Tryptophan imaging of membrane proteins

A theoretical analysis of resonance energy transfer between protein tryptophan and the n-(9-anthroyloxy) (AO) fatty acid probes has been carried out to evaluate its potential use in determining the tryptophan distribution in membrane proteins. The F?rster theory for two-dimensional energy transfer was formulated to calculate multiple donor (tryptophan) transfer efficiencies to ensembles of AO probes at different depths in the bilayer. The variation of transfer efficiency with AO probe depth is found to be a sensitive function of tryptophan position and the protein radius but not the dipole-dipole orientation factor or the decay heterogeneity of the donor. For single tryptophan-containing proteins the model predicts that the tryptophan position can be determined with a precision of about 2 A. Although for multiple tryptophans there is appreciable deterioration in resolution, it is still possible to determine the essential features of the distribution such as its first two moments. The positions determined by this method are the projections of the tryptophan positions on a plane perpendicular to the membrane surface, since the probes distribute uniformly around the protein. To analyze the data, a Monte Carlo approach has been developed to search for tryptophan distributions compatible with the observed efficiencies and to display the results in terms of a tryptophan density map. It is shown that even for cases in which little is known about the quantum yield distribution, significant information can be determined about the tryptophan spatial distribution.     

Anthroyl stearate as a fluorescent probe of chloroplast membranes.

1. A reversible light-induced enhancement of the fluorescence of a "hydrophobic fluorophore", 12-(9-anthroyl)-stearic acid (anthroyl stearate), is observed with chloroplasts supporting phenazine methosulfate, cyclic or 1,1'-ethylene-2,2'-dipyridylium dibromide (Diquat) pseudo-cyclic electron flow; no fluorescence change is observed when methyl viologen or ferricyanide are used as electron acceptors. The stearic acid moiety of anthroyl stearate is important for its localization and fluorescence response in the thylakoid membrane, since structural analogs of anthroyl stearate lacking this group do not show the same response. 2. This effect is decreased under phosphorylating conditions (presence of ADP, Pi, Mg2+), and completely inhibited by the uncoupler of phosphorylation NH4Cl(5-10mM), as well as the ionophores nigericin and gramicidin-D (both at 5 - 10(-8)M). The MgCl2 concentration dependence of the anthroyl stearate enhancement effect is identical to that previously observed for cyclic photophosphorylation, as well as for the formation of a "high energy intermediate". The anthroyl stearate fluorescence enhancement is inhibited by increasing concentrations of ionophores in parallel with the decrease in ATP synthesis, but is essentially unaffected by specific inhibitors (Dio-9 and phlorizin) of photophosphorylation; thus, it appears that anthroyl stearate monitors a component of the "high energy state" of the thylakoid membrane rather than a terminal phosphorylation step. 3. The light-induced anthroyl stearate fluorescence enhancement is suggested to monitor a proton gradient in the energized chloroplast because (a) similar enhancement can be produced by sudden injection of hydrogen ions in a solution of anthroyl stearate; (b) when the proton gradient is dissipated by gramicidin or nigericin light-induced anthroyl stearate fllorescence is eliminated; (c) when the proton gradient is dissipated by tetraphenylboron, light-induced anthroyl stearate fluorescence decreases, and (d) light-induced anthroyl stearate fluorescence change as a function of pH is qualitatively similar to that observed with other probes for a proton gradient (e.g. 9-aminoacridine). Furthermore, anthroyl stearate does not monitor H+ uptake per se because (a) the pH dependence of H+ transport is different from that of the anthroyl stearate fluorescence change, and (b) tetraphenylboron, which does not inhibit H+ uptake, reduces anthroyl stearate fluorescence. Thus, anthroyl stearate appears to be a useful probe of a proton gradient supported by phenazine methosulfate of Diquat catalyzed electron flow and is the first "non-amine" fluorescence probe utilized for this purpose in chloroplasts.     

