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Further characterization of the ciliate photoreceptor (stentorin) and the initial photoprocess
Abstract
The visual sensory system of the unicellular ciliate, Stentor coeruleus, serves as a simple model for studies of light signal transduction in living systems. In previous studies, proton transfer in excited state stentorin, a hypericin derivative, was predicted as an initial photoprocess triggering the light signal transduction chain and subsequently generating the photophobic and phototactic responses in Stentor coeruleus (Song et al., 1980a; 1980b & 1990). In the present studies, two fractions, stentorin-2A and stentorin-2B were isolated from the stentorin-2 complex using FPLC/hydrophobic interaction column chromatography. The latter contains the chromophore covalently bound to an approximately 50 kDa chromoprotein. Partial amino acid sequences were obtained from this 50,000 subunit. Its visible CD and absorption spectra are similar to those of stentorin-2. The steady state and picosecond time-resolved fluorescence spectra of stentorin-2B in H$\sb2$O and D$\sb2$O buffers were similar to those of stentorin-2, suggesting that stentorin-2B isolated from the stentorin-2 complex retains the spectral integrity and primary photoreactivity. The short lifetime fluorescence species (8-17 psec) of stentorin-2 and -2B were found to be the predominant emitting species ($>$90%) in all decay analyses and exhibit little or no solvent deuterium effect. At 15 K upon irradiation with light of longer than 520 nm free radical species of stentorin-2B were detected at 15 K. Photoreaction of hypericin in the presence of dithiodiethanol demonstrates that this hydroxyquinone tends to function as an electron donor in its excited state, reducing dithiodiethanol to mercaptoethanol. This result indicates that electron transfer plays a role in the molecular aspects of the stentorin excited state. A non-radiative process is discussed as a photoredox pathway and a possible mechanism for the primary photoreaction appears to involve electron transfer coupled with proton transfer.
Subject Area
Biophysics|Biochemistry
Recommended Citation
Dai, Renke, "Further characterization of the ciliate photoreceptor (stentorin) and the initial photoprocess" (1994). ETD collection for University of Nebraska-Lincoln. AAI9430164.
https://digitalcommons.unl.edu/dissertations/AAI9430164