ИСТИНА

Cyanobacteria use a unique photoprotection mechanism based on the quenching of the light-harvesting antenna by the interaction with the water-soluble Orange Carotenoid Protein (OCP). OCP is composed of the N and C-terminal domains (NTD, CTD) sharing a single ketocarotenoid. OCP photoactivation is accompanied by the domain separation and ketocarotenoid migration into the NTD. The process can be reversed by the Fluorescence Recovery Protein (FRP), which inactivates OCP by reassembling its domains. While most cyanobacteria have genes of full-sized OCP, some active photosynthesizers (such as the unicellular marine diazotroph Crocosphaera watsonii) do not, instead having only genes of FRP and of OCP domain homologs – HCP (NTD homolog) and CTDH (CTD homolog). The presence of only these three uncharacterized proteins in C. watsonii suggests an alternative photoprotection mechanism based on a putative noncovalent OCP-like species formed by HCP and CTDH with the participation of FRP. By using recombinant proteins, here we show that C. watsonii CTDH (CrCTDH) can dimerize upon extracting ketocarotenoids echinenone and canthaxanthin from membranes and efficiently transfers them to the apoform of OCP from Synechocystis (SynOCP). C. watsonii FRP (CrFRP) inactivates SynOCP, but interacts with it with a lower affinity than SynFRP, suggesting the significant role of SynFRP residues that are not conserved in CrFRP. C. watsonii HCP (CrHCP) could not be obtained in a soluble form, likely due to splitting of the coding sequence. The intact gene was reconstructed by homology using one from a closely related cyanobacterium. Its functionality as a carotenoid donor to the apoform of CrCTDH was also shown. Thus, three novel OCP-related proteins from C. watsonii were successfully obtained and characterized, and their functional activity in vitro was demonstrated. Partly supported by RFBRDFG (205412018; FR1276/61).