Mechanism of Cph1 phytochrome assembly from stopped-flow kinetics and circular dichroism

The kinetics and mechanism of the autocatalytic assembly of holo-Cph1 phytochrome (from Synechocystis) from the apoprotein and the bilin chromophores phycocyanobilin (PCB) and phycoerythrobilin (PEB) were investigated by stopped flow and circular dichroism. At 1:1 stoichiometry, pH 7.9, and 10 degrees C, SVD analysis of the kinetic data for PCB revealed three spectral components involving three transitions with time constants tau(1) approximately 150 ms, tau(2) approximately 2.5 s, and tau(3) approximately 50 s. Tau(1) was associated with a major red shift and transfer of oscillator strength from the Soret region to the 680 nm region. When the sulfhydryl group of cysteine 259 was blocked with iodoacetamide, preventing the formation of a covalent adduct, a noncovalent red-shifted complex (680 nm) was formed with a time constant of 200 ms. Tau(1) could thus be assigned to the formation of a noncovalent complex. The absorption changes during tau(1) are due to the formation of the extended conformation of the linear tetrapyrrole and to its protonation in the binding pocket. From the concentration and pH dependence of the kinetics we obtained a value of 1.5 microM for the K(D) of this noncovalent complex and a value of 8.4 for the pK(a) of the proton donor. The tau(2) component was associated with a blue shift of about 25 nm and was attributed to the formation of the covalent bond (P(r)), accompanied with the loss of the 3-3' double bond to ring A. Tau(3) was due to photoconversion to P(fr). For PEB, which is not photochromic, the formation of the noncovalent complex is faster (tau(1) = 70 ms), but the covalent bond formation is about 80 times slower (tau(2) = 200 s) than with the natural chromophore PCB. The CD spectra of the PCB adduct in the 250-800 nm range show that the chromophore geometries in P(r) and P(fr) are similar to those in plant phytochrome. The opposite rotational strengths of P(r) and P(fr) in the longest wavelength band suggest that the photoisomerization induces a reversal of the chirality. The Cph1 complex with noncovalently bound PCB was still photochromic when cysteine 259 was blocked with IAA or with the bulkier IAF. The covalent linkage to cysteine 259 is thus not required for photoconversion. The CD spectra of the noncovalently bound PCB in P(r)- and P(fr)-like states are qualitatively similar to those of the covalent adducts, suggesting analogous structures in the binding pocket. The noncovalent interactions with the binding pocket are apparently sufficient to hold the chromophore in the appropriate geometry for photoisomerization.     

Light-induced proton release and proton uptake reactions in the cyanobacterial phytochrome Cph1

The P(r) to P(fr) transition of recombinant Synechocystis PCC 6803 phytochrome Cph1 and its N-terminal sensor domain Cph1Delta2 is accompanied by net acidification in unbuffered solution. The extent of this net photoreversible proton release was measured with a conventional pH electrode and increased from less than 0.1 proton released per P(fr) formed at pH 9 to between 0.6 (Cph1) and 1.1 (Cph1Delta2) H(+)/P(fr) at pH 6. The kinetics of the proton release were monitored at pH 7 and pH 8 using flash-induced transient absorption measurements with the pH indicator dye fluorescein. Proton release occurs with time constants of approximately 4 and approximately 20 ms that were also observed in parallel measurements of the photocycle (tau(3) and tau(4)). The number of transiently released protons per P(fr) formed is about one. This H(+) release phase is followed by a proton uptake phase of a smaller amplitude that has a time constant of approximately 270 ms (tau(5)) and is synchronous with the formation of P(fr). The acidification observed in the P(r) to P(fr) transition with pH electrodes is the net effect of these two sequential protonation changes. Flash-induced transient absorption measurements were carried out with Cph1 and Cph1Delta2 at pH 7 and pH 8. Global analysis indicated the presence of five kinetic components (tau(1)-tau(5): 5 and 300 micros and 3, 30, and 300 ms). Whereas the time constants were approximately pH independent, the corresponding amplitude spectra (B(1), B(3), and B(5)) showed significant pH dependence. Measurements of the P(r)/P(fr) photoequilibrium indicated that it is pH independent in the range of 6.5-9.0. Analysis of the pH dependence of the absorption spectra from 6.5 to 9.0 suggested that the phycocyanobilin chromophore deprotonates at alkaline pH in both P(r) and P(fr) with an approximate pK(a) of 9.5. The protonation state of the chromophore at neutral pH is therefore the same in both P(r) and P(fr). The light-induced deprotonation and reprotonation of Cph1 at neutral pH are thus due to pK(a) changes in the protein moiety, which are linked to conformational transitions occurring around 4 and 270 ms after photoexcitation. These transient structural changes may be relevant for signal transduction by this cyanobacterial phytochrome.     

