Cell Press Acta Medica Okayama 2666-1667 3 1 2022 Expression of microbial rhodopsins in Escherichia coli and their extraction and purification using styrene-maleic acid copolymers 101046 EN Keiichi Kojima Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University Yuki Sudo Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University Microbial rhodopsins are photoreceptive membrane proteins showing various light-dependent biological activities. Styrene-maleic acid (SMA) copolymers spontaneously form nanoscale lipid particles containing membrane proteins and associated lipids without detergent, and can be used to characterize membrane molecules. Here, we provide a protocol to functionally express a thermally stable rhodopsin, Rubrobacter xylanophilus rhodopsin, and an unstable rhodopsin, Halobacterium salinarum sensory rhodopsin I, in Escherichia coli. We then describe the preparation of SMA and the extraction and purification of rhodopsin molecules using SMA. <br> For complete details on the use and execution of this protocol, please refer to Ueta et al. (2020). No potential conflict of interest relevant to this article was reported.