Localization of Blepharismin Photosensors and Identification of a Photoreceptor Complex Mediating the Step-up Photophobic Response of the Unicellular Organism, Blepharisma

— Photosensitivity for the step-up photophobic response of Blepharisma is localized in the anterior 1/5 of the cell body. Blepharismin pigment, which is believed to be a photoreceptor pigment mediating the step-up photophobic response of the cells, was separated into five types of blepharismin (BL-1, -2, -3, -4 and -5). Blepharismin-1, -3, -4 and -5 were localized in the posterior 4/5, while BL-2 was located over the entire cell body; the anterior end, which is the photosensitive region, contained only BL-2. The results indicate that a functional photoreceptor pigment mediating the step-up photophobic response is BL-2. Hydroxylapatite chromatography revealed that BL-2 was bound to a 200 kDa membrane protein. We concluded that a photoreceptor mediating the step-up photophobic response was a BL-2/200 kDa protein complex.