A two-component quasi-icosahedral protein nanocompartment with variable shell composition and irregular tiling

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Protein shells, or capsids, are essential for compartmentalization in viruses and cellular organisms.
• Encapsulins are prokaryotic protein-based compartments, typically formed by a single shell protein with viral HK97-fold.
• Previous studies focused on single-component encapsulins, leaving multi-component systems less understood.

Purpose of the Study:

• To characterize a novel two-component encapsulin system from Streptomyces lydicus.
• To investigate the assembly mechanisms and structural properties of its constituent shell proteins.
• To explore the co-assembly of these components into mixed protein shells.

Main Methods:

• Cryo-electron microscopy (cryo-EM) for structure determination.
• 3D classification of assembled shells.
• Crosslinking studies to analyze protein interactions.
• Biochemical characterization of individual component assembly.

Main Results:

• The study identified and characterized a Family 2B two-component encapsulin from Streptomyces lydicus.
• Differential assembly behaviors of the two distinct shell protein components were observed.
• Co-assembly into mixed shells with variable compositions and irregular tiling was demonstrated.
• Cryo-EM structures revealed the detailed architecture of the individual shell proteins and mixed shells.

Conclusions:

• This work expands the known assembly principles of HK97-fold proteins beyond single-component systems.
• The findings reveal a new mode of protein-based compartmentalization using two distinct shell proteins.
• This research provides a foundation for future functional studies and engineering of two-component encapsulins.