Comparative study of the lateral motion of extrinsic probes and anthracene-labelled constitutive phospholipids in the plasma membrane of Chinese hamster ovary cells

9-(2-Anthryl)-nonanoic acid is a new photoactivatable fluorescent probe which has been designed for the study of the lateral diffusion and distribution of lipids in biological membranes by means of the anthracene photodimerization reaction. This anthracene fatty acid can be incorporated metabolically into the glycerophospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol) of Chinese hamster ovary (CHO) cells in culture. The diffusion coefficient of intrinsic lipids in the plasma membrane of these eukaryotic cells can thus be measured using the fluorescence recovery after a photobleaching technique, since illumination of the fluorescent anthracene groups yields non-fluorescent photodimers. For the sake of comparison, the extrinsic lipophilic probes 5-(N-hexadecanoyl)-aminofluorescein, 12-(9-anthroyloxy)-stearic acid, 9-(2-anthryl)-nonanoic acid and a synthetic anthracene-phosphatidylcholine were also used to label the plasma membrane of CHO cells. The diffusion coefficients for the extrinsic and intrinsic probes ranged over 1 - 2 x 10(-9) cm2/s. Small but significant differences were observed between the various probes reflecting differences they exhibit in size and polarity. All the extrinsic probes were free to diffuse, with a mobile fraction close to 100%. In contrast, a fractional recovery of only 75% was observed for the intrinsic anthracene-labelled phospholipids, suggesting that the anthracene fatty acid was metabolically incorporated into membrane lipid regions which were inaccessible to the extrinsic probes.     

An assessment of the fluidity gradient of the lipid bilayer as determined by a set of n-(9-anthroyloxy) fatty acids (n = 2, 6, 9, 12, 16).

The rotational behavior of a set of n-(9-anthroyloxy) fatty acid fluorescent probes is examined in two liquid paraffins and in liposomes composed of dipalmitoyl phosphatidylcholine. As has been observed with other membrane fluorescent probes (Hare, F., and Lussan, C. (1977) Biochim. Biophys. Acta 467, 262-272), the degree of fluorescence depolarization for a given solvent viscosity is dependent on the solvent standard employed. In addition, when the anthroyloxy group is in the terminal position of the acyl chain, it has more rotational freedom than when it is conjugated to positions 6, 9, or 12 where the rotational motion of the fluorophore is similar. When incorporated into lipid bilayers, values of fluorescence polarization reflect the gradient of "fluidity" which extends from the surface to the center of the membrane. The nature of this polarization gradient is discussed in relation to the intrinsic differences between the probes and the anisotropic rotations responsible for depolarization.     

Anthroyl stearate as a fluorescent probe of chloroplast membranes

1. A reversible light-induced enhancement of the fluorescence of a “hydrophobic fluorophore”, 12-(9-anthroyl)-stearic acid (anthroyl stearate), is observed with chloroplasts supporting phenazine methosulfate, cyclic or 1,1′-ethylene-2,2′-dipyridylium dibromide (Diquat) pseudo-cyclic electron flow; no fluorescence change is observed when methyl viologen or ferricyanide are used as electron acceptors. The stearic acid moiety of anthroyl stearate is important for its localization and fluorescence response in the thylakoid membrane, since structural analogs of anthroyl stearate lacking this group do not show the same response.

2. This effect is decreased under phosphorylating conditions (presence of ADP, P_i, Mg²⁺), and completely inhibited by the uncoupler of phosphorylation NH₄Cl (5–10 mM), as well as the ionophores nigericin and gramicidin-D (both at 5 · 10⁻⁸ M). The MgCl₂ concentration dependence of the anthroyl stearate enhancement effect is identical to that previously observed for cyclic photophosphorylation, as well as for the formation of a “high energy intermediate”. The anthroyl stearate fluorescence enhancement is inhibited by increasing concentrations of ionophores in parallel with the decrease in ATP synthesis, but is essentially unaffected by specific inhibitors (Dio-9 and phlorizin) of photophosphorylation; thus, it appears that anthroyl stearate monitors a component of the “high energy state” of the thylakoid membrane rather than a terminal phosphorylation step.

3. The light-induced anthroyl stearate fluorescence enhancement is suggested to monitor a proton gradient in the energized chloroplast because (a) similar enhancement can be produced by sudden injection of hydrogen ions in a solution of anthroyl stearate; (b) when the proton gradient is dissipated by gramicidin or nigericin light-induced anthroyl stearate fluorescence is eliminated; (c) when the proton gradient is dissipated by tetraphenylboron, light-induced anthroyl stearate fluorescence decreases, and (d) light-induced anthroyl stearate fluorescence change as a function of pH is qualitatively similar to that observed with other probes for a proton gradient (e.g. 9-aminoacridine). Furthermore, anthroyl stearate does not monitor H⁺ uptake per se because (a) the pH dependence of H⁺ transport is different from that of the anthroyl stearate fluorescence change, and (b) tetraphenylboron, which does not inhibit H⁺ uptake, reduces anthroyl stearate fluorescence.

