A subgroup of light-driven sodium pumps with an additional Schiff base counterion

Abstract

Light-driven sodium pumps (NaRs) are unique ion-transporting microbial rhodopsins. The major group of NaRs is characterized by an NDQ motif and has two aspartic acid residues in the central region essential for sodium transport. Here we identify a subgroup of the NDQ rhodopsins bearing an additional glutamic acid residue in the close vicinity to the retinal Schiff base. We thoroughly characterize a member of this subgroup, namely the protein ErNaR from Erythrobacter sp. HL-111 and show that the additional glutamic acid results in almost complete loss of pH sensitivity for sodium-pumping activity, which is in contrast to previously studied NaRs. ErNaR is capable of transporting sodium efficiently even at acidic pH levels. X-ray crystallography and single particle cryo-electron microscopy reveal that the additional glutamic acid residue mediates the connection between the other two Schiff base counterions and strongly interacts with the aspartic acid of the characteristic NDQ motif. Hence, it reduces its pKa. Our findings shed light on a subgroup of NaRs and might serve as a basis for their rational optimization for optogenetics.

Calcium-Sensitive Microbial Rhodopsin VirChR1: A Femtosecond to Second Photocycle Study

“…However, E54 in VirChR1 and E54 in TAT rhodopsin are not equivalently located with respect to RSB, which indicates potentially different Ca 2+ binding sites. Residue E54 in TAT rhodopsin is located one helix turn closer to the extracellular side, at the same position as residue E64 in Erythrobacter rhodopsin ( Er NaR) . The detailed structural comparison is shown in Figure S3.…”