PHOTOSENSORY TRANSDUCTION IN CILIATES .2. POSSIBLE ROLE OF G-PROTEIN AND CGMP IN STENTOR-COERULEUS

The heterotrichous ciliate, Stentor coeruleus, exhibits a well-defined photophobic response to a sudden increase in the intensity of visible light. The phobic reactions usually appear with a latency period (i.e . a time delay between the onset of the stimulus and the stop response ). This latency of phobic response was significantly in when the cells were incubated with 8-bromo-guanosine 3', 5'-cyclic monophosphate. In the presence of this nucleotide, a reduction of cell responsiveness ( i.e. the number of photophobically responding cells) was also observed . Similar effects were observed when cells were treated with pertussis toxin, a G-protein activity modulator, and 3'-isobutyl-methylxanthine , an inhibitor of guanosine 3',5'-cyclic monophosphate (cGMP) phosphod iesterase. The G-protein activator fluoroaluminate and 6-anilino-5,8-q uinolinedione(LY 83583) (an effective agent for lowering cellular cGMP levels) showed opposite effects on the cell photophobic response. The se results indirectly suggest that the level of cytoplasmic cGMP, poss ibly modulated by a G-protein-coupled cGMP phosphodiesterase, plays a phototransducing role in Stentor. In addition, using an antiserum rais ed against bovine transducin, a cross-reacting protein with an apparen t molecular mass of 39 kDa was detected on immunoblots. The alpha-subu nit of a Stentor G-protein has also been partially cloned and sequence d. However, the possible coupling between the G-protein and the putati ve phosphodiesterase remains to be established.