Photosensory transduction in ciliates. Role of intracellular pH and comparison between Stentor coeruleus and Blepharisma japonicum

A knockout mutation of a constitutive GPCR in Tetrahymena decreases both G-protein activity and chemoattraction

Although G-protein coupled receptors (GPCRs) are a common element in many chemosensory transduction pathways in eukaryotic cells, no GPCR or regulated G-protein activity has yet been shown in any ciliate. To study the possible role for a GPCR in the chemoresponses of the ciliate Tetrahymena, we have generated a number of macronuclear gene knockouts of putative GPCRs found in the Tetrahymena Genome database. One of these knockout mutants, called G6, is a complete knockout of a gene that we call GPCR6 (TTHERM_00925490). Based on sequence comparisons, the Gpcr6p protein belongs to the Rhodopsin Family of GPCRs. Notably, Gpcr6p shares highest amino acid sequence homologies to GPCRs from Paramecium and several plants. One of the phenotypes of the G6 mutant is a decreased responsiveness to the depolarizing ions Ba²⁺ and K⁺, suggesting a decrease in basal excitability (decrease in Ca²⁺ channel activity). The other major phenotype of G6 is a loss of chemoattraction to lysophosphatidic acid (LPA) and proteose peptone (PP), two known chemoattractants in Tetrahymena. Using microsomal [³⁵S]GTPγS binding assays, we found that wild-type (CU427) have a prominent basal G-protein activity. This activity is decreased to the same level by pertussis toxin (a G-protein inhibitor), addition of chemoattractants, or the G6 mutant. Since the basal G-protein activity is decreased by the GPCR6 knockout, it is likely that this gene codes for a constitutively active GPCR in Tetrahymena. We propose that chemoattractants like LPA and PP cause attraction in Tetrahymena by decreasing the basal G-protein stimulating activity of Gpcr6p. This leads to decreased excitability in wild-type and longer runs of smooth forward swimming (less interrupted by direction changes) towards the attractant. Therefore, these attractants may work as inverse agonists through the constitutively active Gpcr6p coupled to a pertussis-sensitive G-protein.

Target Analysis of Primary Photoprocesses Involved in the Oxyblepharismin-Binding Protein

Target analysis is performed on previously published transient absorption spectra of the 200-kDa oxyblepharismin-binding protein (OBIP) thought to trigger the photophobic response of the ciliate Blepharisma japonicum. The OBIP sample is considered as heterogeneous and made of two distinct classes of chromophore-protein complexes. A so-called nonreactive class is seen to be comparable to free oxyblepharismin in organic solution. Another, reactive, class is shown to undergo a fast picosecond photocycle involving the formation in 4 ps of an intermediate state noted Y1. The spectrum associated to Y1 bears striking similarities with that of the oxyblepharismin radical cation. This element favors the hypothesis that an excited-state intermolecular electron-transfer could be the primary step of the sensory transduction chain of B. japonicum. Proton release is also considered as a possible secondary step. These possibilities support the idea that reactive OBIP functions like an electron or proton pump. We alternatively propose a new hypothesis stating that the fast photocycle of reactive OBIP actually does not generate any photoproduct or protein change of conformation but is involved in another biological function. It would act as a kind of solar screen, providing additional protection to the light-adapted form of B. japonicum in case of excessive illumination.

Spectroscopic study of the chromophore–protein association and primary photoinduced events in the photoreceptor of Blepharisma japonicum

