Structure of the Capsid Size-Determining Scaffold of "Satellite" Bacteriophage P4

Area of Science:

• Structural Biology
• Virology
• Molecular Genetics

Background:

• P4 is a mobile genetic element (MGE) that hijacks the life cycle of helper bacteriophages like P2 for its own propagation, a phenomenon termed 'molecular piracy'.
• When mobilized by P2, P4 forms smaller viral capsids than those produced by P2 alone, a process mediated by the P4-encoded Sid protein.

Purpose of the Study:

• To determine the high-resolution structure of P4 procapsids.
• To build an atomic model for the Sid protein and the gpN capsid protein.
• To understand the mechanism by which Sid redirects capsid size.

Main Methods:

• High-resolution structural determination of P4 procapsids.
• Atomic modeling of Sid and gpN capsid proteins based on structural data.

Main Results:

• Sixty copies of the Sid protein form an intertwined dodecahedral cage around the T=4 P4 procapsid.
• The Sid cage makes specific contact with only one of the four symmetrically non-equivalent copies of the gpN capsid protein.
• The structure explains the effects of previously identified 'sir' and 'nms' mutations in gpN and Sid.

Conclusions:

• The atomic structure of P4 procapsids provides a detailed molecular basis for Sid-mediated capsid size control.
• This work offers insights into the mechanism of molecular piracy and viral assembly.
• The findings suggest a model for how Sid redirects capsid size during P4 propagation.