Chromophore of sensory rhodopsin II from Halobacterium halobium

The photoreceptor sensory rhodopsin II (sR-II) was enriched 120-fold from cell membranes of Halobacterium halobium. The final preparation yields sR-II with a specific content of 3 nmol of sR-II/mg of protein. The spectroscopic measurements were performed on the enriched photoreceptor solubilized in digitonin. In the absolute absorption spectrum of the partially purified receptor, the main peak in the visible range corresponded to sR-II with a maximum at 488 nm. Cytochromes contributed to the spectrum only in a minor band at 415 nm. The extinction coefficient of sR-II was estimated from difference spectra during bleaching with hydroxylamine to be 48,000 M-1 cm-1. The reduced chromophore displayed a pronounced fine structure which is due to the coplanarity of the retinyl residue. The isomeric composition of the chromophore from the enriched photoreceptor was determined in retinal extracts in HPLC. The dark-adapted sR-II contains 80% all-trans- and 20% 13-cis-retinal. After illumination, the ratio changed to 1:1, indicating a trans-cis isomerization during the photocycle of sR-II.