Cyanobacterial photosynthesis: finding the function of the ALC

When ancient cyanobacteria evolved the ability to fix carbon through oxygenic photosynthesis, they transformed life on earth. The oxygen that they produced as a by-product of photosynthesis created an oxygenic atmosphere – allowing the evolution of multicellular life forms. Like plants, cyanobacteria use rubisco to catalyse the carbon-fixing step in photosynthesis. Unfortunately oxygenic conditions reduce the efficiency of rubisco, as it is capable of catalysing wasteful reactions using oxygen instead of carbon dioxide. Cyanobacteria overcome this by concentrating carbon dioxide in microcompartments containing rubisco called carboxysomes.

Fluorescently‐labeled carboxysomes in cyanobacteria contain an activase‐like protein that condenses rubisco in an ATP‐dependent manner. Image courtesy of Sigal Lechno‐Yossef (Lechno‐Yossef et al ., pp. 793–806).

Not only is rubisco efficiency reduced by wasteful reactions with oxygen, it is also capable of forming an inactive complex with its own substrate, ribulose-1,5-bisphosphate (RuBP). Plants have an enzyme called rubisco activase (Rca) that uses ATP hydrolysis to release rubisco from its inactive state by removing inhibitory molecules such as RuBP. While rubisco in cyanobacteria does not appear to be inactivated by RuBP in the same way that it is in plants, a homologous protein to Rca called activase-like cyanobacterial protein (ALC) has been found in a range of diverse cyanobacteria.

As it is unclear whether there is a need for a rubisco activase in cyanobactria, Lechno-Yossef and colleagues set out to find the function of the ALC. They found that the ALC is localised to the carboxysomes and is capable of mediating the clustering of rubisco, which is important for carboxysome formation. They found no evidence of the ALC acting as an activase in the way that Rca does in plants. This indicates that, while the ALC has been proposed to be ancestral to the plant Rca, its role in CO2 fixation is different.

Read the article: Lechno‐Yossef, S., Rohnke, B.A., Belza, A.C.O., Melnicki, M.R., Montgomery, B.L. and Kerfeld, C.A. (2020) Cyanobacterial carboxysomes contain an unique rubisco‐activase‐like protein. New Phytologist, 225: 793-806. doi: 10.1111/nph.16195