Humanin Nanoparticles for Reducing Pathological Factors Characteristic of Age-Related Macular Degeneration.

BACKGROUND: Humanin is a novel neuronal peptide that has displayed potential in the treatment of Alzheimer's Disease through the suppression of inflammatory IL-6 cytokine receptors. Such receptors are found throughout the body, including the eye, suggesting its other potential applications. Age-related Macular Degeneration (AMD) is the leading cause of blindness in the developing world. There is no cure for this disease, and current treatments have several negative side effects associated with them, making finding other treatment options desirable. OBJECTIVE: In this study, the potential applications in treating AMD for a more potent humanin derivative, AGA-HNG, were studied. METHODS: AGA-HNG was synthesized and encapsulated in chitosan Nanoparticles (NPs), which were then characterized for their size, Encapsulation Efficiency (EE), and drug release. Their ability to suppress VEGF secretion and protect against oxidative apoptosis was studied in vitro using ARPE-19 cells. The chitosan NPs exhibited similar anti-VEGF properties and oxidative protection as the free protein while exhibiting superior pharmaceutical characteristics including biocompatibility and drug release. RESULTS: Drug-loaded NPs exhibited a radius of 346nm with desirable pharmacokinetic properties including a stable surface charge (19.5 ± 3.7 mV) and steady drug release capacity. AGA-HNG showed great promise in mediating apoptosis in hypoxic cells. They were also able to significantly reduce VEGF expression in vitro with reduced cellular toxicity compared to the free drug. CONCLUSION: The ability of this drug delivery system to reduce retinal apoptosis with desirable pharmacokinetic and biocompatible properties makes this a promising therapeutic option for AMD.

A structural view of the dissociation of Escherichia coli tryptophanase.

Tryptophanase (Trpase) is a pyridoxal 5'-phosphate (PLP)-dependent homotetrameric enzyme which catalyzes the degradation of L-tryptophan. Trpase is also known for its cold lability, which is a reversible loss of activity at low temperature (2°C) that is associated with the dissociation of the tetramer. Escherichia coli Trpase dissociates into dimers, while Proteus vulgaris Trpase dissociates into monomers. As such, this enzyme is an appropriate model to study the protein-protein interactions and quaternary structure of proteins. The aim of the present study was to understand the differences in the mode of dissociation between the E. coli and P. vulgaris Trpases. In particular, the effect of mutations along the molecular axes of homotetrameric Trpase on its dissociation was studied. To answer this question, two groups of mutants of the E. coli enzyme were created to resemble the amino-acid sequence of P. vulgaris Trpase. In one group, residues 15 and 59 that are located along the molecular axis R (also termed the noncatalytic axis) were mutated. The second group included a mutation at position 298, located along the molecular axis Q (also termed the catalytic axis). Replacing amino-acid residues along the R axis resulted in dissociation of the tetramers into monomers, similar to the P. vulgaris Trpase, while replacing amino-acid residues along the Q axis resulted in dissociation into dimers only. The crystal structure of the V59M mutant of E. coli Trpase was also determined in its apo form and was found to be similar to that of the wild type. This study suggests that in E. coli Trpase hydrophobic interactions along the R axis hold the two monomers together more strongly, preventing the dissociation of the dimers into monomers. Mutation of position 298 along the Q axis to a charged residue resulted in tetramers that are less susceptible to dissociation. Thus, the results indicate that dissociation of E. coli Trpase into dimers occurs along the molecular Q axis.

N-terminal-mediated oligomerization of DnaA drives the occupancy-dependent rejuvenation of the protein on the membrane.

DnaA, the initiator of chromosome replication in most known eubacteria species, is activated once per cell division cycle. Its overall activity cycle is driven by ATP hydrolysis and ADP-ATP exchange. The latter can be promoted by binding to specific sequences on the chromosome and/or to acidic phospholipids in the membrane. We have previously shown that the transition into an active form (rejuvenation) is strongly co-operative with respect to DnaA membrane occupancy. Only at low membrane occupancy is DnaA reactivation efficiently catalysed by the acidic phospholipids. The present study was aimed at unravelling the molecular mechanism underlying the occupancy-dependent DnaA rejuvenation. We found that truncation of the DnaA N-terminal completely abolishes the co-operative transformation between the high and low occupancy states (I and II respectively) without affecting the membrane binding. The environmentally sensitive fluorophore specifically attached to the N-terminal cysteines of DnaA reported on occupancy-correlated changes in its vicinity. Cross-linking of DnaA with a short homobifunctional reagent revealed that state II of the protein on the membrane corresponds to a distinct oligomeric form of DnaA. The kinetic transition of DnaA on the membrane surface is described in the present study by a generalized 2D condensation phase transition model, confirming the existence of two states of DnaA on the membrane and pointing to the possibility that membrane protein density serves as an on-off switch in vivo. We conclude that the DnaA conformation attained at low surface density drives its N-terminal-mediated oligomerization, which is presumably a pre-requisite for facilitated nt exchange.

