Dynamics of Conformational Changes of Arabidopsis Phototropin 1 LOV2 with the Linker Domain

Conformational changes of Arabidopsis phot1-LOV2 with the linker (phot1-LOV2-linker) were investigated from the viewpoint of the changes in molecular volume and molecular diffusion coefficient (D) by time-resolved transient grating (TG) and transient lens (TrL) methods. Although the absorption spectrum change completes within a few microseconds, the D-value detected by the TG method decreased drastically with a time constant of 1.0 ms from 9.2(±0.4) × 10−11 m2/s to 5.0(±0.3) × 10−11 m2/s. This time-dependent D was interpreted in terms of the unfolding of α-helices in the linker region. The change of the α-helices was confirmed by observing the recovery of the circular dichroism intensity. The TrL signal showed that the molecular volume decreases with two time constants; 300 μs and 1.0 ms. The former time constant is close to the previously observed photo-dissociation reaction rate of the phot1-LOV2 (without the linker) dimer, and the latter one agrees well with the rate of the D-change. Considering a similar time constant of the dissociation reaction of the LOV2 dimer, we interpreted these kinetics in terms of the dissociation step of the linker region from the LOV2 domain (T390pre state). After this step, the protein volume and D are decreased significantly with the lifetime of 1.0 ms. The D decrease indicates the increase of the intermolecular interaction between the protein and water molecules. On the basis of these observations, a two-step mechanism of the linker unfolding is proposed.