Resolution of ligand positions by site-directed tryptophan fluorescence in tear lipocalin

Regioselective analysis of heat-induced conformational changes of β-lactoglobulin by quantitative liquid chromatography-mass spectrometry analysis of chemical labeling kinetics

β-Lactoglobulin is a main allergen in cow's milk; its allergenicity is strongly impacted by processing. To understand heat-induced epitope-specific effects, the present study analyzed regiospecific conformational changes of heated native β-lactoglobulin variant A (BLG-A). Complementary fluorescence spectroscopy methods indicated two denaturation phases comprising minor sequential conformational changes (25-75 °C) and complete transitions (80-90 °C). Regioselective conformational changes of BLG-A in the native state (25 °C), sequential (70 °C) and complete transition (90 °C) were determined by quantitative liquid chromatography-mass spectrometry analysis of chemical labeling kinetics covering 14 lysine residues and the N-terminus. Conformational changes in two phases were observed for N-terminus, K8 (both N-terminal chain), K60 (β-sheet C), K75 (β-sheet D), K77 (DE loop), K83 (β-sheet E), K100 and K101 (FG loop). The residues K14 (β-sheet A1), K47 (β-sheet B), K69, K70 (both β-sheet D), and K91 (β-sheet F) were not involved in conformational changes.

Tear Lipocalin and Lipocalin-Interacting Membrane Receptor

Tear lipocalin is a primate protein that was recognized as a lipocalin from the homology of the primary sequence. The protein is most concentrated in tears and produced by lacrimal glands. Tear lipocalin is also produced in the tongue, pituitary, prostate, and the tracheobronchial tree. Tear lipocalin has been assigned a multitude of functions. The functions of tear lipocalin are inexorably linked to structural characteristics that are often shared by the lipocalin family. These characteristics result in the binding and or transport of a wide range of small hydrophobic molecules. The cavity of tear lipocalin is formed by eight strands (A-H) that are arranged in a β-barrel and are joined by loops between the β-strands. Recently, studies of the solution structure of tear lipocalin have unveiled new structural features such as cation-π interactions, which are extant throughout the lipocalin family. Lipocalin has many unique features that affect ligand specificity. These include a capacious and a flexible cavity with mobile and short overhanging loops. Specific features that confer promiscuity for ligand binding in tear lipocalin will be analyzed. The functions of tear lipocalin include the following: antimicrobial activities, scavenger of toxic and tear disruptive compounds, endonuclease activity, and inhibition of cysteine proteases. In addition, tear lipocalin binds and may modulate lipids in the tears. Such actions support roles as an acceptor for phospholipid transfer protein, heteropolymer formation to alter viscosity, and tear surface interactions. The promiscuous lipid-binding properties of tear lipocalin have created opportunities for its use as a drug carrier. Mutant analogs have been created to bind other molecules such as vascular endothelial growth factor for medicinal use. Tear lipocalin has been touted as a useful biomarker for several diseases including breast cancer, chronic obstructive pulmonary disease, diabetic retinopathy, and keratoconus. The functional possibilities of tear lipocalin dramatically expanded when a putative receptor, lipocalin-interacting membrane receptor was identified. However, opposing studies claim that lipocalin-interacting membrane receptor is not specific for lipocalin. A recent study even suggests a different function for the membrane protein. This controversy will be reviewed in light of gene expression data, which suggest that tear lipocalin has a different tissue distribution than the putative receptor. But the data show lipocalin-interacting membrane receptor is expressed on ocular surface epithelium and that a receptor function here would be rational.

The chain order of binary unsaturated lipid bilayers modulated by aromatic-residue-containing peptides: an ATR-FTIR spectroscopy study

It highlights the importance of aromatic residues in influencing peptide binding to the membrane, demonstrates that the stability of the membranes depends on the lipid composition and the sequence, structural context, and orientation of the peptides.

Exploring the process-structure-function relationship of horseradish peroxidase through investigation of pH- and heat induced conformational changes

Given the importance of peroxidase as an indicator for the preservation of vegetables by heat treatment, the present study is focused on enzyme behavior under different pH and temperature conditions, in terms of process-structure-function relationships. Thus, the process-structure-function relationship of peroxidase was investigated by combining fluorescence spectroscopy, in silico prediction methods and inactivation kinetic studies. The fluorescence spectra indicated that at optimum pH value, the Trp(117) residue is not located in the hydrophobic core of the protein. Significant blue- and red-shifts were obtained at different pH values, whereas the heat-treatment did not cause significant changes in Trp and Tyr environment. The ANS and quenching experiments demonstrated a more flexible conformation at lower pH and respectively at higher temperature. On the other hand molecular dynamics simulations at different temperatures highlighted that the secondary structure appeared better preserved against temperature, whereas the tertiary structure around the heme was more affected. Temperature dependent changes in the hydrogen bonding and ion paring involving amino acids from the heme-binding region (His(170) and Asp(247)) might trigger miss-coordination of the heme iron atom by His(170) residue and further enzyme activity loss.

