Correct Assembly of the Bacteriophage T5 Procapsid Requires Both the Maturation Protease and the Portal Complex

Area of Science:

• Virology
• Structural Biology
• Molecular Biology

Background:

• Coliphage T5 possesses a 90-nm icosahedral capsid (T=13) enclosing a 121kbp double-stranded DNA genome.
• Capsid assembly shares similarities with phage HK97, utilizing a major head protein with a scaffold (∆-domain) cleaved by a maturation protease.
• The major head protein alone yields poor-quality capsid-like particles, indicating the necessity of other components.

Purpose of the Study:

• To investigate the roles of the portal protein and maturation protease in coliphage T5 capsid assembly.
• To elucidate the mechanism by which these components influence the formation of the T=13 icosahedral structure.

Main Methods:

• Generation and overexpression of constructs containing the major head protein with wild-type or inactive protease and portal proteins in Escherichia coli.
• Cryo-electron microscopy reconstruction of prohead I particles.
• Analysis of protein interactions and structural organization.

Main Results:

• An inactive protease mutant was found to trigger the assembly of the major head protein, likely via binding to the ∆-domain.
• The portal protein was identified as a key regulator, ensuring the correct T=13 icosahedral geometry.
• Cryo-EM revealed density consistent with the ∆-domain and the inactivated protease within prohead I particles.

Conclusions:

• Coliphage T5 capsid assembly is more complex than previously understood, involving intricate regulation by portal and protease components.
• The findings highlight specific roles for the protease in initiating assembly and the portal protein in achieving the correct geometric organization.
• This study provides new insights into the structural mechanisms governing bacteriophage capsid formation.