Interactions and structural variability of β-carboxysomal shell protein CcmL

Area of Science:

• Structural biology
• Microbial biochemistry

Background:

• CcmL is a protein proposed to localize at the vertices of beta-carboxysomal shells.
• Understanding CcmL structure and interactions is key to carboxysome assembly.

Purpose of the Study:

• To elucidate the structures of CcmL orthologs from Nostoc sp. PCC 7120 and Thermosynechococcus elongatus BP-1.
• To investigate the structural basis for CcmL's role in beta-carboxysome assembly.

Main Methods:

• X-ray crystallography was used to determine the structures of CcmL orthologs.
• Förster resonance energy transfer (FRET) experiments were conducted to study CcmL-CcmK2 interactions.

Main Results:

• Structures of Nostoc and T. elongatus CcmL were determined, broadly similar to known orthologs.
• Nostoc CcmL exhibited conformational variability in loops at the base of the pentamer, forming a distinct protomer arrangement.
• A back-to-back decameric organization was observed in crystals, but likely not in solution.
• Weak interactions between CcmL and CcmK2 were detected at high CcmK2 concentrations (>100 μM).

Conclusions:

• CcmL's structural plasticity may influence its incorporation into carboxysomes.
• CcmL likely interacts with CcmK2 at specific, potentially defective, sites on the growing carboxysome shell.