The C-terminus of bacteriorhodopsin is a random coil.

The 21 amino acids which can be selectively removed from the carboxyl terminus of bacteriorhodopsin by proteolytic treatment are disordered in 2-dimensional arrays of the protein present in purple membranes. This C-terminal portion of the molecule may be involved in the efficiency and rate of light-driven proton uptake, although its presence is not required for pumping activity. In this study, the secondary structure of the C-terminus of bacteriorhodopsin has been determined by examining circular dichroism (CD) difference spectra derived from native and digested samples. In low ionic strength media, this part of the molecule appears to form a random coil-like structure. To examine if this structure is related to the structure found under the high ionic strength condition present in halobacteria, the CD spectra of native purple membranes in water and in 4 M salt solutions were compared. They were found to be identical, suggesting the conformation of the C-terminus in vivo may also be a random coil.