Humanin Derivatives Inhibit Necrotic Cell Death in Neurons.

Humanin and its derivatives are peptides known for their protective antiapoptotic effects against Alzheimer's disease. Herein, we identify a novel function of the humanin-derivative AGA(C8R)-HNG17 (namely, protection against cellular necrosis). Necrosis is one of the main modes of cell death, which was until recently considered an unmoderated process. However, recent findings suggest the opposite. We have found that AGA(C8R)-HNG17 confers protection against necrosis in the neuronal cell lines PC-12 and NSC-34, where necrosis is induced in a glucose-free medium by either chemohypoxia or by a shift from apoptosis to necrosis. Our studies in traumatic brain injury models in mice, where necrosis is the main mode of neuronal cell death, have shown that AGA(C8R)-HNG17 has a protective effect. This result is demonstrated by a decrease in a neuronal severity score and by a reduction in brain edema, as measured by magnetic resonance imaging (MRI). An insight into the peptide's antinecrotic mechanism was attained through measurements of cellular ATP levels in PC-12 cells under necrotic conditions, showing that the peptide mitigates a necrosis-associated decrease in ATP levels. Further, we demonstrate the peptide's direct enhancement of the activity of ATP synthase activity, isolated from rat-liver mitochondria, suggesting that AGA(C8R)-HNG17 targets the mitochondria and regulates cellular ATP levels. Thus, AGA(C8R)-HNG17 has potential use for the development of drug therapies for necrosis-related diseases, for example, traumatic brain injury, stroke, myocardial infarction, and other conditions for which no efficient drug-based treatment is currently available. Finally, this study provides new insight into the mechanisms underlying the antinecrotic mode of action of AGA(C8R)-HNG17.

Structures of Escherichia coli tryptophanase in holo and 'semi-holo' forms.

Two crystal forms of Escherichia coli tryptophanase (tryptophan indole-lyase, Trpase) were obtained under the same crystallization conditions. Both forms belonged to the same space group P43212 but had slightly different unit-cell parameters. The holo crystal form, with pyridoxal phosphate (PLP) bound to Lys270 of both polypeptide chains in the asymmetric unit, diffracted to 2.9 Šresolution. The second crystal form diffracted to 3.2 Šresolution. Of the two subunits in the asymmetric unit, one was found in the holo form, while the other appeared to be in the apo form in a wide-open conformation with two sulfate ions bound in the vicinity of the active site. The conformation of all holo subunits is the same in both crystal forms. The structures suggest that Trpase is flexible in the apo form. Its conformation partially closes upon binding of PLP. The closed conformation might correspond to the enzyme in its active state with both cofactor and substrate bound in a similar way as in tyrosine phenol-lyase.

Serine protease inhibitors interact with IFN-γ through up-regulation of FasR; a novel therapeutic strategy against cancer.

Among the many immunomodulatory and anti-tumor activities, IFN-γ up-regulates tumor cell death mediated by Fas receptor (FasR). Our and several other studies have demonstrated the involvement of trypsin-like proteases (TLPs) in the mode of action of IFN-γ. In the present study, we tried to unravel the role of serine proteases in IFN-γ induced Fas-mediated cell death. Our present results show that both tosyl-l-Lysine chloromethylketone (TLCK), a trypsin like protease inhibitor and tosyl-l-phenylalanine chloromethylketone (TPCK) - a chymotrypsin like protease (CLP) inhibitor, sensitize HeLa cells to Fas-mediated cell death. The combined effect of these protease inhibitors with anti-Fas was stronger than additive. In contrast, elastase inhibitor III (EI), which also contains the chloromethyl ketone moiety, was not active. Furthermore, co-addition of TLCK or TPCK with IFN-γ markedly enhanced Fas-induced cell death. IFN-γ led to up-regulation of FasR on its own, which was further enhanced by the co-addition of TLCK or TPCK. This was evident both by increased expression of Fas receptor on cell surface and by elevated Fas mRNA level. This study may provide the basis for the design of a novel combinatory therapeutic strategy that could enhance the eradication of tumors.

