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

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 (DpB) was ∼11% lower than that (DpG) of the ground state (pG) species. The difference in D (DpG − DpB) 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 α-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 α-helix was estimated to be 0.3 × 10−12 kg/s. The conformational change in the N-terminal group upon photoillumination is discussed.