Multicolored protein conformation states in the photocycle of transducer-free sensory rhodopsin-I

Color-discriminating retinal configurations of sensory rhodopsin I by photo-irradiation solid-state NMR spectroscopy

SRI (sensory rhodopsin I) can discriminate multiple colors for the attractant and repellent phototaxis. Studies aimed at revealing the color-dependent mechanism show that SRI is a challenging system not only in photobiology but also in photochemistry. During the photoreaction of SRI, an M-intermediate (attractant) transforms into a P-intermediate (repellent) by absorbing blue light. Consequently, SRI then cycles back to the G-state. The photoreactions were monitored with the (13)C NMR signals of [20-(13)C]retnal-SrSRI using in situ photo-irradiation solid-state NMR spectroscopy. The M-intermediate was trapped at -40 °C by illumination at 520 nm. It was transformed into the P-intermediate by subsequent illumination at 365 nm. These results reveal that the G-state could be directly transformed to the P-intermediate by illumination at 365 nm. Thus, the stationary trapped M- and P-intermediates are responsible for positive and negative phototaxis, respectively.

Structural water cluster as a possible proton acceptor in the adduct decay reaction of oat phototropin 1 LOV2 domain

LOV domains (Light, Oxygen, Voltage) are the light-sensory modules of phototropins, the blue-light photoreceptor kinases in plants, and of a wide variety of flavoproteins found in all three domains of life. These 12 kDa modules bind a flavin chromophore (FMN or FAD) noncovalently and undergo a photochemical activation in which the sulfur atom of a conserved cysteine forms an adduct to the C(4a) carbon of the flavin. The adduct breaks spontaneously in a base-catalyzed reaction involving a rate-limiting proton-transfer step, regenerating the dark state in seconds. This photocycle involves chromophore and protein structural changes that activate the C-terminal serine/threonine kinase. Previous studies (Biochemistry 2007, 46, 7016-7021) showed that decreased hydration obtained at high glycerol concentrations stabilizes the adduct state in a manner similar to that attained at low temperatures, resulting in much longer adduct decay times. This kinetic effect was attributed to an increased protein rigidity that hindered structural fluctuations necessary for the decay reaction. In this work, we studied the adduct decay kinetics of oat phototropin 1 (phot1) LOV2 at varying hydration using a specially designed chamber that allowed for measurement of UV-visible and FTIR spectra of the same samples. Therefore, we obtained LOV protein concentrations, adduct decay kinetics, and the different populations of bound water by deconvolution of the broad water absorption peak around 3500 cm(-1). A linear dependence of the adduct decay rate constant on the concentration of double and triple hydrogen-bonded waters strongly suggests that the adduct decay is a pseudo-first-order reaction in which both the adduct and the strongly bound waters are reactants. We suggest that a cluster of strongly bound water functions as the proton acceptor in the rate-limiting step of adduct decay.

FT-IR Difference Spectroscopy Elucidates Crucial Interactions of Sensory Rhodopsin I with the Cognate Transducer HtrI

The phototaxis receptor sensory rhodopsin I (SRI) from Halobacterium salinarum interacts with its cognate transducer (HtrI) forming a transmembrane complex. After light excitation of the chromophore all-trans retinal, SRI undergoes structural changes that are ultimately transmitted to HtrI. The interaction of SRI with HtrI results in the closure of the receptor's proton pathway, which renders the photocycle recovery kinetics of SRI pH-independent. We demonstrate on heterologously expressed and reconstituted SRI-HtrI fusion proteins that the transmembrane part of HtrI (residues 1-52) as well as the downstream cytoplasmic part (residues 53-147) exhibit conformational changes after light excitation. The sum of these conformational changes is similar to those observed in the fusion constructs SRI-HtrI 1-71 and SRI-HtrI 1-147, which display pH-independent receptor kinetics. These results indicate the occurrence of spatially distinct conformational changes that are required for functional signal transmission. Kinetic and spectroscopic analysis of HtrI point mutants of Asn53 provides evidence that this residue is involved in the receptor-transducer interaction. We suggest that Asn53 plays a role similar to that of Asn74 of the HtrII from Natronobacterium pharaonis, the latter forming a hydrogen bond to the receptor within the membrane.

