Аннотация:The photoprotective mechanism of non-photochemical quenching (NPQ) in cyanobacteria relies on the interplay between the Orange Carotenoid Protein (OCP) and the Fluorescence Recovery Protein (FRP). Illumination with blue-green light converts OCP from the orange, inactive state OCPO into the red-shifted, active state OCPR that can quench phycobilisome (PBs) fluorescence. Upon binding of FRP, OCPR is converted to OCPO and dissociates from PBs, however, the mode and site of OCPR/FRP interaction remain elusive. Recently, we introduced the purple OCPW288A mutant as a fully competent model for the signaling OCPR state. Here, we compared the thermodynamic stability of OCPW288A, OCP wildtype and the corresponding apoproteins by differential scanning calorimetry. Furthermore, we utilized site-directed fluorescence labeling of OCP to generate fluorescent OCP proteins for fluorescence correlation spectroscopy (FCS). Our results show that OCPW288A proteins melt at lower temperatures than the corresponding wildtype proteins, with significantly larger unfolding enthalpies. Furthermore, OCPW288A has a (1.6±0.4)-fold larger hydrodynamic radius than OCPO. Whereas the addition of FRP did not change the diffusion behavior of OCPO, a substantial compaction of the OCPW288A mutant and of the OCP-apoprotein was observed by FCS. These results show that sufficiently stable complexes between FRP and OCPW288A or OCP-apoprotein are formed to be detected by FCS. A 1:1 complex formation with a micromolar apparent dissociation constant between OCP-apoprotein and FRP was corroborated by size-exclusion chromatography. Beyond the established OCP/FRP interaction underlying NPQ cessation, the OCP-apoprotein/FRP interaction suggests a more general role of FRP as a scaffold protein for OCP maturation.