Mechanism of fluorescence concentration quenching of carboxyfluorescein in liposomes: energy transfer to nonfluorescent dimers

When 5(6)-carboxyfluorescein (6CF) is encapsulated in liposomes at 0.2 M, 97-98% of the fluorescence is quenched. We have studied the mechanism of this effect. The dye-liposome system is a special case of concentration quenching of dyes, a phenomenon recognized for 100 years. Absorption spectra of encapsulated dye show that 6CF dimerizes, and the dimer is nonfluorescent. The dimerization constant was estimated, and it was concluded that dimerization can account for only part of the quenching. In 6CF solutions, the fluorescence lifetime decreased drastically as concentration was changed over the narrow range 0.02-0.05 M, a finding which was attributed to energy transfer to dimers. Inhibition of dimerization by propylene glycol also inhibited the shortening of lifetime. F?rster critical transfer distances were calculated to be 51 and 57 A for monomer-monomer and monomer-dimer transfer, respectively. Monomer-monomer transfer was demonstrated directly by steady-state or time-resolved anisotropy experiments, while transfer to dimer was modeled by using sulforhodamine B, which has a critical transfer distance like that for the dimer and also quenches 6CF emission. No direct evidence for collisional self-quenching of 6CF could be found, although a model compound, salicylate, did quench weakly. For xanthene dyes, the rate of energy transfer is much faster than that for quenching collisions, implying that collisional quenching in the usual 6CF-liposome system is insignificant. The reason why 6CF is not 100% quenched in liposomes is attributed to dye interaction with lipid as evidenced by (i) multiexponential decay of 6CF in liposomes with a long component of 3-4 ns, (ii) inhibition of dimerization in liposomes, (iii) partial protection of dye from quenching by KI, (iv) differing amounts of dimerization in liposomes made from different kinds of phospholipid, and (v) enhancement of fluorescence lifetime in the presence of Triton X-100.     

Two independent, light-sensing two-component systems in a filamentous cyanobacterium.

Two ORFs, cphA and cphB, encoding proteins CphA and CphB with strong similarities to plant phytochromes and to the cyanobacterial phytochrome Cph1 of Synechocystis sp. PCC 6803 have been identified in the filamentous cyanobacterium Calothrix sp. PCC7601. While CphA carries a cysteine within a highly conserved amino-acid sequence motif, to which the chromophore phytochromobilin is covalently bound in plant phytochromes, in CphB this position is changed into a leucine. Both ORFs are followed by rcpA and rcpB genes encoding response regulator proteins similar to those known from the bacterial two-component signal transduction. In Calothrix, all four genes are expressed under white light irradiation conditions, albeit in low amounts. For heterologous expression and convenient purification, the cloned genes were furnished with His-tag encoding sequences at their 3' end and expressed in Escherichia coli. The two recombinant apoproteins CphA and CphB bound the chromophore phycocyanobilin (PCB) in a covalent and a noncovalent manner, respectively, and underwent photochromic absorption changes reminiscent of the P(r) and P(fr) forms (red and far-red absorbing forms, respectively) of the plant phytochromes and Cph1. A red shift in the absorption maxima of the CphB/PCB complex (lambda(max) = 685 and 735 nm for P(r) and P(fr), respectively) is indicative for a noncovalent incorporation of the chromophore (lambda(max) of P(r), P(fr) of CphA: 663, 700 nm). A CphB mutant generated at the chromophore-binding position (Leu246-->Cys) bound the chromophore covalently and showed absorption spectra very similar to its paralog CphA, indicating the noncovalent binding to be the only cause for the unexpected absorption properties of CphB. The kinetics of the light-induced P(fr) formation of the CphA-PCB chromoprotein, though similar to that of its ortholog from Synechocystis, showed differences in the kinetics of the P(fr) formation. The kinetics were not influenced by ATP (probing for autophosphorylation) or by the response regulator. In contrast, the light-induced kinetics of the CphB-PCB complex was markedly different, clearly due to the noncovalently bound chromophore.     

