A RHODOPSIN-LIKE MOLECULE ON THE PLASMA-MEMBRANE OF FABREA-SALINA

Immunofluorescence was used to reveal the presence of a rhodopsin-like molecule on the plasma membrane of the photoresponsive ciliate Fabrea salina. The ciliate, Bred on a glass support, showed a spontaneous re d-orange fluorescence. Cells incubated with a fluoresceinated, anti-bo vine opsin, sheep polyclonal antiserum showed instead a green fluoresc ence typical of fluorochrome excited at 436 nm, whereas cells incubate d with a fluoresceinated, nonimmune sheep serum kept their original re d fluorescence. Excess bovine opsin did not inhibit the antibody react ion, but detergent-coated purified bovine opsin was proven able to bin d to the cell membrane. Since rhodopsin belongs to a gene-related prot ein family, which includes adrenergic or cholinergic receptors, the ce ll response to acute exposure to muscarinic and adrenergic agonist and antagonist drugs was investigated. Acetylcholine, atropine, hexametho nium, noradrenaline, or phenoxybenzamine in the culture medium (10(-4) M) did not influence motion behaviour nor phototaxis of Fabrea salina . These Endings raise the possibility that a rhodopsin-like molecule i s present on the surface of Fabrea salina. Nevertheless, they do not p rove that this surface antigen is a rhodopsin, or that this molecule i s responsible for photoresponsiveness in this organism.