Blepharisma japonicum is a ciliated protozoan exhibiting a strong step-up photophobic response upon illumination. The photoreceptor chromophores responsible for this response have been identified to be hypericin-like chromophores (blepharismin and oxyblepharismin), complexed to a 200 kDa non-water-soluble protein. The present work opens up new perspectives on the primary phototransduction steps of B. japonicum's light perception through a joined approach by steady-state fluorescence spectroscopy, time-resolved fluorescence anisotropy and sub-picosecond transient absorption spectroscopy. The free chromophore of the light-adapted form of the cell (oxyblepharismin) was studied in various solvents and its spectroscopic properties, as well as its primary excited-state reactivity, compared with those of the corresponding pigment-protein complex, extracted by phosphate-concentration-step chromatography on a hydroxyapatite column. Fluorescence anisotropy together with SDS PAGE electrophoresis results confirm that oxyblepharismin is non-covalently bound to the apoprotein and show that, in the excited state, it is free to rotate in all directions within the binding site where it experiences a large local viscosity. Time-resolved anisotropy measurements on aromatic amino acids confirm that the molecular weight of the protein is of the order of 200 kDa. Although showing very similar steady-state spectra, free oxyblepharismin and its protein complex have noticeably different excited-state behaviours. In particular, the protein complex exhibits a pronounced short-lived absorption feature in the 640--750 nm range, decaying biexponentially in 4 ps and 56 ps. Those decays, also observed in other spectral regions, are not found in the corresponding kinetics of the isolated pigment in solution. This early behaviour of the protein complex might be the signature of the primary phototransduction process, possibly involving an electron transfer from the pigment to a neighbouring protein acceptor residue as it had been suggested in previous studies.

Photoreception and photomovements of microorganisms

Many freely motile microorganisms can perceive and transduce external photic stimuli to the motor apparatus, eventually moving, by means of various behavioural strategies, into environments in which the illumination conditions are the most favourable for their life. In different microorganisms, a wide range of chromophores operate as light detectors, each of them set in a special molecular pocket that, in its turn, can be linked to another component of the transduction chain. The diverse photosensors are organized in special (and in many cases dedicated) photoreceptor units or subcellular organelles. The main molecular mechanisms connecting the early event of photon absorption to the formation of the signalling state down to the dark steps of the transduction chain are discussed in a selected number of case examples. The possible importance of an intensive multidisciplinary approach to these problems in an evolutionary perspective is finally briefly outlined.

Sensory perception and transduction of UV-B radiation by the ciliate Blepharisma japonicum

A key question to answer studying the biological effects of ultraviolet radiation on planktonic micro-organisms is whether they can perceive UV-B radiation as a sensory signal, likewise they do with visible light. We have faced this problem performing an individual-cell analysis of Blepharisma japonicum photomotile responses to UV-B stimuli. Our results on spectral responsiveness and on the effects of a photoresponse inhibitor indicate that B. japonicum is capable to perceive and transduce UV-B radiation as an environmental sensory stimulus, which it escapes from gathering in shadowed areas. Similar UV-B avoidance motile reactions could serve as a behavioural defence mechanism contributing to avoid harmful overexposure to UV-B.

Spectroscopic and photoacoustic studies of hypericin embedded in liposomes as a photoreceptor model

In photoresponsive ciliates, like Blepharisma japonicum and Stentor coeruleus, the photoreceptor pigments responsible for photomotile reactions are hypericin-type chromophores packed in highly osmiophilic subpellicular granules. Lipopsomes loaded with hypericin can constitute a simple model system, appropriate for understanding the primary light-induced molecular events triggering the sensory chain in these microorganisms. Optical absorption, steady-state and time-resolved fluorescence and pulsed photoacoustic calorimetry have been used to measure spectral distributions, fluorescence lifetimes, radiative and radiationless transition quantum yields of hypericin when assembled into egg L-alpha-phosphatidylcholine liposomes. With respect to hypericin ethanol solutions, both absorption and fluorescence maxima are 5 nm red shifted when the pigment is inserted into the lipidic microenvironment, regardless of the hypericin local concentration. Increasing by 100 times the hypericin local concentration decreases the relative fluorescence quantum yield by a factor of around 150 and the fraction of thermally released energy, conversely, increases from 0.6 to 0.9. From the analysis of fluorescence lifetimes and their relative amplitudes it appears that a subnanosecond living component is predominant at the highest hypericin local concentrations.