Exceptional Heterogeneous Elasticity and One-Way Valve Mechanism of the PHi29 Head-Tail Connector

Abstract

The bacteriophage Φ29 DNA packaging motor, comprising ATPase, pRNA and head-tail connector, transports viral DNA inside the prohead against a maximum pressure of ∼60 atm. Two recently discussed transport models, push-and-roll (Yu et al. 2010) and one-way-revolution (Zhao et al. 2013), propose that the ATPase pushes the DNA directly into the procapsid; additionally, the latter model postulates that the connector acts as a one-way valve and restricts DNA leakage by specific loop interactions (residues N229-N246). Here, we focus on the connector's role in translocation and how it affects DNA conformation. Specifically, how such a one-way valve withstands the large pressure difference and how the connector loop-residues affect the function. To address these questions, we performed equilibrium and force-probe molecular dynamics simulations of the connector with and without DNA. We observe that the connector deforms DNA, which untwists, over-twists and compresses. Remarkably, comparison of the obtained DNA compression with FRET-FCS measurements of the T4 bacteriophage motor (Ray et al. 2010), revealed to be common characteristic of the head-tail bacteriophages. Further, the Young's modulus of the connector central region is comparable to that of structural proteins like collagen, and the obtained heterogonous connector stiffness resembles composite materials. These exceptional elastic properties enable the connector to withstand both longitudinal and lateral pressure generated by the packed DNA and the procapsid, respectively. Furthermore, pushing the DNA into the procapsid requires less force than pulling it out. Upon three loop-residue mutations (K234A.K235A.R237A), the required forces for pushing and pulling become similar, which supports the residues' essential role in the one-way-valve function. Our results corroborate the connector's one-way valve function, whereas rotation and/or revolution motions of DNA proposed in both models remain open for future investigation.