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.