Regulation of autophagy by α1-antitrypsin: "a foe of a foe is a friend".

Autophagy is involved in both the cell protective and the cell death process but its mechanism is largely unknown. The present work unravels a novel intracellular mechanism by which the serpin α1-antitrypsin (AAT) acts as a novel negative regulator of autophagic cell death. For the first time, the role of intracellularly synthesized AAT, other than in liver cells, is demonstrated. Autophagic cell death was induced by N-α-tosyl-L-phenylalanine chloromethyl ketone (TPCK) and tamoxifen. By utilizing a fluorescently tagged TPCK analog, AAT was "fished out" (pulled out) as a TPCK intracellular protein target. The interaction was further verified by competition binding experiments. Both inducers caused downregulation of AAT expression associated with activation of trypsin-like proteases. Furthermore, silencing AAT by siRNA induced autophagic cell death. Moreover, AAT administration to cultured cells prevented autophagic cell death. This new mechanism could have implications in the treatment of diseases by the regulation of AAT levels in which autophagy has a detrimental function. Furthermore, the results imply that the high synthesis of endogenous AAT by cancer cells could provide a novel resistance mechanism of cancer against autophagic cell death.

Cardiolipin plays a role in KCN-induced necrosis.

Cardiolipin (CL) is a unique anionic, dimeric phospholipid found almost exclusively in the inner mitochondrial membrane and is essential for the function of numerous enzymes that are involved in mitochondrial energy metabolism. While the role of cardiolipin in apoptosis is well established, its involvement in necrosis is enigmatic. In the present study, KCN-induced necrosis in U937 cells was used as an experimental model to assess the role of CL in necrosis. KCN addition to U937 cells induced reactive oxygen species (ROS) formation, while the antioxidants inhibited necrosis, indicating that ROS play a role in KCN-induced cell death. Further, CL oxidation was confirmed by the monomer green fluorescence of 10-N-nonyl acridine orange (NAO) and by TLC. Utilizing the red fluorescence of the dimeric NAO, redistribution of CL in mitochondrial membrane during necrosis was revealed. We also showed that the catalytic activity of purified adenosine triphosphate (ATP) synthase complex, known to be modulated by cardiolipin, decreased following KCN treatment. All these events occurred at an early phase of the necrotic process prior to rupture of the cell membrane. Furthermore, CL-deficient HeLa cells were found to be resistant to KCN-induced necrosis as compared with the wild type cells. We suggest that KCN, an effective reversible inhibitor of cytochrome oxidase and thereby of the respiratory chain leads to ROS increase, which in turn oxidizes CL (amongst other membrane phospholipids) and leads to mitochondrial membrane lipid reorganization and loss of CL symmetry. Finally, the resistance of CL-deficient cells to necrosis further supports the notion that CL, which undergoes oxidation during necrotic cell death, is an integral part of the milieu of events taking place in mitochondria leading to membrane disorganization and mitochondrial dysfunction.

Conformational changes and loose packing promote E. coli Tryptophanase cold lability.

BACKGROUND: Oligomeric enzymes can undergo a reversible loss of activity at low temperatures. One such enzyme is tryptophanase (Trpase) from Escherichia coli. Trpase is a pyridoxal phosphate (PLP)-dependent tetrameric enzyme with a Mw of 210 kD. PLP is covalently bound through an enamine bond to Lys270 at the active site. The incubation of holo E. coli Trpases at 2 degrees C for 20 h results in breaking this enamine bond and PLP release, as well as a reversible loss of activity and dissociation into dimers. This sequence of events is termed cold lability and its understanding bears relevance to protein stability and shelf life. RESULTS: We studied the reversible cold lability of E. coli Trpase and its Y74F, C298S and W330F mutants. In contrast to the holo E. coli Trpase all apo forms of Trpase dissociated into dimers already at 25 degrees C and even further upon cooling to 2 degrees C. The crystal structures of the two mutants, Y74F and C298S in their apo form were determined at 1.9A resolution. These apo mutants were found in an open conformation compared to the closed conformation found for P. vulgaris in its holo form. This conformational change is further supported by a high pressure study. CONCLUSION: We suggest that cold lability of E. coli Trpases is primarily affected by PLP release. The enhanced loss of activity of the three mutants is presumably due to the reduced size of the side chain of the amino acids. This prevents the tight assembly of the active tetramer, making it more susceptible to the cold driven changes in hydrophobic interactions which facilitate PLP release. The hydrophobic interactions along the non catalytic interface overshadow the effect of point mutations and may account for the differences in the dissociation of E. coli Trpase to dimers and P. vulgaris Trpase to monomers.

