Isolation and properties of the native chromoprotein halorhodopsin

The native chromoprotein of the light-driven chloride pump halorhodopsin (HR) was isolated from Halobacterium halobium strain L-33 which lacks bacteriorhodopsin but contains 'slow cycling rhodopsin-like pigment' (SR). A membrane fraction was prepared in low salt and dissolved in a high salt medium by the detergents Lubrol PX or octylglucoside. These conditions destroyed the chromophore of SR but not the HR pigment. Chromatography on phenyl-Sepharose and hydroxylapatite produced, in 60% yield, a 230-fold enriched monomeric chromoprotein with an apparent mol. wt. of 20,000. The chromoprotein was stable in 1 M NaCl and 1% octylglucoside and remained stable upon removal of detergent. It reacted with borohydride in the dark and with hydroxylamine in the light. The absorption maximum of the light-adapted state is at 580 + 2 nm and its molar extinction approximately 50,000/M/cm. Upon illumination in the presence of detergent it was converted into a 410 nm absorbing species with concomitant release of protons. A thermal reconversion to the 580 nm species occurred with a half time of 76 s at -6 degrees C. Blue light absorbed by the photoproduct accelerated the re-conversion as well as the re-uptake of protons. Removal of the detergent prevented the light-induced formation of the 410 nm species. Under these conditions a photochemical behaviour similar to that in intact cells and cell vesicles, i.e., a photocycle in the 10-20 ms range was observed. These findings form the basis for functional reconstitution of HR.