Thus, anthroyl stearate appears to be a useful probe of a proton gradient supported by phenazine methosulfate or Diquat catalyzed electron flow and is the first “non-amine” fluorescence probe utilized for this purpose in chloroplasts.     

Interaction of rifampicin and isoniazid with large unilamellar liposomes: spectroscopic location studies

The location of isoniazid and rifampicin, two tuberculostatics commonly used for the treatment of Mycobacterium tuberculosis and Mycobacterium avium complex infectious diseases, in bilayers of dimyristoyl-L-alpha-phosphatidylcholine (DMPC) and dimyristoyl-L-a-phosphatidylglycerol (DMPG) have been studied by 1H NMR and fluorimetric methods. Steady-state fluorescence intensity and fluorescence energy transfer studies between rifampicin and a set of functionalized probes [n-(9-anthroyloxy)stearic acids, n=2, 12] reveal that, in both systems, isoniazid is located at the membrane surface whereas rifampicin is deeply buried inside the lipid bilayers. Steady-state fluorescence anisotropy studies performed with the probes 1,6-diphenyl-1,3,5-hexatriene (DPH) and trimethylammonium-diphenylhexa-triene (TMA-DPH), not only corroborate the above results, but also show that no changes in membrane fluidity were detected in either liposome. The 1H NMR results, in DMPC liposomes, confirm the location of rifampicin near the methylene group of the acyl chains of the lipid bilayers.     

Transverse location of anthracyclines in lipid bilayers. Paramagnetic quenching studies

Quenching of anthracycline fluorescence by a series of spin-labeled fatty acids was used to probe the transverse location of the drug in phosphatidylcholine bilayers in the form of small unilamellar vesicles. Stern-Volmer plots of the quenching data indicate that the fluorophore moiety of the anthracycline is intercalated into the hydrocarbon region of the bilayer, with deeper penetration observed in fluid-phase than in solid-phase vesicles. 31P-NMR parameters (T1 and nuclear Overhauser enhancement (NOE] are unaffected by the presence of drug, consistent with a binding site removed from the interfacial region. Comparison of intensity (F0/F) plots with lifetime (tau 0/tau) data shows that the predominant mechanism of anthracycline quenching by membrane-bound nitroxides is static. Since the membrane-bound drug is also accessible to quenching by I-, the binding site in the membrane must create a channel which is accessible to solvent. Two other fluorescent probes, 12-(9-anthroyloxy)stearate (12-AS) and diphenylhexatriene (DPH), were employed to confirm the results obtained with the anthracyclines, giving quenching data representative of their location in the bilayer.     

Fluorescent lipid probes in the study of viral membrane fusion

Fluorescent lipid probes are widely used in the observation of viral membrane fusion, providing a sensitive method to study fusion mechanism(s). Due to the wealth of data concerning liposome fusion, a variety of fusion assays has been designed including fluorescent probe redistribution, fluorescence dequenching, fluorescence resonance energy transfer and photosensitized labeling. These methods can be tailored for different virus fusion assays. For instance, virions can be loaded with membrane dye which dequenches at the moment of membrane merger. This allows for continuous observation of fusion and therefore kinetic information can be acquired. In the case of cells expressing viral envelope proteins, dye redistribution studies of lipidic and water-soluble fluorophores yield information about fusion intermediates. Lipid probes can be metabolically incorporated into cell membranes, allowing observation of membrane fusion in vitro with minimal chance of flip flop, non-specific transfer and formation of microcrystals. Fluorescent lipid probes have been incorporated into liposomes and/or reconstituted viral envelopes, which provide a well-defined membrane environment for fusion to occur. Interactions of the viral fusion machinery with the membrane can be observed through the photosensitized labeling of the interacting segments of envelope proteins with a hydrophobic probe. Thus, fluorescent lipid probes provide a broad repertoire of fusion assays and powerful tools to produce precise, quantitative data in real time required for the elucidation of the complex process of viral fusion.     

