Efficient femtosecond energy transfer from carotenoid to retinal in gloeobacter rhodopsin-salinixanthin complex

The retinal proton pump xanthorhodopsin (XR) was recently found to function with an attached carotenoid light harvesting antenna, salinixanthin (SX). It is intriguing to discover if this departure from single chromophore architecture is singular or if it has been adopted by other microbial rhodopsins. In search of other cases, retinal protein encoding genes in numerous bacteria have been identified containing sequences corresponding to carotenoid binding sites like that in XR. Gloeobacter rhodopsin (GR), exhibiting particularly close homology to XR, has been shown to attach SX, and fluorescence measurements suggest SX can function as a light harvesting (LH) antenna in GR as well. In this study, we test this suggestion in real time using ultrafast transient absorption. Results show that energy transfer indeed occurs from S2 of SX to retinal in the GR-SX composite with an efficiency of ∼40%, even higher than that in XR. This validates the earlier fluorescence study, and supports the notion that many microbial retinal proteins use carotenoid antennae to harvest light.