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

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.

[Optogenetics: A Therapeutic Option for Advanced Retinal Dystrophies]

Optogenetics refers to the genetic modification of cells to express light-sensitive proteins, which mediate ion flow or secondary signalling cascades upon light exposure. Channelrhodopsin, the most famous example, is an unselective cation channel, which opens when exposed to blue light, thus mediating the depolarisation of the expressing cell. Along with other light-sensitive proteins such as the chloride pump eNpHR, which mediates light-activated hyperpolarisation, the optogenetic toolset offers a wide range of non-invasive single cell manipulations. Due to the direct modulation of the membrane potential, the in-vivo and in-vitro application of optogenetics in neuronal cells seemed to be of outstanding interest. Soon it became evident that these tools are well-suited to treat retinas of patients suffering from photoreceptor degeneration, independently of the underlying mutation. The ectopic expression of channelrhodopsin or eNpHR may cause inactive photoreceptors or other, intact cells of the retina to become sensitive to light. Thus, the most basic function of the retina, the perception of light, can be restored. This review gives a short overview of the retinal structure as well as its physiological and pathological function as the primary light-perceiving tissue. We will focus on different optogenetic strategies to restore visual function in previously blind retinas.