New tryptophanase inhibitors: towards prevention of bacterial biofilm formation.

Tryptophanase (tryptophan indole-lyase, Tnase, EC 4.1.99.1), a bacterial enzyme with no counterpart in eukaryotic cells, produces from L-tryptophan pyruvate, ammonia and indole. It was recently suggested that indole signaling plays an important role in the stable maintenance of multicopy plasmids. In addition, Tnase was shown to be capable of binding Rcd, a short RNA molecule involved in resolution of plasmid multimers. Binding of Rcd increases the affinity of Tnase for tryptophan, and it was proposed that indole is involved in bacteria multiplication and biofilm formation. Biofilm-associated bacteria may cause serious infections, and biofilm contamination of equipment and food, may result in expensive consequences. Thus, optimal and specific factors that interact with Tnase can be used as a tool to study the role of this multifunctional enzyme as well as antibacterial agents that may affect biofilm formation. Most known quasi-substrates inhibit Tnase at the mM range. In the present work, the mode of Tnase inhibition by the following compounds and the corresponding Ki values were: S-phenylbenzoquinone-L-tryptophan, uncompetitively, 101 microM; alpha-amino-2-(9,10-anthraquinone)-propanoic acid, noncompetitively, 174 microM; L-tryptophane-ethylester, competitively, 52 microM; N-acetyl-L-tryptophan, noncompetitively, 48 microM. S-phenylbenzoquinone-L-tryptophan and alpha-amino-2-(9,10-anthraquinone)-propanoic acid were newly synthesized.

Structural insights into cold inactivation of tryptophanase and cold adaptation of subtilisin S41.

A wide variety of enzymes can undergo a reversible loss of activity at low temperature, a process that is termed cold inactivation. This phenomenon is found in oligomeric enzymes such as tryptophanase (Trpase) and other pyridoxal phosphate dependent enzymes. On the other hand, cold-adapted, or psychrophilic enzymes, isolated from organisms able to thrive in permanently cold environments, have optimal activity at low temperature, which is associated with low thermal stability. Since cold inactivation may be considered "contradictory" to cold adaptation, we have looked into the amino acid sequences and the crystal structures of two families of enzymes, subtilisin and tryptophanase. Two cold adapted subtilisins, S41 and subtilisin-like protease from Vibrio, were compared to a mesophilic and a thermophilic subtilisins, as well as to four PLP-dependent enzymes in order to understand the specific surface residues, specific interactions, or any other molecular features that may be responsible for the differences in their tolerance to cold temperatures. The comparison between the psychrophilic and the mesophilic subtilisins revealed that the cold adapted subtilisins have a high content of acidic residues mainly found on their surface, making it charged. The analysis of the Trpases showed that they have a high content of hydrophobic residues on their surface. Thus, we suggest that the negatively charged residues on the surface of the subtilisins may be responsible for their cold adaptation, whereas the hydrophobic residues on the surface of monomeric Trpase molecules are responsible for the tetrameric assembly, and may account for their cold inactivation and dissociation.

The reactivation of DnaA(L366K) requires less acidic phospholipids supporting their role in the initiation of chromosome replication in Escherichia coli.

DnaA(L366K), in concert with a wild-type DnaA (wtDnaA) protein, restores the growth of Escherichia coli cells arrested in the absence of adequate levels of cellular acidic phospholipids. In vitro and in vivo studies showed that DnaA(L366K) alone does not induce the initiation of replication, and wtDnaA must also be present. Hitherto the different behavior of wt and mutant DnaA were not understood. We now demonstrate that this mutant may be activated at significantly lower concentrations of acidic phospholipids than the wild-type protein, and this may explain the observed growth restoration in vivo.

The structure of apo tryptophanase from Escherichia coli reveals a wide-open conformation.

