Of ion pumps, sensors and channels - perspectives on microbial rhodopsins between science and history

We present a historical overview of research on microbial rhodopsins ranging from the 1960s to the present date. Bacteriorhodopsin (BR), the first identified microbial rhodopsin, was discovered in the context of cell and membrane biology and shown to be an outward directed proton transporter. In the 1970s, BR had a big impact on membrane structural research and bioenergetics, that made it to a model for membrane proteins and established it as a probe for the introduction of various biophysical techniques that are widely used today. Halorhodopsin (HR), which supports BR physiologically by transporting negatively charged Cl⁻ into the cell, is researched within the microbial rhodopsin community since the late 1970s. A few years earlier, the observation of phototactic responses in halobacteria initiated research on what are known today as sensory rhodopsins (SR). The discovery of the light-driven ion channel, channelrhodopsin (ChR), serving as photoreceptors for behavioral responses in green alga has complemented inquiries into this photoreceptor family. Comparing the discovery stories, we show that these followed quite different patterns, albeit the objects of research being very similar. The stories of microbial rhodopsins present a comprehensive perspective on what can nowadays be considered one of nature's paradigms for interactions between organisms and light. Moreover, they illustrate the unfolding of this paradigm within the broader conceptual and instrumental framework of the molecular life sciences. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.