Phytochrome Cph1 from the cyanobacterium Synechocystis PCC6803. Purification, assembly, and quaternary structure.

The phytochrome Cph1 from the cyanobacterium Synechocystis PCC6803 forms holoprotein adducts with close spectral similarity to plant phytochromes when autoassembled in vitro with bilin chromophores. Cph1 is a 85-kDa protein that acts as a light-regulated histidine kinase seemingly involved in 'two-component' signalling. This paper describes the improvement of Cph1 purification, estimation of the extinction coefficient of holo-Cph1, spectral analyses of the assembly procedure and studies on quaternary structure. During assembly with the natural chromophore phycocyanobilin (PCB), a red-shifted intermediate is observed. A similar result was obtained when phycoerythrobilin was used as chromophore. As shown by SDS/PAGE and Zn2+ fluorescence, the covalent attachment of PCB is blocked by 1 mM iodoacetamide, a cysteine-derivatizing agent. When PCB was incubated with blocked apo-Cph1, again a shoulder at longer wavelengths appeared. It is therefore proposed that the long-wavelength-absorbing form represents the protonated, noncovalently bound bilin. Biliverdin, which is neither protonated nor covalently attached, undergoes spectral changes in its blue-absorbing band upon incubation with apo-Cph1. On the basis of these data we therefore propose a three-step model for phytochrome autoassembly. Size-exclusion chromatography revealed different mobilities for the apoprotein, red-absorbing Cph1-PCB and far-red-absorbing Cph1-PCB. The major peaks of both holoprotein adducts had apparent molecular masses approximately 200 kDa, a result in agreement with the notion that autophosphorylation in sensory histidine kinases requires dimerization. When Cph1-PCB was further purified by preparative native electrophoresis, the mobility on size-exclusion chromatography was approximately 100 kDa, and it was found to have lost its kinase activity, results implying that the material had lost its capacity to dimerize.     

Characterization of fluorescence quenching in bifluorophoric protease substrates

NorFES is a relatively rigid, bent undecapeptide which contains an amino acid sequence that is recognized by the serine protease elastase (AspAlaIleProNle downward arrow SerIleProLysGlyTyr ( downward arrow indicates the primary cleavage site)). Covalent attachment of a fluorophore on each side of NorFES's elastase cleavage site enables one to use a change of fluorescence intensity as a measure of enzymatic activity. In this study two bichromophoric NorFES derivatives, D-NorFES-A and D-NorFES-D, were prepared in which D (donor) was tetramethylrhodamine and A (acceptor) was rhodamine-X, two chromophores with characteristics suitable for energy transfer. Absorption and fluorescence spectra were obtained with both the intact and cleaved homodoubly, heterodoubly and singly labeled derivatives. It was found that both the homo and hetero doubly-labeled derivatives form ground-state complexes which exhibit exciton bands. The hetero labeled derivative exhibits little or no resonance energy transfer. Spectral measurements were also done in urea, which partially disrupts ground-state dimers.     