Fluorescence studies of dipalmitoylphosphatidylcholine vesicles reconstituted with the glycoprotein of vesicular stomatitis virus

The vesicular stomatitis virus glycoprotein (G) was reconstituted into dipalmitoylphosphatidylcholine (DPPC) vesicles by detergent dialysis. The DPPC gel to liquid-crystalline phase transition of the DPPC-G protein vesicles was monitored by the fluorescence anisotrophy of trans-paranaric acid, 16-(9-anthroyloxy)palmitoylglucocerebroside, 1,6-diphenyl-1,3,5-hexatriene, and 4-heptadecyl-7-hydroxycoumarin. The DPPC transition temperature measured by all four fluorescent probes was lowered in the presence of the G protein and the DPPC gel state was disordered by the G protein as evidenced by a decreased fluorescence anisotropy for all four probes below the phase-transition temperature. A possible ordering of the DPPC liquid-crystalline state by the G protein was indicated by an increased anisotropy of trans-paranaric acid and 16-(9-anthroyloxy)palmitoylglucocerebroside in the liquid-crystalline state of DPPC-G protein vesicles. The G protein in addition affected the ionization of the 4-heptadecyl-7-hydroxycoumarin in lipid vesicles, increasing the apparent pK of the probe from 9.05 to 9.45.     

The activation energy of incorporation of extrinsic probes in model vesicles

Time dependence of fluorescence enhancement of probes after addition to lipid vesicles has been used to investigate the position of chromophores in the lipid bilayer. Incorporation studies of a series of $\text{n-(9-}\text{anthroyloxy}$) fatty acids ($\text{n}\text{= 2, 2, 12}\text{and}\text{16}$) and 1,6-diphenylhexatriene in dipalmitoyl phosphatidylcholine vesicles are described. The activation energies for incorporation of these several lipid-mimic type fluorescent probes have been measured. Results show that the activation energy is a function of the distance of the anthracene moiety (chromophore) from the polar end of the probe and the length of the acyl portion of the probe. An average insertion energy of 0.6 kcal/carbon is seen for these fatty acid probes. The activation energy of 1,6-diphenylhexatriene, a factor of 2 greater than that of 16-(9-anthroyloxy)palmitic acid, is consistent with locating 1,6-diphenyl-hexatriene in the middle of the bilayer.     

Properties and the locations of a set of fluorescent probes sensitive to the fluidity gradient of the lipid bilayer

The synthesis and properties of a set of four fluorescent probes (n-(9-anthroyloxy) fatty acids, n = 2, 6, 9, 12) sensitive to the fluidity gradient of the lipid bilayer are described. Fluorescent quenching experiments show that the probes locate at a graded series of depths in the bilayer. A fifth probe, methyl-9-anthroate, locates near the bilayer centre. As an example of their application, the probes are used to study the phase transitions of dipalmitoyl phosphatidyl-choline. Changes in the rotational relaxation times of the probes across the transitions are more pronounced at the centre of the bilayer than at the surface.     

Kinetic model of protein-mediated ligand transport: influence of soluble binding proteins on the intermembrane diffusion of a fluorescent fatty acid

The mechanism (or mechanisms) whereby fatty acids and other amphipathic compounds are transported from the plasma membrane to intracellular sites of biotransformation remains poorly defined. In an attempt to better characterize the role of cytosolic binding proteins in this process, a kinetic model of intermembrane ligand transport was developed in which diffusional transfer of ligand between membrane and protein is assumed. The model was tested by utilizing stopped-flow techniques to monitor the transfer of the fluorescent fatty acid analogue, 12-anthroyloxy stearate (12-AS), between model membrane vesicles. Studies were conducted in the presence or absence of bovine serum albumin (BSA), liver fatty acid-binding protein (L-FABP), and intestinal fatty acid-binding protein (I-FABP) in order to determine the effect of soluble proteins on the rate of intermembrane ligand transfer. As predicted by the model, the initial velocity of 12-AS arrival at the acceptor membrane increases in an asymptotic manner with the acceptor concentration. Furthermore, probe transfer velocity was found to decline asymptotically with increasing concentrations of BSA or L-FABP, proteins that exhibit diffusional transfer kinetics. This observation was found to hold true independent of whether donor or acceptor vesicles were preequilibrated with the protein. In contrast, 12-AS transfer velocity exhibited a linear correlation with the concentration of I-FABP, a protein that is thought to transport fatty acids, at least in part, via a collisional mechanism. Taken together, these findings validate the derived kinetic model of protein-mediated ligand transport and further suggest that the mechanism of ligand-protein interaction is a key determinant of the effect of cytosolic proteins on intracellular ligand diffusion.     

