Photosensory transduction in unicellular eukaryote Blepharisma

In the ciliated protozoan Blepharisma, step-up photophobic response is medi ated by a novel type of photosensory complex of pink-colored pigment "bleph arismins" and 200-kDa membrane protein contained in the pigment granules lo cated just beneath the plasma membrane. We found that the fluorescence inte nsity of isolated blepharismins decreased prominently with a decrease of H concentration in the surrounding medium. In the present study, therefore, we utilized the endogenous pigment blepharismins as the pH indicator. Light stimulation evoked a sudden decrease in fluorescence intensity in a photos ensitive anterior portion of the cell, suggesting that a drop in H+ concent ration occurred in the anterior region. The result indicates that the photo signal is transduced into cytoplasmic signaling of H+ translocation across the outer membrane surrounding the pigment granules, so that cytosolic H+ c oncentration in the vicinity of plasma membrane might be increased. J. Exp. Zool. 289:467-471, 2001. (C) 2001 Wiley-Liss, Inc.