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Published on: March 24, 2010
The bacteriophage straight phi29 portal motor can package DNA against a large internal force
D E Smith1, S J Tans, S B Smith
1Department of Physics, University of California, Berkeley, California 94720, USA.
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.
Insights
Bacteriophage phi29 portal motor uses ATP hydrolysis to generate force, packaging DNA against significant resistance. This motor achieves high forces and processivity, crucial for viral infection and DNA ejection.
Area of Science:
- Molecular Biology
- Virology
- Biophysics
Background:
- Viruses require efficient genome packaging for infection.
- Bacteriophage phi29 uses a portal complex to package its DNA into a capsid.
- Overcoming DNA's entropic, electrostatic, and bending energies is essential for dense packaging.
Purpose of the Study:
- To investigate the force-generating capabilities of the bacteriophage phi29 portal complex.
- To characterize the motor activity during DNA packaging.
- To understand the forces involved in DNA confinement within the capsid.
Main Methods:
- Utilized optical tweezers to apply force to single DNA molecules during packaging.
- Measured force generation and movement of the portal complex.
- Established the force-velocity relationship of the motor.
Main Results:
- Demonstrated the portal complex acts as a force-generating motor, withstanding loads up to 57 pN.
- Observed high processivity with movements exceeding 5 microm.
- Identified force-dependent rate-limiting steps and force buildup due to DNA confinement (approx. 50 pN).
Conclusions:
- The bacteriophage phi29 portal complex is a powerful molecular motor.
- Internal forces generated during packaging may play a role in DNA ejection.
- Understanding these forces provides insight into viral infection mechanisms.
