The small protein CP12: a protein linker for supramolecular complex assembly

Area of Science:

• Plant molecular biology
• Photosynthesis research
• Protein structure and function

Background:

• CP12 is a chloroplast protein found in higher plants.
• It is part of a core complex with phosphoribulokinase (PRK) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH).
• The precise role of CP12 in this complex assembly was previously undetermined.

Purpose of the Study:

• To investigate the role of CP12 in the assembly of the PRK-GAPDH-CP12 complex.
• To determine the structural and functional differences between oxidized and reduced CP12.
• To elucidate the mechanism by which CP12 facilitates complex formation.

Main Methods:

• Cloning and expression of Chlamydomonas reinhardtii CP12 in E. coli.
• Reconstitution assays and surface plasmon resonance (SPR) binding studies.
• Nuclear Magnetic Resonance (NMR) and circular dichroism (CD) spectroscopy.

Main Results:

• Oxidized CP12, but not reduced CP12, functions as a linker in complex assembly.
• CP12 associates with GAPDH, inducing a conformational change.
• Oxidized CP12 is structured and flexible, while reduced CP12 is unstructured.
• CP12 exhibits properties of intrinsically unstructured proteins.

Conclusions:

• CP12 acts as a crucial, redox-sensitive linker protein in the PRK-GAPDH complex assembly.
• The structural transition of CP12 upon reduction/oxidation is key to its function.
• CP12 represents a plant-specific family of intrinsically unstructured proteins involved in regulating macromolecular complexes.