In vitro identification of rhodopsin in the green alga Chlamydomonas

The unicellular alga Chlamydomonas can detect both intensity and direction of the ambient light and adjust its swimming speed and direction accordingly. On the basis of physiological experiments, the functional photoreceptor for this visual process has recently shown to be a rhodopsin. We here report the in vitro identification of endogenous retinal and a rhodopsin in Chlamydomonas cell extracts and purified membrane preparations. The rhodopsin absorption spectrum has fine structure with the maximum at 495 nm and matches the action spectra for the behavioral light responses. The rhodopsin can be bleached and subsequently reconstituted with exogenous retinal. Labeling with [3H]retinal occurs in the final preparation only with a single protein with a molecular weight of 32,000. We conclude that this protein is the visual photoreceptor in Chlamydomonas.