Constitutive and agonist-induced dimerizations of the P2Y1 receptor: relationship to internalization and scaffolding

In living cells, P2Y(1) receptor dimerization was quantitated by an improved version of fluorescence resonance energy transfer donor photobleaching analysis. 44% of the P2Y(1) receptors expressed in HEK293 cell membranes exist as dimers in the resting state, inducible by agonist exposure to give 85-100% dimerization. Monomer and constitutive dimers are fully active. Agonist-induced dimerization follows desensitization and is fully reversible upon withdrawal of agonist. Receptor dimers are required for internalization at 37 degrees C but are not sufficient; at 20 degrees C dimerization also occurs, but endocytosis is abolished. Removal of the C-terminal 19 amino acids abolished both dimerization and internalization, whereas full activation by agonists was retained up to a loss of 39 amino acids, confirming active monomers. This receptor is known to bind through its last four amino acids (DTSL) to a scaffolding protein, Na/H exchanger regulatory factor-2, which was endogenous here, and DTSL removal blocked constitutive dimerization specifically. Distinction should therefore be made between the following: 1) constitutive dimers tethered to a scaffolding protein, together with effector proteins, within a signaling micro-domain, and 2) free dimers in the cell membrane, which here are inducible by agonist exposure. For the class A G-protein-coupled receptors, we suggest that the percentages of free monomers, and in many cases of induced free dimers, commonly become artifactually increased; this would arise from an excess there of the receptor over its specific scaffold and from a lack of the native targeting of the receptor to that site.     

Spectroscopic studies on human serum albumin and methemalbumin: optical, steady-state, and picosecond time-resolved fluorescence studies, and kinetics of substrate oxidation by methemalbumin

The nature of the heme environment in methemalbumin, the Fe(III) protoporphyrin IX (heme)-human serum albumin (HSA) complex, was investigated by optical spectroscopy. Comparison of the optical spectra of methemalbumin, ferro-hemalbumin in the absence and presence of 2-methylimidazole, and their carbon monoxide derivatives with horseradish peroxidase (HRP) and its corresponding derivatives indicates that histidine is not present in the first coordination sphere of heme in methemalbumin and that the protein is devoid of a well-defined heme cavity. The complex exhibits peroxidase activity by catalyzing oxidation of 2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) by hydrogen peroxide. Its activity ( K(M)=433 microM, molar catalytic activity=0.33 s(-1)), however, is considerably lower compared to HRP, indicating differences in the heme environments. Fluorescence intensity decays of Trp214 in HSA and methemalbumin, best fitted to a three-exponential model, gave the lifetimes 7.03 ns (30%), 3.17 ns (38%), and 0.68 ns (32%) for HSA and 8.04 ns (1.7%), 2.42 ns (19.7%), and 0.64 ns (78.6%) for methemalbumin. These lifetime values were further confirmed by a model-independent maximum entropy method. Similarity in the lifetimes and variations in the amplitudes suggest that while conformational heterogeneity of HSA is unperturbed on heme binding, redistribution of the populations of the three conformations occurs and the sub-state associated with the shortest lifetime dominates the total population by approximately 80%. Decay associated spectra (DAS) indicate that the observed lifetime variation with wavelength is predominantly due to ground state heterogeneity, though solvent dipolar relaxation also contributes. Time-resolved fluorescence anisotropy measurements of the Trp214 residue yielded information on motion within the protein together with the whole protein molecule. The binding of heme did not affect the rotational correlation time of the albumin molecule (approximately 20 ns). However, the motion of tryptophan within the protein matrix increased by a factor of approximately 3 (0.46 ns to 0.15 ns). This indicates that while the overall hydrodynamic volume of the albumin molecule remained the same, tryptophan underwent a more rapid internal rotation because of the efficient energy transfer to the bound heme. Optical studies, analysis of lifetime measurements, DAS, and anisotropy measurements together suggest that heme binds to a surface residue. The rapid internal motion of Trp214 during its excited state lifetime for the approximately 80% populated conformer of methemalbumin allows the orientation factor, kappa(2), to approach the average value of 2/3. From the time-resolved fluorescence measurements and the energy transfer calculations on methemalbumin, a Trp214-heme distance of 22 A was deduced.     

