ИСТИНА

Carotenoids are essential components of all photosynthetic organisms and many non­photosynthetic organisms such as fungi and animals. These molecules have pronounced photoprotective and antioxidant properties and participate in visual sensation. Cyanobacteria utilize carotenoids to dissipate excessive light energy absorbed by their light harvesting complexes, phycobilisomes, through the action of the water­soluble photoactive Orange Carotenoid Protein (OCP), which is composed of the N­ and C­terminal domains coordinating the carotenoid. OCP is downregulated by the so­called Fluorescence Recovery Protein (FRP); however, many cyanobacteria possess no genes for FRP. Maturation of OCP carotenoprotein has to involve the extraction of carotenoids from the membrane, but the mechanism of this process is largely unknown. Recently, it was shown that the naturally expressed C­terminal domain homolog of OCP (called CTDH) can extract carotenoid molecules from the cell membrane and transfer them to OCP and/or the N­terminal domain homologs of OCP (called HCPs). Here we show that the carotenoid transfer between CTDH and OCP­related proteins can go in the opposite direction and revealed the corresponding metastable heterodimeric intermediates. Moreover, we discovered a lightdependent carotenoid transfer process when OCP photoactivation led to the domain separation and transient carotenoid migration into CTDH, associated by protein dimerization, whereas in the dark the reversal of this process was observed. Such an observation suggests that CTDHs may participate not only in OCP and HCP maturation but also in modulation of the OCP activity through the light­dependent carotenoid arrest. Our hypothesis was supported by the CTDH­dependent recovery of phycobilisome fluorescence after the OCP action. These results suggest a new possible mechanism of the OCP activity modulation in cyanobacteria where FRP is absent. Partially supported by RFBR (18­04­00691).