INITIAL SPECTROSCOPIC CHARACTERIZATION OF THE CILIATE PHOTORECEPTOR STENTORIN

Stentorin serves as the primary photosensor in the single cell ciliate , Stenter coeruleus, for its photophobic and phototactic responses to light of visible wavelengths. We separated two subunits, stentorin-2A and -2B, from the previous stentorin complex ('stentorin-2') of greate r than half a million molecular mass isolated from the photoreceptor o rganelle (pigment granule). Stentorin-2B bears the chromophore covalen tly linked to an approx, 50 kDa apoprotein, as determined by SDS-urea- PAGE. Partial amino acid sequences were obtained from this 50 kDa subu nit. Its visible and CD spectra were found to be similar to those of s tentorin-2, The steady-state and time-resolved fluorescence spectra of stentorin-2B, in H2O and D2O buffers, were also similar to those of s tentorin-2. This suggests that the 50 kDa subunit retains the spectral integrity and primary photoreactivity of the stentorin-complex. The p icosecond time-resolved fluorescence study revealed that the short pic osecond emission component (tau(F) congruent to 8-10 ps) was the predo minant emitting species in stentorin-2B and -2, followed by longer dec aying species, No deuterium solvent effect was seen in this fast-decay ing species. The possible mechanism for the primary photoreaction appe ars to involve electron transfer coupled with proton transfer,