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Conformational Changes of PYP Monitored by Diffusion Coefficient: Effect of N-Terminal -Helices

Javaid Shahbaz Khan ^*, Yasushi Imamoto , Miki Harigai , Mikio Kataoka  and Masahide Terazima ^*

^* Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan; and Graduate School of Materials Science, Nara Institute of Science and Technology, Nara 630-0192, Japan

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

Conformational changes in the light illuminated intermediate (pB) of photoactive yellow protein (PYP) were studied from a viewpoint of the diffusion coefficient (D) change of several N-truncated PYPs, which lacked the N-terminal 6, 15, or 23 amino acid residues (T6, T15, and T23, respectively). For intact PYP (i-PYP), D of pB (D_pB) was 11% lower than that (D_pG) of the ground state (pG) species. The difference in D (D_pG – D_pB) decreased upon cleavage of the N-terminal region in the order of i-PYP>T6>T15>T23. This trend clearly showed that conformational change in the N-terminal group is the main reason for the slower diffusion of pB. This slower diffusion was interpreted in terms of the unfolding of the two -helices in the N-terminal region, increasing the intermolecular interactions due to hydrogen bonding with water molecules. The increase in friction per one residue by the unfolding of the -helix was estimated to be 0.3 x 10^–12 kg/s. The conformational change in the N-terminal group upon photoillumination is discussed.

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