Exploring protein solution structure: Second moments of fluorescent spectra report heterogeneity of tryptophan rotamers

Trp fluorescent spectra appear as a log-normal function but are usually analyzed with λmax, full width at half maximum, and the first moment of incomplete spectra. Log-normal analyses have successfully separated fluorescence contributions from some multi-Trp proteins but deviations were observed in single Trp proteins. The possibility that disparate rotamer environments might account for these deviations was explored by moment spectral analysis of single Trp mutants spanning the sequence of tear lipocalin as a model. The analysis required full width Trp spectra. Composite spectra were constructed using log-normal analysis to derive the inaccessible blue edge, and the experimentally obtained spectra for the remainder. First moments of the composite spectra reflected the site-resolved secondary structure. Second moments were most sensitive for spectral deviations. A novel parameter, derived from the difference of the second moments of composite and simulated log-normal spectra correlated with known multiple heterogeneous rotamer conformations. Buried and restricted side chains showed the most heterogeneity. Analyses applied to other proteins further validated the method. The rotamer heterogeneity values could be rationalized by known conformational properties of Trp residues and the distribution of nearby charged groups according to the internal Stark effect. Spectral heterogeneity fits the rotamer model but does not preclude other contributing factors. Spectral moment analysis of full width Trp emission spectra is accessible to most laboratories. The calculations are informative of protein structure and can be adapted to study dynamic processes.

Double tryptophan exciton probe to gauge proximal side chains in proteins: augmentation at low temperature

The circular dichroic (CD) exciton couplet between tryptophans and/or tyrosines offers the potential to probe distances within 10 Å in proteins. The exciton effect has been used with native chromophores in critical positions in a few proteins. Here, site-directed mutagenesis created double tryptophan probes for key sites of a protein (tear lipocalin). For tear lipocalin, the crystal and solution structures are concordant in both apo- and holo-forms. Double tryptophan substitutions were performed at sites that could probe conformation and were likely within 10 Å. Far-UV CD spectra of double Trp mutants were performed with controls that had noninteracting substituted tryptophans. Low temperature (77 K) was tested for augmentation of the exciton signal. Exciton coupling appeared with tryptophan substitutions at positions within loop A-B (28 and 31, 33), between loop A-B (28) and strand G (103 and 105), as well as between the strands B (35) and C (56). The CD exciton couplet signals were amplified 3-5-fold at 77 K. The results were concordant with close distances in crystal and solution structures. The exciton couplets had functional significance and correctly assigned the holo-conformation. The methodology creates an effective probe to identify proximal amino acids in a variety of motifs.

pH-induced structural changes of tyrosinase from Agaricus bisporus using fluorescence and in silico methods

BACKGROUND

Tyrosinases are involved in enzymatic browning reactions in damaged fruits during post-harvest handling and processing. The overall structure of tyrosinase from Agaricus bisporus mushrooms at different pH values was monitored using fluorescence spectroscopy and molecular dynamics simulations.

RESULTS

When the pH value was increased from 6.0 to 9.0, the protein passed through several structural intermediates, including the tetramer, trimer and dimer stages. Changes in the secondary and tertiary structure of tyrosinase at neutral pH were outlined after running molecular dynamics simulations. A detailed check at the single-molecule level by means of molecular modeling tools suggested that the most important contribution to the fluorescence intensity is given by the H subunits with seven Trp and nine Tyr residues exposed to the solvent, whereas the lectin-like folded L subunits have only six Trp and three Tyr residues, of which only Trp(15) , Trp(59) and Trp(93) are partially exposed to the solvent.

CONCLUSIONS

The results indicated that the enzyme was sensitive to pH. The experimental results revealed the unfolding of the native tetrameric enzyme in acidic pH range, causing exposure of the hydrophobic residues.

New insights into heat induced structural changes of pectin methylesterase on fluorescence spectroscopy and molecular modeling basis

Heat-induced structural changes of Aspergillus oryzae pectin methylesterase (PME) were studied by means of fluorescence spectroscopy and molecular modeling, whereas the functional enzyme stability was monitored by inactivation studies. The fluorescence spectroscopy experiments were performed at two pH value (4.5 and 7.0). At both pH values, the phase diagrams were linear, indicating the presence of two molecular species induced by thermal treatment. A red shift of 7 nm was observed at neutral pH by increasing temperature up to 60°C, followed by a blue shift of 4 nm at 70°C, suggesting significant conformational rearrangements. The quenching experiments using acrylamide and iodide demonstrate a more flexible conformation of enzyme with increasing temperature, especially at neutral pH. The experimental results were complemented with atomic level observations on PME model behavior after performing molecular dynamics simulations at different temperatures. The inactivation kinetics of PME in buffer solutions was fitted using a first-order kinetics model, resulting in activation energy of 241.4±7.51 kJ mol(-1).

