The C-terminus of a diatom plant-like cryptochrome influences the FAD redox state and binding of interaction partners

Abstract

A plant-like cryptochrome of diatom microalgae, CryP, acts as a photoreceptor involved in transcriptional regulation. It contains FAD and 5,10-methenyltetrahydrofolate as chromophores. Here, we demonstrate that the unstructured C-terminal extension (CTE) of CryP has an influence on the redox state of the flavin. In CryP lacking the CTE, the flavin is in the oxidized state, while it is a neutral radical (FADH •) in the full-length protein. When the CTE of CryP is coupled to another diatom cryptochrome that naturally binds FADox, this chimera also binds FADH •. In full-length CryP, FADH • is the most stable redox state and oxidation to FADox is extremely slow, whereas reduction to FADH2 is reversible in the dark in about an hour. We also identified novel interaction partners of this algal CRY and characterized two of them in depth regarding their binding activities. BolA, a putative transcription factor, binds to monomeric as well as to dimeric CryP via the CTE, independent of the redox state of the flavin. In contrast, an unknown protein, ID42612, occurring solely in heterokont algae, only binds to CryP dimers. This binding is independent of the CTE and shows slight differences in strength depending on the flavin’s redox state.