Characterization of NTQ-type Microbial Rhodopsin

초록

Rhodopsins are a family of membrane-embedded photoactive retinylidene proteins. One opsin gene was identified in α-proteobacterium (HTCC2506) that is isolated from western Sargasso Sea. It is very similar to Xanthorhodopin(XR) based on the comparison of primary sequences among other ion pumping rhodopsins. In this study, we performed a basic characterization of this rhodopsin using UV-VIS spectroscopy, pH titration, proton pumping activity, and photocycle measurements. We figured out that it has λmax of 523 nm at pH 7 and 533 nm at pH 10, and the pKa of its proton acceptor is around 5.0. Based on the fast photocycling rate of M and O intermediates, it seems to be a typical ion pumping rhodopsin. By the way, we doubted that the function of this rhodopsin is indirect proton transportation. Through this idea, we tried to show anion dependence first. One interesting thing with this rhodopsin is that it shows large red-shift with absence of Cl ion (543nm at pH7). But, there is no significant difference in pumping activity by existence of Cl ion. Additionally, we made and analysed 4 mutants N110D, T114D, Q121E, and D243N which are thought as important residues in proton transport pathway.