Photoreceptor current and photoorientation in Chlamydomonas mediated by 9-demethylchlamyrhodopsin

Green flagellates possess rhodopsin-like photoreceptors involved in control of their behavior via generation of photocurrents across the plasma membra ne. Chlamydomonas mutants blocked in retinal biosynthesis are "blind," but they can be rescued by the addition of exogenous. retinoids. Photosignaling by chlamyrhodopsin regenerated with 9-demethylretinal was investigated by recording photocurrents from single cells and cell suspensions, and by meas uring phototactic orientation. The addition of a saturating concentration o f this analog led to reconstitution of all receptor molecules. However, sen sitivity of the photoreceptor current in cells reconstituted with the analo g was smaller compared with retinal-reconstituted cells, indicating a decre ased signaling efficiency of the analog receptor protein. Suppression of th e photoreceptor current in double-flash experiments was smaller and its rec overy faster with 9-demethylretinal than with retinal, as it would be expec ted from a decreased PC amplitude in the analog-reconstituted cells. Cells reconstituted with either retinal or the analog displayed negative phototax is at low light and switched to positive one upon an increase in stimulus i ntensity, as opposed to the wild type. The reversal of the phototaxis direc tion in analog-reconstituted cells was shifted to a higher fluence rate com pared with cells reconstituted with retinal, which corresponded to the decr eased signaling efficiency of 9-demethylchlamyrhodopsin.