The M Intermediate of Pharaonis Phoborhodopsin Is Photoactive

Abstract

The retinal protein phoborhodopsin (pR) (also called sensory rhodopsin II) is a specialized photoreceptor pigment used for negative phototaxis in halobacteria. Upon absorption of light, the pigment is transformed into a short-wavelength intermediate, M, that most likely is the signaling state (or its precursor) that triggers the motility response of the cell. The M intermediate thermally decays into the initial pigment, completing the cycle of transformations. In this study we attempted to determine whether M can be converted into the initial state by light. The M intermediate was trapped by the illumination of a water glycerol suspension of phoborhodopsin from Natronobacterium pharaonis called pharaonis phoborhodopsin ( ppR) with yellow light (>450 nm) at −50°C. The M intermediate absorbing at 390 nm is stable in the dark at this temperature. We found, however, that M is converted into the initial (or spectrally similar) state with an absorption maximum at 501 nm upon illumination with 380-nm light at −60°C. The reversible transformations ppR ⇔ M are accompanied by the perturbation of tryptophan(s) and probably tyrosine(s) residues, as reflected by changes in the UV absorption band. Illumination at lower temperature (−160°C) reveals two intermediates in the photoconversion of M, which we termed M′ (or M′ 404) and ppR′ (or ppR′ 496). A third photoproduct, ppR′ 504, is formed at −110°C during thermal transformations of M′ 404 and ppR′ 496. The absorption spectrum of M′ 404 (maximum at 404 nm) consists of distinct vibronic bands at 362, 382, 404, and 420 nm that are different from the vibronic bands of M at 348, 368, 390, and 415 nm. ppR′ 496 has an absorption band that is shifted to shorter wavelengths by 5 nm compared to the initial ppR, whereas ppR′ 504 is redshifted by at least 3 nm. As in bacteriorhodopsin, photoexcitation of the M intermediate of ppR and, presumably, photoisomerization of the chromophore during the M → M′ transition result in a dramatic increase in the proton affinity of the Schiff base, followed by its reprotonation during the M′ → ppR′ transition. Because the latter reaction occurs at very low temperature, the proton is most likely taken from the counterion (Asp 75) rather than from the bulk. The phototransformation of M reveals a certain heterogeneity of the pigment, which probably reflects different populations of M or its photoproduct M′. Photoconversion of the M intermediate provides a possible pathway for photoreception in halobacteria and a useful tool for studying the mechanisms of signal transduction by phoborhodopsin (sensory rhodopsin II).