence Code	with/from	Aspect	Extension	Notes	Status
9CAUD:S5Y730		GO:0019073: viral DNA genome packaging	ECO:0000314:		P		The portal complex can work against large internal forces. Internal forces and the rate of DNA packaging were measured. In addition to demonstrating the phi29 motor complex produces force when DNA is being packaged into the capsid. Usage of microspheres were utilized by attaching it to partly prepackaged complexes and to the tethered end of the DNA that was captured in the optical trap. Optical tweezers measured the forces acting upon the packaging on DNA while the other microsphere was held by a micropipette (Fig 1a). The force increases when the portal motor pulls in the tethered DNA and less force when the DNA is not being pulled in (Fig. 2). Stalling of DNA packaging occurs at higher forces (Fig. 3). Build up of internal force is created when more DNA is packaged in which it exerts opposing forces causing the motor to slow. Internal forces are also created in order to initiate the injection of phage genomic DNA into a bacterium.	complete CACAO 10872

References

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