Cryo-EM Structure and Assembly of an Extracellular Contractile Injection System

Area of Science:

• Structural biology
• Microbiology
• Nanotechnology

Background:

• Contractile injection systems (CISs) are cell-puncturing nanomachines.
• CISs share evolutionary origins with contractile tail bacteriophages.
• The Photorhabdus virulence cassette (PVC) is an extracellular CIS found in bacteria and archaea.

Purpose of the Study:

• To determine the cryo-electron microscopy (cryo-EM) structure of an intact PVC from P. asymbiotica.
• To elucidate the structural features and potential functional mechanisms of the PVC.
• To provide insights into the broader family of CISs and their applications.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structure of the PVC.
• Structural analysis of the multi-megadalton device.

Main Results:

• The PVC structure, exceeding 10 MDa, resembles a simplified T4 phage tail.
• It features a hexagonal baseplate with six fibers and a capped 117 nm sheath-tube trunk.
• Three variants of tube and sheath proteins suggest functional specialization, with a terminal cap locking the sheath in a pre-contraction state.

Conclusions:

• The determined PVC structure offers a framework for understanding CIS mechanisms.
• The findings highlight the PVC's potential as a delivery tool for biological and therapeutic applications.
• This research advances our knowledge of nanoscale biological machinery and its engineering potential.