α-Carboxysome Size Is Controlled by the Disordered Scaffold Protein CsoS2

Area of Science:

• Biochemistry
• Structural Biology
• Microbiology

Background:

• Carboxysomes are essential protein microcompartments for bacterial carbon dioxide (CO2) assimilation via the CO2 concentrating mechanism (CCM).
• These structures assemble into large icosahedral shells encapsulating key enzymes like Rubisco and carbonic anhydrase, but the precise control of their size and composition remains unclear.

Purpose of the Study:

• To investigate the role of the scaffolding protein CsoS2 in regulating the size of alpha-carboxysomes.
• To elucidate the molecular mechanisms by which CsoS2 influences carboxysome assembly and size determination.

Main Methods:

• Analysis of the disordered scaffolding protein CsoS2 and its interactions with carboxysome shell proteins.
• Investigating the distinct binding modes of CsoS2's peptide repeats to the carboxysome shell.

Main Results:

• The size of alpha-carboxysomes is determined by the scaffolding protein CsoS2.
• CsoS2 possesses two classes of peptide repeats that bind differently to the shell, with size controlled by the number of these interactions.
• A model is proposed where CsoS2 repeat classes bind to shell hexamers, influencing local curvature and overall size.

Conclusions:

• CsoS2 acts as a key regulator of alpha-carboxysome size.
• Specific and repeated interactions between CsoS2 and shell proteins collectively ensure the large, uniform size of alpha-carboxysomes.
• This provides a framework for understanding the precise assembly control of bacterial microcompartments.