CO2 acquisition and assimilation in microalgae: Regulation by conformational dynamics and supramolecular complexes

In aquatic environments, the CO2 concentration is lower that the Michaelis constant for RuBisCO, and microalgae have evolved specific CO2 concentrating mechanisms (CCM) that increase the local CO2 concentration nearby the RuBisCO active site. Recently, we have identified a new class of carbonic anhydrase, the iota carbonic anhydrase (ι-CA) that participates to the biophysical CCM of a marine diatom, Thalassiosira pseudonana[1]. ι-CA presents unique specificities in its catalysis of CO2 ↔ HCO3– conversion: it does not require Zn2+; can work either independently of metal or with Mn2+; it oligomerises in an atypical drone-like shape[2]. Upon acquisition, the CO2 is assimilated via the Calvin-Benson-Basham cycle, that is regulated by a range of processes that are well studied in Paris, including redox post-translational modifications, metabolite availability, or the association of supramolecular complexes. I will present you our particular interest on a small disordered protein that is the master of the CBB cycle : CP12. We have monitored in-situ in Chlamydomonas reinhardtii cell extracts the conformational transitions of CP12 that orchestrate the formation of the supramolecular complex with two CBB enzymes: the phosphoribulokinase and the glyceraldehyde 3-phosphate dehydrogenase[3]. Our resent data as well as that of others illustrates that CP12 function expend beyond the dark-down regulation of CBB enzymes[4].

[1] E.L. Jensen, R. Clement, A. Kosta, S.C. Maberly, B. Gontero, A new widespread subclass of carbonic anhydrase in marine phytoplankton, ISME J. 13 (2019) 2094–2106. https://doi.org/10.1038/s41396-019-0426-8.

[2] E.L. Jensen, V. Receveur-Brechot, M. Hachemane, L. Wils, P. Barbier, G. Parsiegla, B. Gontero, H. Launay, Structural contour map of the iota carbonic anhydrase from the diatom Thalassiosira pseudonana using a multiprong approach, International Journal of Molecular Sciences. 22 (2021) 8723. https://doi.org/10.3390/ijms22168723.

[3] H. Launay, H. Shao, O. Bornet, F.-X. Cantrelle, R. Lebrun, V. Receveur-Brechot, B. Gontero, Flexibility of oxidized and reduced states of the chloroplast regulatory protein CP12 in isolation and in cell extracts, Biomolecules. 11 (2021) 701. https://doi.org/10.3390/biom11050701.

[4] C. Gérard, R. Lebrun, E. Lemesle, L. Avilan, K.S. Chang, E. Jin, F. Carrière, B. Gontero, H. Launay, Reduction in Phosphoribulokinase Amount and Re-Routing Metabolism in Chlamydomonas reinhardtii CP12 Mutants, International Journal of Molecular Sciences. 23 (2022) 2710. https://doi.org/10.3390/ijms23052710.