Figure 2

PR structure with endogenous cysteine residues and other important features. Ribbon representation of the PR solution NMR structure (PDB: 2L6X20) showing the retinal cofactor (in magenta), the three endogenous cysteines (in cyan), including C176 that was mutated to a serine, and asparagine N221 that was mutated to a cysteine (in green). Lipid bilayer boundaries are indicated and based on the OPM (orientation of proteins in membranes) database calculations (http://opm.phar.umich.edu).