Electrically controlling and optically observing the membrane potential of supported lipid bilayers

Supported lipid bilayers are a well-developed model system for the study of membranes and their associated proteins, such as membrane channels, enzymes, and receptors. These versatile model membranes can be made from various components, ranging from simple synthetic phospholipids to complex mixtures of constituents, mimicking the cell membrane with its relevant physiochemical and molecular phenomena. In addition, the high stability of supported lipid bilayers allows for their study via a wide array of experimental probes. In this work, we describe a platform for supported lipid bilayers that is accessible both electrically and optically, and demonstrate direct optical observation of the transmembrane potential of supported lipid bilayers. We show that the polarization of the supported membrane can be electrically controlled and optically probed using voltage-sensitive dyes. Membrane polarization dynamics is understood through electrochemical impedance spectroscopy and the analysis of an equivalent electrical circuit model. In addition, we describe the effect of the conducting electrode layer on the fluorescence of the optical probe through metal-induced energy transfer, and show that while this energy transfer has an adverse effect on the voltage sensitivity of the fluorescent probe, its strong distance dependency allows for axial localization of fluorescent emitters with ultrahigh accuracy. We conclude with a discussion on possible applications of this platform for the study of voltage-dependent membrane proteins and other processes in membrane biology and surface science.     

Shape of the fluidity gradient in the plasma membrane of living HeLa cells

The shape of the fluidity gradient of the outer hemi-leaflet of the plasma membrane of living HeLa cells was determined using a series of n-(9-anthroyloxy) fatty acid probes where n = 2, 3, 6, 7, 9, 10, 11, 12, and 16. Fluorescence uptake and steady-state anisotropy values were obtained with a flow cytometer capable of continuous recording over time of vertical and horizontal emission intensities, and of the output of these intensities as calculated anisotropy values. The fluorescence uptake of all of the membrane probes was rapid up to about 15 min. The magnitudes of the uptake of fluorescence were, for the n-(9-anthroyloxy) series, in the order 2 greater than 3 greater than 6 greater than 7 greater than 9 greater than 10 greater than 11 = 12 = 16. Anisotropy values were constant from 5 to 30 min after addition of the various probes, and the magnitudes were in the order 7 greater than 6 greater than 9 = 10 greater than 2 = 3 greater than 11 greater than 12 greater than 16, indicative of the shape of the fluidity gradient. No differences were noted between the values obtained with 12-(9-anthroyloxy) stearic acid and 12- (9-anthroyloxy) oleate. The kinetics of anisotropy exhibited by those probes with the anthroyloxy group in positions deeper than 9, where initially higher values declined until equilibrium was reached, were probably indicative of an energy barrier at the approximate depth sensed by 7 AS.     

A fluorescent, genetically-encoded voltage probe capable of resolving action potentials

There is a pressing need in neuroscience for genetically-encoded, fluorescent voltage probes that can be targeted to specific neurons and circuits to allow study of neural activity using fluorescent imaging. We created 90 constructs in which the voltage sensing portion (S1-S4) of Ciona intestinalis voltage sensitive phosphatase (CiVSP) was fused to circularly permuted eGFP. This led to ElectricPk, a probe that is an order of magnitude faster (taus ～1-2 ms) than any currently published fluorescent protein-based voltage probe. ElectricPk can follow the rise and fall of neuronal action potentials with a modest decrease in fluorescence intensity (～0.7% ΔF/F). The probe has a nearly linear fluorescence/membrane potential response to both hyperpolarizing and depolarizing steps. This is the first probe based on CiVSP that captures the rapid movements of the voltage sensor, suggesting that voltage probes designed with circularly permuted fluorescent proteins may have some advantages.     

Evaluation of hydralazine and procainamide effects on fibroblast membrane fluidity

In this study the membrane fluidity of fibroblasts under different pharmacological treatment was investigated. Two drugs, hydralazine and procainamide, were used to treat the immortalized mouse NIH 3T3 and hamster B14 fibroblasts. Membrane lipid dynamics was measured by fluorescence spectroscopy and electron spin resonance techniques. Two kinds of fluorescent probes (TMA-DPH and 12-(9-anthroyloxy)-stearic acid (12-AS)) and two spin labels (5-doxylstearic acid (5-DS) and 12-doxylstearic acid (12-DS)) were used to monitor fluidity in the upper polar and in the hydrophobic core regions of the lipid bilayer. The drugs influenced the membrane hydrophobic core, of which hydralazine induced fluidization and procainamide increased the rigidity. The membrane fluidity at the surface of the lipid bilayer was not modified by the drugs which indicates that both drugs intercalated mainly into the inner core of the cell membrane.     

