Rubisco forms a lattice inside alpha-carboxysomes

Area of Science:

• Cellular Biology
• Structural Biology
• Biochemistry

Background:

• Prokaryotic microcompartments are crucial for biological processes but their complex structures remain poorly understood.
• Structural heterogeneity hinders a complete understanding of microcompartment architecture and function.
• Alpha-carboxysomes are key microcompartments involved in carbon fixation.

Purpose of the Study:

• To elucidate the in vivo structure and organization of Rubisco cargo within alpha-carboxysomes.
• To understand how Rubisco is arranged to optimize carbon fixation efficiency.
• To investigate the balance between order and disorder in microcompartment protein organization.

Main Methods:

• Cryo-electron tomography was used to image alpha-carboxysomes at high resolution.
• Subtomogram averaging was employed to determine the arrangement of Rubisco within the carboxysome.
• Analysis focused on the spatial organization of Rubisco, CsoSCA, and CsoS2.

Main Results:

• Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) polymerizes in vivo into fibrils within the alpha-carboxysome, facilitated by its small subunit.
• These Rubisco fibrils pack into a lattice exhibiting six-fold pseudo-symmetry.
• This ordered yet flexible arrangement maintains access to Rubisco active sites and binding sites for CsoSCA and CsoS2, even at high protein concentrations (>800 μM).

Conclusions:

• The study provides a high-resolution structural characterization of Rubisco cargo organization within alpha-carboxysomes.
• The findings reveal a novel mechanism of enzyme polymerization and lattice formation for efficient carbon fixation.
• This work offers insights into the principles governing protein organization within biological microcompartments.