Electronic excitation transfer in the complex of lumazine protein with bacterial bioluminescence intermediates

Fluorescence dynamics measurements have been made on the bioluminescence reaction intermediates using Photobacterium leiognathi, Vibrio fischeri, and Vibrio harveyi luciferases, both alone and in mixtures with Photobacterium phosphoreum lumazine protein. Each luciferase produces a "fluorescent transient" intermediate on reaction with the bioluminescence substrates, FMNH2, tetradecanal, and O2, and all have a fluorescence quantum yield about 0.3, with a predominant lifetime around 10 ns. The P. leiognathi luciferase fluorescent transient has a rotational correlation time of 79 ns at 2 degrees C, as expected for the rotational diffusion of a 77-kDa macromolecule. In the presence of lumazine protein however a faster correlation time of about 3 ns predominates. This rapid channel of anisotropy loss is attributed to energy transfer from the flavin intermediate bound on the luciferase to the lumazine ligand, reflects the presence of protein-protein complexation, and is greatest in the case of P. leiognathi, but not at all for V. fischeri. This fact is consistent with the strong influence of lumazine protein on the bioluminescence reaction of P. leiognathi, and not at all with V. fischeri. The rate of energy transfer is of order 10(9) s-1, much greater than the 10(8) s-1 fluorescence rate of the donor. Thus the bioluminescence excitation of lumazine protein could occur by a similar photophysical mechanism of interprotein energy transfer from a chemically excited fluorescent transient donor to the lumazine acceptor.     

Low-temperature energy transfer in LHC-II trimers from the Chl a/b light-harvesting antenna of photosystem II.

Temperature dependence in electronic energy transfer steps within light-harvesting antenna trimers from photosystem II was investigated by studying Chl a pump-probe anisotropy decays at several wavelengths from 675 to 682 nm. The anisotropy lifetime is markedly sensitive to temperature at the longest wavelengths (680-682 nm), increasing by factors of 5 to 6 as the trimers are cooled from room temperature to 13 K. The temperature dependence is muted at 677 and 675 nm. This behavior is modeled using simulations of temperature-broadened Chl a absorption and fluorescence spectra in spectral overlap calculations of Förster energy transfer rates. In this model, the 680 nm anisotropy decays are dominated by uphill energy transfers from 680 nm Chl a pigments at the red edge of the LHC-II spectrum; the 675 nm anisotropy decays reflect a statistical average of uphill and downhill energy transfers from 676-nm pigments. The measured temperature dependence is consistent with essentially uncorrelated inhomogeneous broadening of donor and acceptor Chl a pigments.     

Automated fluorescence lifetime imaging plate reader and its application to Förster resonant energy transfer readout of Gag protein aggregation

Fluorescence lifetime measurements can provide quantitative readouts of local fluorophore environment and can be applied to biomolecular interactions via Förster resonant energy transfer (FRET). Fluorescence lifetime imaging (FLIM) can therefore provide a high content analysis (HCA) modality to map protein‐protein interactions (PPIs) with applications in drug discovery, systems biology and basic research. We present here an automated multiwell plate reader able to perform rapid unsupervised optically sectioned FLIM of fixed and live biological samples and illustrate its potential to assay PPIs through application to Gag protein aggregation during the HIV life cycle. We demonstrate both hetero‐FRET and homo‐FRET readouts of protein aggregation and report the first quantitative evaluation of a FLIM HCA assay by generating dose response curves through addition of an inhibitor of Gag myristoylation. Z ′ factors exceeding 0.6 are realised for this FLIM FRET assay. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

