Department of Ophthalmology, University Hospital Bonn, Medical Faculty, Bonn, Germany.
Institute of Physical and Theoretical Chemistry, Goethe University Frankfurt, 60438, Frankfurt am Main, Germany.
Nat Commun. 2024 Apr 10;15(1):3119. doi: 10.1038/s41467-024-47469-0.
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.
光驱动的钠离子泵（NaR）是一种独特的离子转运微生物视蛋白。NaR 的主要类型以 NDQ 基序为特征，在中央区域有两个天冬氨酸残基，对钠离子的转运至关重要。在这里，我们鉴定出一个在视黄醛席夫碱附近有额外谷氨酸残基的 NDQ 视蛋白亚组。我们详细研究了该亚组的一个成员，即来自Erythrobacter sp. HL-111 的蛋白 ErNaR，并表明额外的谷氨酸残基导致钠离子泵活性对 pH 值的敏感性几乎完全丧失，这与之前研究的 NaR 不同。即使在酸性 pH 值下，ErNaR 也能有效地转运钠离子。X 射线晶体学和单颗粒冷冻电子显微镜揭示了额外的谷氨酸残基介导了其他两个席夫碱反离子之间的连接，并与特征性 NDQ 基序的天冬氨酸残基强烈相互作用。因此，它降低了其 pKa。我们的发现揭示了 NaR 的一个亚组，并可能为它们的理性优化提供依据，以用于光遗传学。