The crystal structure of apo tryptophanase from Escherichia coli (space group F222, unit-cell parameters a = 118.4, b = 120.1, c = 171.2 A) was determined at 1.9 A resolution using the molecular-replacement method and refined to an R factor of 20.3% (R(free) = 23.2%). The structure revealed a significant shift in the relative orientation of the domains compared with both the holo form of Proteus vulgaris tryptophanase and with another crystal structure of apo E. coli tryptophanase, reflecting the internal flexibility of the molecule. Domain shifts were previously observed in tryptophanase and in the closely related enzyme tyrosine phenol-lyase, with the holo form found in an open conformation and the apo form in either an open or a closed conformation. Here, a wide-open conformation of the apo form of tryptophanase is reported. A conformational change is also observed in loop 297-303. The structure contains a hydrated Mg(2+) at the cation-binding site and a Cl(-) ion at the subunit interface. The enzyme activity depends on the nature of the bound cation, with smaller ions serving as inhibitors. It is hypothesized that this effect arises from variations of the coordination geometry of the bound cation.

Intrinsic fluorescence polarization of amniotic fluid: II. Toward a noninvasive method for the determination of fetal lung maturity.

The present study compares two methods for the determination of fetal lung maturity: the novel intrinsic fluorescence polarization ratio (IFPR) and the commercial TDx-FLMII. Amniotic fluid (AF) samples were collected from 69 women during the second and third trimesters of singleton pregnancies. Thirty-three samples were tested for IFPR only after centrifugation, and the rest were examined both before and after centrifugation. Of the latter 33 samples, 29 were assessed for lung maturity with the TDx-FLMII method as well. The results showed that IFPR values decreased with the advance in gestational age (r = 0.77, p < 0.05, n = 69). A significant correlation was found between IFPR of centrifuged and noncentrifuged samples (r = 0.94, p < 0.05, n = 36). A significant correlation was demonstrated between IFPR and TDx-FLMII values of centrifuged (r = 0.75, p < 0.05, n = 29) and noncentrifuged (r = 0.63, p < 0.05, n = 29) samples and moreover, samples considered mature by TDx-FLMII had low values of IFPR (n = 10). It can be concluded that the IFPR method can utilize noncentrifuged AF, thus suggested as a potential noninvasive method.

Membrane-catalyzed nucleotide exchange on DnaA. Effect of surface molecular crowding.

DnaA is the initiator protein for chromosomal replication in bacteria; its activity plays a central role in the timing of the primary initiations within the Escherichia coli cell cycle. A controlled, reversible conversion between the active ATP-DnaA and the inactive ADP forms modulates this activity. In a DNA-dependent manner, bound ATP is hydrolyzed to ADP. Acidic phospholipids with unsaturated fatty acids are capable of reactivating ADP-DnaA by promoting the release of the tightly bound ADP. The nucleotide dissociation kinetics, measured in the present study with the fluorescent derivative 3'-O-(N-methylantraniloyl)-5'-adenosine triphosphate, was dependent on the density of DnaA on the membrane in a cooperative manner: it increased 5-fold with decreased protein density. At all surface densities the nucleotide was completely released, presumably due to protein exchange on the membrane. Distinct temperature dependences and the effect of the crowding agent Ficoll suggest that two functional states of DnaA exist at high and low membrane occupancy, ascribed to local macromolecular crowding on the membrane surface. These novel phenomena are thought to play a major role in the mechanism regulating the initiation of chromosomal replication in bacteria.

Crystallization and preliminary X-ray analysis of the apo form of Escherichia coli tryptophanase.

Tryptophanase from Escherichia coli is a pyridoxal phosphate-dependent homotetrametic enzyme with a subunit weight of 52 kDa. It has been crystallized in the apo form by the hanging-drop vapour-diffusion method using polyethylene glycol 400 as a precipitant and magnesium chloride as an additive. The crystals belong to the orthorhombic space group F222, with unit-cell parameters a = 118.4, b = 120.1, c = 171.2 A. A 97.8% complete data set to 1.9 A resolution was collected at a rotating-anode source from a single frozen crystal. Packing-density considerations agree with a monomer in the asymmetric unit with a solvent content of 55%. Tryptophanase mutants W330F and Y74F were crystallized under the same conditions and the crystals diffracted to a resolution limit of 1.9 A. Data sets of wild-type crystals soaked with L-tryptophan or pyridoxal phosphate were collected, as well as of Y74F mutant soaked with both.

Phosphatidylethanolamine and phosphatidylglycerol are segregated into different domains in bacterial membrane. A study with pyrene-labelled phospholipids.