Mutation-induced blocker permeability and multiion block of the CFTR chloride channel pore

Chloride permeation through the cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel is blocked by a broad range of anions that bind tightly within the pore. Here we show that the divalent anion Pt(NO₂)₄²⁻ acts as an impermeant voltage-dependent blocker of the CFTR pore when added to the intracellular face of excised membrane patches. Block was of modest affinity (apparent K_d 556 μM), kinetically fast, and weakened by extracellular Cl⁻ ions. A mutation in the pore region that alters anion selectivity, F337A, but not another mutation at the same site that has no effect on selectivity (F337Y), had a complex effect on channel block by intracellular Pt(NO₂)₄²⁻ ions. Relative to wild-type, block of F337A-CFTR was weakened at depolarized voltages but strengthened at hyperpolarized voltages. Current in the presence of Pt(NO₂)₄²⁻ increased at very negative voltages in F337A but not wild-type or F337Y, apparently due to relief of block by permeation of Pt(NO₂)₄²⁻ ions to the extracellular solution. This “punchthrough” was prevented by extracellular Cl⁻ ions, reminiscent of a “lock-in” effect. Relief of block in F337A by Pt(NO₂)₄²⁻ permeation was only observed for blocker concentrations above 300 μM; as a result, block at very negative voltages showed an anomalous concentration dependence, with an increase in blocker concentration causing a significant weakening of block and an increase in Cl⁻ current. We interpret this effect as reflecting concentration-dependent permeability of Pt(NO₂)₄²⁻ in F337A, an apparent manifestation of an anomalous mole fraction effect. We suggest that the F337A mutation allows intracellular Pt(NO₂)₄²⁻ to enter deeply into the CFTR pore where it interacts with multiple binding sites, and that simultaneous binding of multiple Pt(NO₂)₄²⁻ ions within the pore promotes their permeation to the extracellular solution.

Intramolecular proton transfers and structural changes during the photocycle of the LOV2 domain of phototropin 1

The phototropins are a family of membrane-associated flavoproteins that function as the primary blue light receptors regulating phototropism, chloroplast movements, stomatal opening, and leaf expansion in plants. Phot1, a member of this family, contains two FMN-binding domains, LOV1 and LOV2, within the N-terminal region and a C-terminal serine-threonine protein kinase domain. Light irradiation of oat phot1 LOV2 produces a cysteinyl adduct (Cys-39) at the flavin C(4a) position, which decays thermally back to the dark state. We measured pH and isotope effects on the photocycle. Between pH 3.7 and 9.5, adduct formation showed minimal pH dependence, and adduct decay showed only slight pH dependence, indicating that the pK values of mechanistically relevant groups are outside this range. LOV2 showed a nearly 5-fold slowing of adduct formation in D(2)O relative to H(2)O, indicating that the rate-limiting step involves proton transfer(s). Light-induced changes in the far UV CD spectrum of LOV2 revealed putative protein structural perturbations. The light minus dark CD difference spectrum resembles an inverted alpha-helix spectrum, suggesting that alpha-helicity is reversibly lost upon light irradiation. Decay kinetics for CD spectral changes in the far UV region occur at the same rate as those in the visible region, indicating synchronous relaxation of protein and chromophore structures.

Conformational rearrangements in light-harvesting complex II accompanying light-induced chlorophyll a fluorescence quenching

Light-induced chlorophyll a (Chl a) fluorescence quenching was studied in light-harvesting complex of photosystem II (LHCII). Fluorescence intensity decreased by ca. 20% in the course of 20 min illumination (412 nm, 36 micromol m(-2) s(-1)) and was totally reversible within 30 min dark adaptation. The pronounced quenching was observed only in LHCII in an aggregated form and exclusively in the presence of molecular oxygen. Structural rearrangement of LHCII correlated to the quenching was monitored by measuring changes in UV-Visible light absorption spectra, and by measuring Fourier-transform infrared spectroscopy (FTIR) in the Amide I region of the protein (1600-1700 cm(-1)). The light-induced structural rearrangement of LHCII was interpreted as a partial disaggregation of the complex based on the decrease in the light scattering signal and the characteristic features observed in the FTIR spectra: the relative increase in the intensity of the band at 1653 cm(-1), corresponding to a protein in the alpha-helical structure at the expense of the band centered at 1621 cm(-1), characteristic of aggregated forms. The fact that the light-driven isomerization of the all-trans violaxanthin to the 13-cis form was not observed under the non-oxygenic conditions coincided with the lack of large-scale conformational reorganization of LHCII. The kinetics of this large-scale structural effect does not correspond to the light-induced fluorescence quenching, in contrast to the kinetics of structural changes in LHCII observable at low oxygen concentrations. Photo-conversion of 5% of the pool of all-trans violaxanthin to 9-cis isomer was observed under such conditions. Possible involvement of the violaxanthin isomerization in the process of structural rearrangements and excitation quenching in LHCII is discussed.