R-phycocyanin II, a new phycocyanin occurring in marine Synechococcus species. Identification of the terminal energy acceptor bilin in phycocyanins

A new member of the phycocyanin family of phycobiliproteins, R-phycocyanin II (R-PC II) has been discovered in several strains of marine Synechococcus sp. R-PC II has absorption maxima at 533 and 554 nm, a subsidiary maximum at 615 nm, and a fluorescence emission maximum at 646 nm. It is the first phycoerythrobilin (PEB)-containing phycocyanin of cyanobacterial origin. The purified protein is made up of alpha and beta subunits in equal amounts and is in an (alpha beta)2 aggregation state. The alpha and beta subunits of this protein are homologous to the corresponding subunits of previously described C- and R-phycocyanins as assessed by amino-terminal sequence determination and analyses of sequences about sites of bilin attachment. R-PC II carries phycocyanobilin (PCB) at beta-84 and PEB at alpha-84 and beta-155 (residue numbering is that for C-phycocyanin), whereas in C-phycocyanin PCB is present at all three positions. In R-phycocyanin, the bilin distribution is alpha-84 (PCB), beta-84 (PCB), beta-155 (PEB). In both R-phycocyanin and R-phycocyanin II excitation at 550 nm, absorbed primarily by PEB groups, leads to emission at 625 nm from PCB. These comparative data support the conclusion that the invariant beta-84 PCB serves as the terminal energy acceptor in phycocyanins.     

二聚化:受体酪氨酸激酶活化的重要机制

受体酪氨酸激酶家族是一类具有内源性蛋白酪氨酸激酶活性的生长因子受体。它们具有相似的分子结构 ,其配体介导的受体活化主要是通过二聚化的机制来实现的。配体介导同源或异源的受体二聚化 ,不同的配体以不同的机制介导受体的二聚化。本文介绍了受体酪氨酸激酶家族不同亚类受体在其配体介导下二聚化的机制 ,并着重介绍了表皮生长因子受体家族各成员间的异二聚化及其引起的胞内信号转导途径的多样化     

Femtosecond energy transfer and spectral equilibration in bacteriochlorophyll a--protein antenna trimers from the green bacterium Chlorobium tepidum.

Femtosecond energy transfer processes in a bacteriochlorophyll a-protein antenna complex from the green sulfur bacterium Chlorobium tepidum have been studied by one-color, two-color, and broadband absorption difference spectroscopy. Much of the spectral excitation equilibration in this antenna occurs with 350 to 450 fs kinetics. The anisotropy decay functions r(t) exhibit two major lifetime components, 100 to 130 fs and 1.7 to 2.0 ps. The short component lifetimes may represent single-step energy transfer kinetics in this antenna; the long component is similar to the anisotropy decay observed in earlier picosecond pump-probe experiments.     

Effects of i-propanol on the structural dynamics of Thermomyces lanuginosa lipase revealed by tryptophan fluorescence

Influence of isopropanol (iPrOH) on the structural dynamics of Thermomyces lanuginosa lipase (TLL) was studied by steady-state, time-resolved, and stopped-flow fluorescence spectroscopy, monitoring the intrinsic emission of Trp residues. The fluorescence of the four Trps of the wild-type enzyme report on the global changes of the whole lipase molecule. To monitor the conformational changes in the so-called "lid," an alpha-helical surface loop, the single Trp mutant W89m (W117F, W221H, W260H) was employed. Circular dichroism (CD) spectra revealed that iPrOH does not cause major alterations in the secondary structures of the wild-type TLL and W89m. With increasing [iPrOH], judged by the ratio of emission intensities at 350 nm and 330 nm, the average microenvironment of the Trps in the wild-type TLL became more hydrophobic, whereas Trp89 of W89m moved into a more hydrophilic microenvironment. Time-resolved fluorescence measurements revealed no major changes to be induced by iPrOH neither in the shorter fluorescence lifetime component (tau(1) = 0.5--1.2 ns) for the wild-type TLL nor in the longer fluorescence lifetime component (tau(2) = 4.8--6.0 ns) in the wild-type TLL and the W89m mutant. Instead, for W89m on increasing iPrOH from 25% to 50% the value for tau(1) increased significantly, from 0.43 to 1.5 ns. The shorter correlation time phi(1) of W89m had a minimum of 0.08 ns in 25% iPrOH. Judged from the residual anisotropy r(infinity) the amplitude of the local motion of Trp89 increased upon increasing [iPrOH] 10%. Stopped-flow fluorescence spectroscopy measurements suggested the lid to open within approximately 2 ms upon transfer of W89m into 25% iPrOH. Steady-state anisotropies and longer correlation times revealed increasing concentrations of iPrOH to result also in the formation of dimers as well as possibly also higher oligomers by TLL.     

