Mistic-fused expression of algal rhodopsins in Escherichia coli and its photochemical properties

Abstract Background Since algal rhodopsins, the eukaryotic seven-transmembrane proteins, are generally difficult to express in Escherichia coli , eukaryotic cells have been used for heterologous expression. Mistic, a membrane-associated protein that was originally discovered in Bacillus subtilis , has been shown to improve the expression levels of many foreign integral membrane proteins in E. coli when used as a fusion partner linked to the N-terminus of cargo proteins. Methods Here, we expressed two algal rhodopsins with N- and C-terminal Mistic domains in E. coli – Acetabularia rhodopsin I (ARI) and Chlamydomonas sensory rhodopsin B (CSRB, channel rhodopsin 2). UV/VIS spectroscopy, pH titration of proton acceptor residue, laser-induced photolysis and electrophysiological measurement were used for investigating important residues in proton transport and spectroscopic characters of the proteins. Results Protein yield of two algal rhodopsins was enhanced, obtaining 0.12 mg of Mistic-ARI and 0.04 mg of Mistic-CSRB per liter of culture. Spheroplast expression Mistic-ARI had outward proton-pumping activity, indicating protein functionality. Asp89 of ARI changed its protonation state by light absorption, and Asp100 was important for O 600 formation. Electrophysiology revealed that both residues took part in proton transport. The spectroscopic analyses of Mistic-CSRB revealed its characteristics. Conclusions Fusion to the membrane-integrating protein Mistic can enhance overexpression of eukaryotic type I rhodopsins in E. coli . General Significance These findings indicate that Mistic fusion and E. coli expression method could be an effective, low cost technique for studying eukaryotic membrane proteins. This may have useful implications, for example, in studying structural characteristics and optogenetics for rhodopsins.