Microbial rhodopsins are light-sensitive proteins that recruit retinal as the chromophore. Rhodopsins most commonly serve as light-driven ion pumps that appear in diverse photo(hetero)trophic organisms, including the most abundant microbes on Earth. Curiously, in addition to retinal, some rhodopsin pumps were discovered to have a separate carotenoid antenna that extends the range of the absorption spectra of the protein. Such rhodopsins have been known for almost 20 years, yet the isolated cases of rhodopsin proton pumps remained just that: a curious phenomenon of marginal ecological significance. In hope of finding more such cases, some attempts by other groups were made in the past using ketolated carotenoids, the only type of antenna carotenoids known in rhodopsins. We tried an entirely different approach and found something unexpected. We designed a workflow that uses environmental samples to fish out from a native pool of carotenoids the ones that might be able to serve as antennas for rhodopsin pumps.
Lake Kinneret-derived rhodopsin is used as a "bait" to fish from Lake Kinneret juice (i.e., carotenoid extract from the native microbial community) potential antenna.
Other members of the lab received this approach with a great deal of skepticism, to say the least (nobody believed this environmental carotenoids juice was worth something). Luckily for us, it worked like magic! Using this workflow, we found several rhodopsins that use hydroxylated carotenoids (e.g., zeaxanthin and lutein) as an antenna.
Sampling site: Kinneret Lake (Sea of Galilee)
We did more experiments checking a panel of rhodopsin pumps predicted to have a window enabling the interaction between the two chromophores, and discovered that the ability to bind hydroxylated carotenoid antennas appear to be quite common. In fact, based on our experiments, we predict that antenna-containing rhodopsins are very abundant in nature. We now estimate that more than a third of the rhodopsins in the oceans and even more in lakes can bind an antenna.
Water sample collection in Lake Kinneret
These antennas were neglected for almost 20 years; we now understand that they may have a substantial impact on rhodopsin phototrophy worldwide.