Role of the second intradiscal loop of peripherin/rds in homo and hetero associations

Peripherin/rds (P/rds) is a disk rim protein that assembles into homo and hetero complexes with its nonglycosylated homologue, Rom-1, to maintain the integrity of the photoreceptor outer segment. Mutations in the rds gene have been identified in a variety of human retinal degenerative diseases. More than 70% of these mutations are located in the second intradiscal (D2) loop, highlighting the functional importance of this region. This study examines the involvement of different regions of the D2 loop in protein associations using a GST pull-down assay and a heterologous coexpression system. The pull-down assay suggests an association of the N-terminal portion (Phe(120)-Phe(187)) of the D2 loop with Rom-1 as well as with other P/rds molecules. Through peptide competition experiments, the region between Cys(165) and Asn(182) of the D2 loop has been identified as the domain for these associations. In a COS-1 cell heterologous expression system, coexpression of the D2 loop along with the intact P/rds and Rom-1 hindered the association of the two full-length proteins. In contrast to the homo association of P/rds molecules, it seems that the hetero association of P/rds with Rom-1 has a more stringent structural requirement. This work defines the crucial domain of the D2 loop, which mediates homo and hetero associations, specifically the regions that lay between Cys(165) and Asn(182). Elucidation of the molecular mechanisms behind the protein-protein associations of P/rds and its partners may reveal the pathogenic defects arising from the most common mutations in this gene.     

多管藻中R-藻蓝蛋白能量传递途径及动力学

以蛋白亚基复性技术和皮秒级时间分辨荧光光谱,研究海洋红藻多管藻中R－藻蓝蛋白（R－PC）单体和三聚体内能量传递过程。利用亚基复性技术对分离后的β亚基复性,以R－藻蓝蛋白单体和β亚基之间的差谱获得α亚基的吸收光谱。皮秒级时间分辨三维谱图（时间、波长和强度）直观地显示出藻红胆素发色团向藻蓝胆素发色团的能量传递;根据时间分辨测量结果的组份解析,对R藻蓝蛋白单体和三聚体内能量传递途径和相关传递参数进行了指认和讨论;对观察到的单体与三聚体能量传递组份特性的差别提出了解释。与C－藻蓝蛋白光谱对比,R－藻蓝蛋白独特的色团组成使其更有效地捕获与传递光能。     

Conformational transitions in the calcium adenosinetriphosphatase studied by time-resolved fluorescence resonance energy transfer

