A New Direction for Light: Schizorhodopsins beyond the Asgard Archaea

Microbial rhodopsins represent a highly diverse family of light-sensitive membrane proteins that use the chromophore retinal to capture photons. A wide range of functions are known among these proteins, but one particularly puzzling group consists of inward proton pumps. Although proton gradients across the membrane created by outward-pumping rhodopsins enable the synthesis of ATP in the photoheterotrophic microbes that inhabit sunlit waters, the existence of pumps that seemingly do the exact opposite remains a bit of a paradox.

Our study, titled "Novel light-driven schizorhodopsins from Antarctic Minisyncoccota (Patescibacteria) and cyanobacteria," adds a new piece to the puzzle by focusing on the colorfully named group of “schizorhodopsins,” inward proton pumps initially identified in Asgard archaea. We demonstrate that schizorhodopsins actually appear in highly diverse bacteria as well, characterize two such bacterial proteins, and dissect their mechanism of inward proton pumping. At first glance, the schizorhodopsin-possessing bacteria share very little with each other and come from vastly different evolutionary lineages; yet, one recurrent pattern is their appearance in the Antarctic. That schizorhodopsins provide a yet-unknown selective advantage to these Antarctic microbes is best illustrated by filamentous photosynthetic cyanobacteria and several lineages of tiny unicellular heterotrophic patescibacteria: both share the same schizorhodopsin clades in this extreme environment, and there is clear evidence of gene exchange events between the two groups.

In the artwork for the cover of the May 19 issue of Biophysical Journal, we tried to capture the connection between bacterial schizorhodopsins and Antarctica. It depicts a rhodopsin pump embedded within a cellular lipid bilayer. In the center of the image, a beam of sunlight strikes the protein, driving it to pump protons (represented by glowing H⁺ spheres) inward across the membrane and into the cell's interior. The background features a frigid Antarctic landscape complete with floating sea ice, distant glaciers, and a vibrant aurora australis. Finally, in the lower left, silhouettes of bacteria gather, representing the microbes whose survival is boosted by these puzzling pumps.

—María del Carmen Marín, Masae Konno, Andrey Rozenberg, Oded Béjà, and Keiichi Inoue