Cytosol-Membrane Interface of Human Erythrocytes: A Resonance Energy Transfer Study

The resonance energy transfer from donors embedded in the membrane of erythrocytes to the cytosol hemoglobin has been measured by comparing the donors' fluorescence decay in ghosts and in intact cells. A series of n - (9-anthroyloxy) stearic acids (n-AS) (n = 2, 6, 9, 12) and similar probes were used as donors, and their locations within the outer leaflet of the phospholipid bilayer were determined from their average efficiency of energy transfer, <T>. The energy transfer data for several membrane probes were analyzed according to a simple semiempirical model, in which the heme acceptors are assumed to form a semiinfinite continuum beyond a plane, whose normal distance (d) from particular donors may be determined if the heme density in the cytosol boundary layer is known. The hemoglobin concentration in the erythrocytes was varied by suspending the cells in buffers of different ionic strengths. This made it possible to study the ionic strength dependence of the heme concentration averaged over the cell (h_c), as well as that in the boundary layer (h_b). Both level off above approximately 600 mosM, as does the ratio h_b/h_c. By using the maximum heme concentration that can be obtained in osmotically shrunken cells as a limiting value, h_b is estimated to be 17 mM or less, under physiological conditions; and from the measured <T> for various probes, the distance d was found to range from 40 Å for 2-AS to 31 Å for 12-AS and 26 Å for 9-vinyl anthracene (9-VA). It is concluded that the hydrophobic probe 9-VA is located near the center of the phospholipid bilayer and that the cytosol hemoglobin is in contact with the inner membrane surface, or nearly so. This conclusion is valid for oxy- and deoxy-hemoglobin, and is shown to be independent of several systematic errors that might arise from the simple assumptions of the model used. The steady-state fluorescence anisotropy of the probes was found to decrease as they approach the bilayer's central plane. The methodology developed here may be used to extend studies of cytosol membrane interactions in ghost systems to intact cells, and is useful in the investigation of the morphology of normal and pathological intact erythrocytes.     

Association of the internal membrane protein with the lipid bilayer in influenza virus. A study with the fluorescent probe 12-(9-anthroyl)-stearic acid

The lipid fluorescent probe 12-(9-anthroyl)-stearic acid was introduced into the lipid bilayer of influenza virus particles. Fluorescent energy transfer was observed from the viral protein to the probe. This transfer persisted after removal of the glycoprotein spikes which cover the outside of the viral particle, demonstrating that the energy donor was an internal protein. It was concluded that the energy donor was the non-glycosylated membrane protein (M protein), the major protein component of the spikeless particle. Analysis of the emission spectrum of the spikeless particle excited at 275 nm shows that a substantial portion of the fluorescence arises from tyrosine residues, in contrast to most other proteins which contain both tryptophan and tyrosine.It is suggested that the donor residue(s) are located no more than 11 Å exterior to the bilayer surface, and that a portion of the M protein may penetrate into the bilayer.     

Interaction of a hydrophobic fluorescent probe with mouse embryo fibroblasts

Fluorescence photomicrographs show that the hydrophobic fluorescent probe 1-anilinonaphthalene-8-sulfonate (ANS) binds to hydrophobic components of intact 3T3 cells. Cells exposed to ANS exhibit fluorescence in the cytoplasm, intense nuclear membrane fluorescence, and well-defined fluorescent nucleoli. Fluorescence titrations of 3T3 cells with ANS show a decrease in fluorescence intensity, a blue shift of native cell emission with increasing ANS concentration and the appearance of a new peak due to ANS fluorescence. These fluorescence effects are ascribed to energy transfer processes involving bound ANS and the tryptophan and tyrosine residues of cellular proteins. ANS bound to 3T3 cells appears to quench the long wavelength component of the cellular tryptophan fluorescence, resulting in an unmasking of tryptophan and tyrosine emission at shorter wavelengths.