We have used time-resolved fluorescence to study proposed conformational transitions in the Ca-ATPase in skeletal sarcoplasmic reticulum (SR). Resonance energy transfer was used to measure distances between the binding sites of 5-[[2-[(iodoacetyl)amino]ethyl]amino]naphthalene-1-sulfonic acid (IAEDANS) and fluorescein 5-isothiocyanate (FITC) as a function of conditions proposed to affect the enzyme's conformation. When 1.0 +/- 0.15 IAEDANS is bound per Ca-ATPase, most (76 +/- 4%) of the probes have an excited-state lifetime (tau) of 18.6 +/- 0.5 ns, and the remainder have a lifetime of 2.5 +/- 0.9 ns. When FITC is bound to a specific site on each IAEDANS-labeled enzyme, most of the long-lifetime component is quenched into two short-lifetime components, indicating energy transfer that corresponds to two donor-acceptor distances. About one-third of the quenched population has a lifetime tau = 11.1 +/- 2.5 ns, corresponding to a transfer efficiency E = 0.40 +/- 0.07 and a donor-acceptor distance R1 = 52 +/- 3 A. The remaining two-thirds exhibit lifetimes in the range of 1.2-4.2 ns, corresponding to a second distance 31 A less than or equal to R2 less than or equal to 40 A. Addition of Ca2+ (in the micromolar to millimolar range), or vanadate (to produce a phosphoenzyme analogue), had no effect on the donor-acceptor distances. Addition of decavanadate results in the quenching of IAEDANS fluorescence but has no effect on the energy-transfer distance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lipid domain structure correlated with membrane protein function in placental microvillus vesicles

Membrane fluidity properties of placental microvillus membrane vesicles (MVV) were determined from fluorescence anisotropy (r), dynamic depolarization, and lifetime heterogeneity studies of diphenylhexatriene (DPH), trimethylamino-DPH (TMA-DPH), and cis- and trans-parinaric acids (c-PnA and t-PnA). Plots of r against temperature for DPH and TMA-DPH in MVV had slope discontinuities at 26 degrees C (Tc, transition temperature); however, analysis of r in terms of probe rotational rate (R), limiting anisotropy (r infinity), and lifetime (tau) revealed that DPH reported a phase transition because of changes in r infinity, whereas the phase transition observed by TMA-DPH occurred primarily because of changes in R. Heterogeneity analysis using phase and modulation lifetimes at three frequencies showed that DPH and TMA-DPH lifetimes were homogeneous in MVV. Both long (greater than 25 ns) and short (less than 6 ns) lifetime components were detected for c-PnA and t-PnA in MVV, corresponding to the probes in solid and fluid lipid phases. The fractional amplitude of the long lifetimes (solid phase) decreased from 0.86 to 0.12 with increasing temperature (5-55 degrees C) as the membrane passed through the phase transition, with 50% of the change occurring at 27 degrees C (c-PnA) or 33 degrees C (t-PnA). The activation energies for alkaline phosphatase, aminopeptidase M, and sodium-proton antiporter activities all showed discontinuities in the temperature range 27-31 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Monitoring of ligand-independent dimerization and ligand-induced conformational changes of melatonin receptors in living cells by bioluminescence resonance energy transfer

Several G protein-coupled receptors have been shown to exist as homo-and hetero-oligomeric complexes in living cells. However, the link between ligand-induced receptor activation and its oligomerization state as well as the proportion of the total receptor population that can engage in oligomeric complexes remain open questions. Here, the closely related human MT1 and MT2 melatonin receptors (MT1R, MT2R) were used to address these issues. Bioluminescence resonance energy transfer (BRET) experiments in living HEK 293 cells revealed that these receptors form homo- and hetero-oligomers. Constitutive energy transfer was observed for all receptor combinations at physiological expression levels and could be detected in single cell BRET experiments. Inhibition of the energy transfer by dilution of the BRET partners identified MT1R and MT2R dimers as the predominant receptor species, and this oligomerization state did not change upon agonist and antagonist binding. Agonists, neutral antagonists, and inverse agonists all promoted increases in BRET values for MT2R but not for MT1R homodimers in living cells and isolated plasma membranes. This indicates that no correlation could be inferred between the receptor activation state and the dimerization state of the receptor. This also suggests that ligand-promoted BRET increases represent specific ligand-induced conformational changes of pre-existing dimers rather then increased dimerization. The observation that ligands favored the energy transfer within the hetero-oligomer from MT1R to MT2R but not in the reverse orientation, from MT2R to MT1R, supports this view.