Demonstration of 2:2 stoichiometry in the functional SRI-HtrI signaling complex in Halobacterium membranes by gene fusion analysis

A fusion protein in which the C-terminus of Halobacterium salinarum sensory rhodopsin I (SRI) is connected by a flexible linker to the N-terminus of its transducer (HtrI) was constructed and expressed in H. salinarum. The fusion protein mediated attractant responses to orange light and repellent responses to UV/violet light that were comparable to those produced by the wild-type SRI-HtrI complex. Immunoblot analysis of H. salinarum membrane proteins demonstrated intact fusion protein and no detectable proteolytic cleavage products. Rapid oxidative cross-linking of a monocysteine mutant in the HtrI domain confirmed that the fusion protein exists as a homodimer in the membrane. HtrI-free SRI and HtrI-complexed SRI have been shown previously to exhibit large differences in the pH dependence of their photocycle kinetics and in the pK(a) of Asp76 that controls a pH-dependent spectral transition in SRI. These differences were used to assess whether only one or both SRI domains in the fusion protein were complexed properly to the HtrI homodimer. Measurement of the photochemical activity, the photocycle kinetics, and the absorption spectra at various pH values established that both SRI domains are complexed to HtrI in the fusion protein, and therefore the stoichiometry is 2:2. Closer examination of the HtrI effect on SRI revealed that Asp76 titration in HtrI-free SRI fits two pK(a) values, with 98% and 2% of the molecules titrating with pK(a)'s of 7 and 9, respectively. The same two pK(a)'s of Asp76 are evident in HtrI-complexed SRI, but with 13% with pK(a) of 7 and 87% with pK(a) of 9 and a similar bias toward the pK(a) of 9 in the fusion protein. Titration of the fusion protein with Ala substitution at Arg73, a residue in the photoactive site, in the SRI domain indicates that a basic residue at Arg73 is necessary for the lower pK(a) to be observed. A model in which Arg73 plays a role in the HtrI effect on SRI is discussed.

The photocycle of a flavin-binding domain of the blue light photoreceptor phototropin

The plant blue light receptor, phot1, a member of the phototropin family, is a plasma membrane-associated flavoprotein that contains two ( approximately 110 amino acids) flavin-binding domains, LOV1 and LOV2, within its N terminus and a typical serine-threonine protein kinase domain at its C terminus. The LOV (light, oxygen, and voltage) domains belong to the PAS domain superfamily of sensor proteins. In response to blue light, phototropins undergo autophosphorylation. E. coli-expressed LOV domains bind riboflavin-5'-monophosphate, are photochemically active, and have major absorption peaks at 360 and 450 nm, with the 450 nm peak having vibronic structure at 425 and 475 nm. These spectral features correspond to the action spectrum for phototropism in higher plants. Blue light excitation of the LOV2 domain generates, in less than 30 ns, a transient approximately 660 nm-absorbing species that spectroscopically resembles a flavin triplet state. This putative triplet state subsequently decays with a 4-micros time constant into a 390 nm-absorbing metastable form. The LOV2 domain (450 nm) recovers spontaneously with half-times of approximately 50 s. It has been shown that the metastable species is likely a flavin-cysteine (Cys(39) thiol) adduct at the flavin C(4a) position. A LOV2C39A mutant generates the early photoproduct but not the adduct. Titrations of LOV2 using chromophore fluorescence as an indicator suggest that Cys(39) exists as a thiolate.