Carotenoid binding in Gloeobacteria rhodopsin provides insights into divergent evolution of xanthorhodopsin types

Engineering of thermal stability in the recombinant xanthorhodopsin from Salinibacter ruber

Solubilization in detergents is a widely used technique for the isolation of membrane proteins and the study of their properties. Unfortunately, protein stability in detergent micelles can sometimes be compromised. We encountered this issue with xanthorhodopsin (XR) from Salinibacter ruber, which had been previously engineered for expression in Escherichia coli cells. To explore the factors affecting stability and to enhance thermal stability of recombinant XR preparations following solubilization of membranes using n-dodecyl-β-D-maltopyranoside and nickel-affinity chromatography, we developed a series of hybrid proteins based on the homology between XR and a stable rhodopsin from Gloeobacter violaceus (GR). Functional studies of these hybrids and measurements of their melting temperatures revealed the structural elements of XR that account for its notable difference in stability compared to GR, despite their high overall homology of approximately 50 % identical residues. In particular, XR variants with an engineered loop between transmembrane helices D and E, similar to that in GR, demonstrated enhanced stability. However, we found that replacing the DE loop affects carotenoid binding. Additionally, two hybrid proteins containing the C and D helices from GR exhibited increased stability as well as improved photocycle and proton transport rates. In conclusion, we have demonstrated that optimizing the amino acid sequence of xanthorhodopsin from S. ruber based on its homology with Gloeobacter rhodopsin is an effective approach to enhance its thermal stability in vitro and improve its potential for optogenetic applications.

Unveiling the critical role of K+ for xanthorhodopsin expression in E. coli

Xanthorhodopsin (XR), a retinal-binding 7-transmembrane protein isolated from the eubacterium Salinibacter ruber, utilizes two chromophores (retinal and salinixanthin (SAL)) as an outward proton pump and energy-donating carotenoid. However, research on XR has been impeded owing to limitations in achieving heterogeneous expression of stable forms and high production levels of both wild-type and mutants. We successfully expressed wild-type and mutant XRs in Escherichia coli in the presence of K+. Achieving XR expression requires significant K+ and a low inducer concentration. In particular, we highlight the significance of Ser-159 in helix E located near Gly-156 (a carotenoid-binding position) as a critical site for XR expression. Our findings indicate that replacing Ser-159 with a smaller amino acid, alanine, can enhance XR expression in a manner comparable to K+, implying that Ser-159 poses a steric hindrance for pigment formation in XR. In the presence of K+, the proton pumping and photocycle of the wild-type and mutants were characterized and compared; the wild-type result suggests similar properties to the first reported XR isolation from the S. ruber membrane fraction. We propose that the K+ gradient across the cell membrane of S. ruber serves to uphold the membrane potential of the organism and plays a role in the expression of proteins, such as XR, as demonstrated in our study. Our findings deepen the understanding of adaptive protein expression, particularly in halophilic organisms. We highlight salt selection as a promising strategy for improving protein yield and functionality.

Proton-pumping photoreceptor controls expression of ABC transporter by regulating transcription factor through light

Light is a significant factor for living organisms with photosystems, like microbial rhodopsin-a retinal protein that functions as an ion pump, channel, and sensory transduction. Gloeobacter violaceus PCC7421, has a proton-pumping rhodopsin gene, the Gloeobacter rhodopsin (GR). The helix-turn-helix family of transcriptional regulators has various motifs, and they regulate gene expression in the presence of various metal ions. Here, we report that active proton outward pumping rhodopsin interacted with the helix-turn-helix transcription regulator and regulated gene expression. This interaction is confirmed using ITC analysis (KD of 8 μM) and determined the charged residues required. During in vitro experiments using fluorescent and luciferase reporter systems, ATP-binding cassette (ABC) transporters and the self-regulation of G. violaceus transcriptional regulator (GvTcR) are regulated by light, and gene regulation is observed in G. violaceus using the real-time polymerase chain reaction. These results expand our understanding of the natural potential and limitations of microbial rhodopsin function.