Expression of microbial rhodopsins in Escherichia coli and their extraction and purification using styrene-maleic acid copolymers

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. For complete details on the use and execution of this protocol, please refer to Ueta et al. (2020).