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Conformational Changes in the N-Terminal Region of Photoactive Yellow Protein: A Time-Resolved Diffusion Study

Yuji Hoshihara ^*, Yasushi Imamoto , Mikio Kataoka , Fumio Tokunaga  and Masahide Terazima ^*

^* Department of Chemistry, Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan; Graduate School of Materials Science, Nara Institute of Science and Technology, Nara 630-0192, Japan; and Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

Correspondence: Address reprint requests to Masahide Terazima, E-mail: mterazima{at}kuchem.kyoto-u.ac.jp.

The kinetics of conformational change in the N-terminal region of photoactive yellow protein (PYP) was studied by the time-resolved diffusion measurement. The transient grating signal that represented the protein diffusion of the ground state and pB state depended on the observation time range. An analysis of the signal based on the time-dependent diffusion coefficient clearly showed that protein diffusion changed with a time constant of 170 µs, corresponding to the pR₂ pB' transition. Since a previous diffusion study of N-terminal truncated PYPs had revealed that the change in the diffusion coefficient reflected the unfolding of the -helices in the N-terminal region of PYP, the results indicate that this unfolding took place at the same rate as the pR₂ pB' transition. This demonstrates that the response of the conformational change in the N-terminal region was quite fast, probably due to changes in a specific hydrogen-bonding network of this domain.