To detect and characterize membrane domains that have been proposed to exist in bacteria, two kinds of pyrene-labelled phospholipids, 2-pyrene-decanoyl-phosphatidylethanolamine (PY-PE) and 2-pyrene-decanoyl-phosphatidylglycerol (PY-PG) were inserted into Escherichia coli or Bacillus subtilis membrane. The excimerization rate coefficient, calculated from the excimer-to-monomer ratio dependencies on the probe concentration, was two times higher for PY-PE than for PY-PG at 37 degrees C. This was ascribed to different local concentrations rather than to differences in mobility. The extent of mixing between the two fluorescent phospholipids, estimated by formation of their heteroexcimer, was found very low both in E. coli and B. subtilis, in contrast to model membranes. In addition, these two pyrene derivatives exhibited different temperature phase transitions and different detergent extractability, indicating that the surroundings of these phospholipids in bacterial membrane differ in organization and order. Inhibition of protein synthesis, leading to condensation of nucleoid and presumably to dissipation of membrane domains, indeed resulted in increased formation of heteroexcimers, broadening of phase transitions and equal detergent extractability of both probes. It is proposed that in bacterial membranes these phospholipids are segregated into distinct domains that differ in composition, proteo-lipid interaction and degree of order; the proteo-lipid domain being enriched by PE.

Intrinsic fluorescence polarization of amniotic fluid: evaluation of human fetal lung maturity.

This article reports a novel approach for the evaluation of fetal lung maturity based on fluorescence polarization (FP). The technique determines the intrinsic fluorescence polarization ratio (IFPR) of the amniotic fluid (AF). In vitro measurements of the IFPR indicate a clear dichotomy: high values for young pregnancies and low values for mature pregnancies. The new method has the potential to be a noninvasive procedure because the excitation of the AF and the collection of its fluorescence emission can be performed through the intact cervical amniotic membranes.

Low-frequency electromagnetic fields induce a stress effect upon higher plants, as evident by the universal stress signal, alanine.

15N NMR analysis reveals alanine production in Duckweed plants exposed to low intensity sinusoidally varying magnetic fields (SVMF) at 60 and 100Hz, and fed by 15N-labeled ammonium chloride. Alanine does not accumulate in the absence of SVMF. Addition of vitamin C, a radical scavenger, reduced alanine production by 82%, indicating the roll of free radicals in the process. Alanine accumulation in plants and animals in response to exposure to a variety of stress conditions, including SVMF, is a general phenomenon. It is proposed that alanine is a universal first stress signal expressed by cells.

Coexistence of domains with distinct order and polarity in fluid bacterial membranes.

In this study we sought the detection and characterization of bacterial membrane domains. Fluorescence generalized polarization (GP) spectra of laurdan-labeled Escherichia coli and temperature dependencies of both laurdan's GP and fluorescence anisotropy of 1,3-diphenyl-1,3,5-hexatriene (DPH) (rDPH) affirmed that at physiological temperatures, the E. coli membrane is in a liquid-crystalline phase. However, the strong excitation wavelength dependence of rlaurdan at 37 degrees C reflects membrane heterogeneity. Time-resolved fluorescence emission spectra, which display distinct biphasic redshift kinetics, verified the coexistence of two subpopulations of laurdan. In the initial phase, <50 ps, the redshift in the spectral mass center is much faster for laurdan excited at the blue edge (350 nm), whereas at longer time intervals, similar kinetics is observed upon excitation at either blue or red edge (400 nm). Excitation in the blue region selects laurdan molecules presumably located in a lipid domain in which fast intramolecular relaxation and low anisotropy characterize laurdan's emission. In the proteo-lipid domain, laurdan motion and conformation are restricted as exhibited by a slower relaxation rate, higher anisotropy and a lower GP value. Triple-Gaussian decomposition of laurdan emission spectra showed a sharp phase transition in the temperature dependence of individual components when excited in the blue but not in the red region. At least two kinds of domains of distinct polarity and order are suggested to coexist in the liquid-crystalline bacterial membrane: a lipid-enriched and a proteolipid domain. In bacteria with chloramphenicol (Cam)-inhibited protein synthesis, laurdan showed reduced polarity and restoration of an isoemissive point in the temperature-dependent spectra. These results suggest a decrease in membrane heterogeneity caused by Cam-induced domain dissipation.