posted on 2018-07-20, 00:00authored byKimitoshi Takeda, Masahide Terazima
Phytochromes
are red and far-red light sensor proteins found in
many organisms. Photoisomerization of a chromophore triggers subsequent
reactions leading to conformational changes that are essential for
signaling. Conformational changes of the N-terminal sensor domain
of cyanobacterium phytochrome 1 from Synechocystis sp. PCC6803 (Cph1Δ2) were studied in this report primarily
by the transient grating (TG) method in the time domain. Although
the reaction kinetics monitored by ultraviolet–visible absorption
changes were insensitive to variations in pH, it was found for the
first time that the diffusion coefficient and molecular volume monitored
by the TG method are significantly dependent on pH. This pH sensitivity
indicates that the conformational changes of Cph1Δ2 during the
reaction are modulated by pH and was explained consistently by two
contributions from two isomers (Pr-I and Pr-II) possessing different
protonation states of His260. Thus, His260 is responsible for the
conformational heterogeneity of Pr, as discussed previously, and this
protonation difference leads to different conformational changes of
the sensor domain of Cph1 during the reaction. The photoreaction dynamics
of the Pr-I state was studied in detail, and a significant diffusion
change was observed, which was attributed to a change in the orientation
(quaternary structure) of the dimer with a time constant of 400 μs.
Furthermore, it was revealed that this reorientation of the dimer
was induced by a conformational change in the tongue region. New site-directed
mutations to change the dimer–monomer equilibrium were reported.