Effects of N- and C-terminal regions on oligomeric formation of blue light sensor protein SyPixD

A sensor of blue-light using flavin adenine dinucleotide (BLUF) is a typical blue light photoreceptor domain that is found in many photosensor proteins in bacteria and some eukaryotic algae. SyPixD in Synechocystis is one of the well-studied BLUF proteins. In the dark state, it forms a decamer and, upon photoexcitation, a dissociation reaction takes place to yield dimers. Such change in the intermolecular interactions of the protomers is important for the biological function. The effect of the N- and C-terminal sequences on the stability of SyPixD oligomeric states and photoreactions of SyPixD were studied to understand how the oligomeric form is maintained with weak interaction. It was found that a few residues that frequently persist at the N-terminus after removing a tag for purification are sensitive to the stability of the decamer structure. Even two or three residues at the N-terminus considerably reduces decamer stability, whereas four or more residues completely prevent decamer formation. Unexpectedly, truncating C-terminal sequences, which locate far from any protomer interface and of which structure is undetermined in crystal structure, also destabilizes the decamer structure. This destabilization is also apparent from the dissociation reaction dynamics detected by the transient grating and transient absorption measurements. The dissociation reaction is faster and the yield increases when the C-terminus does not contain seven amino acid residues. Photoexcitation induces a conformational change in the C-terminus of the decamer but not the dimer.