Salt effect on the selective photoinduced dimerization of a BLUF domain of EB1

Abstract

The BLUF domain of EB1 in Bldp1 (eBLUF) exhibits a unique photoinduced selective dimerization; that is, the dimer of light- and dark-state protomers (LD-dimer) is much more stable than that of two dark-state protomers (DD-dimer) and two light-state protomers (LL-dimer). This characteristic feature of eBLUF could be appropriate for controlling protein–protein interactions using light as an optogenetic tool. In this study, a prominent salt effect was observed and investigated in detail for this dimerization reaction. The cations significantly increased the stability of the dimers, whereas the anion effect was almost negligible. This result suggests that the binding of a cation to the negatively charged residues in eBLUF increases the stability by reducing the Coulombic repulsion, which prevents dimerization. Using the unique ion influences on the dimer and some single-point mutants, a possible dimerization site was investigated. The key residue for the dominant Coulombic repulsion was identified as Asp (D137), and is located near the end of the C-terminal. Hence, the dimerization site should be close to this position. A docking simulation supports this structure. This study confirms that the stability of the dimer of the D137N mutant of eBLUF is much higher than that of the wild-type eBLUF under the same conditions, along with higher selectivity.