Interhelical interactions between D92 and C218 in the cytoplasmic domain regulate proton uptake upon N-decay in the proton transport of Acetabularia rhodopsin II

Publication date: Available online 11 April 2018
Source:Journal of Photochemistry and Photobiology B: Biology
Author(s): Jun Tamogami, Takashi Kikukawa, Keisuke Ohkawa, Noboru Ohsawa, Toshifumi Nara, Makoto Demura, Seiji Miyauchi, Tomomi Kimura-Someya, Mikako Shirouzu, Shigeyuki Yokoyama, Kazumi Shimono, Naoki Kamo
Acetabularia rhodopsin II (ARII or Ace2), an outward light-driven algal proton pump found in the giant unicellular marine alga Acetabularia acetabulum, has a unique property in the cytoplasmic (CP) side of its channel. The X-ray crystal structure of ARII in a dark state suggested the formation of an interhelical hydrogen bond between C218^ARII and D92^ARII, an internal proton donor to the Schiff base (Wada et al., 2011). In this report, we investigated the photocycles of two mutants at position C218^ARII: C218A^ARII which disrupts the interaction with D92^ARII, and C218S^ARII which potentially forms a stronger hydrogen bond. Both mutants exhibited slower photocycles compared to the wild-type pump. Together with several kinetic changes of the photoproducts in the first half of the photocycle, these replacements led to specific retardation of the N-to-O transition in the second half of the photocycle. In addition, measurements of the flash-induced proton uptake and release using a pH-sensitive indium-tin oxide electrode revealed a concomitant delay in the proton uptake. These observations strongly suggest the importance of a native weak hydrogen bond between C218^ARII and D92^ARII for proper proton translocation in the CP channel during N-decay. A putative role for the D92^ARII-C218^ARII interhelical hydrogen bond in the function of ARII is discussed.

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