A simple model-free method for direct assessment of fluorescent ligand binding by linear spectral summation

Fluorescent tagged ligands are commonly used to determine binding to proteins. However, bound and free ligand concentrations are not directly determined. Instead the response in a fluorescent ligand titration experiment is considered to be proportional to the extent of binding and, therefore, the maximum value of binding is scaled to the total protein concentration. Here, a simple model-free method is presented to be performed in two steps. In the first step, normalized bound and free spectra of the ligand are determined. In the second step, these spectra are used to fit composite spectra as the sum of individual components or linear spectral summation. Using linear spectral summation, free and bound 1-Anilinonaphthalene-8-Sulfonic Acid (ANS) fluorescent ligand concentrations are directly calculated to determine ANS binding to tear lipocalin (TL), an archetypical ligand binding protein. Error analysis shows that the parameters that determine bound and free ligand concentrations were recovered with high certainty. The linear spectral summation method is feasible when fluorescence intensity is accompanied by a spectral shift upon protein binding. Computer simulations of the experiments of ANS binding to TL indicate that the method is feasible when the fluorescence spectral shift between bound and free forms of the ligand is just 8 nm. Ligands tagged with environmentally sensitive fluorescent dyes, e.g., dansyl chromophore, are particularly suitable for this method.

Cation-π interactions in lipocalins: structural and functional implications

The cation-π interaction impacts protein folding, structural stability, specificity, and molecular recognition. Cation-π interactions have been overlooked in the lipocalin family. To fill this gap, these interactions were analyzed in the 113 crystal and solution structures from the lipocalin family. The cation-π interactions link previously identified structurally conserved regions and reveal new motifs, which are beyond the reach of a sequence alignment algorithm. Functional and structural significance of the interactions were tested experimentally in human tear lipocalin (TL). TL, a prominent and promiscuous lipocalin, has a key role in lipid binding at the ocular surface. Ligand binding modulation through the loop AB at the "open" end of the barrel has been erroneously attributed solely to electrostatic interactions. Data revealed that the interloop cation-π interaction in the pair Phe28-Lys108 contributes significantly to stabilize the holo-conformation of the loop AB. Numerous energetically significant and conserved cation-π interactions were uncovered in TL and throughout the lipocalin family. Cation-π interactions, such as the highly conserved Trp17-Arg118 pair in TL, were educed in low temperature experiments of mutants with Trp to Tyr substitutions.

Tear lipocalin: structure and function

Lipocalins are a family of diverse low molecular weight proteins that act extracellularly. They use multiple recognition properties that include 1) ligand binding to small hydrophobic molecules, 2) macromolecular complexation with other soluble macromolecules, and 3) binding to specific cell surface receptors to deliver cargo. Tear lipocalin (TLC) is a major protein in tears and has a large ligand-binding cavity that allows the lipocalin to bind an extensive and diverse set of lipophilic molecules. TLC can also bind to macromolecules, including the tear proteins lactoferin and lysozyme. The receptor to which TLC binds is termed tear lipocalin-interacting membrane receptor (LIMR). LIMR appears to work by endocytosis. TLC has a variety of suggested functions in tears, including regulation of tear viscosity, binding and release of lipids, endonuclease inactivation of viral DNA, binding of microbial siderophores (iron chelators used to deliver essential iron to bacteria), serving as a biomarker for dry eye, and possessing anti-inflammatory activity. Additional research is warranted to determine the actual functions of TLC in tears and the presence of its receptor on the ocular surface.

Excited protein states of human tear lipocalin for low- and high-affinity ligand binding revealed by functional AB loop motion

Human tear lipocalin (TL), a prominent member of lipocalin family, exhibits functional and structural promiscuity. The plasticity of loop regions modulates entry to the ligand pocket at the "open" end of the eight-stranded beta-barrel. Site-directed multi-distance measurements using fluorescence resonance energy transfer between functional loops register two excited protein states for low- and high-affinity ligand binding. At low pH, the longest loop AB adopts the conformation of the low-affinity excited protein state that matches the crystal structure of holo-TL at pH 8. A "crankshaft" like movement is detected for the loop AB in a low pH transition. At pH 7.3 the holo-protein assumes a high-affinity excited protein state, in which the loop AB is more compact (RMS=3.1A). In the apo-holo transition, the reporter Trp 28 moves about 4.5A that reflects a decrease in distance between Glu27 and Lys108. This interaction fixes the loop AB conformation for the high-affinity mode. No such movement is detected at low pH, where Glu27 is protonated. Data strongly indicate that the protonation state of Glu27 modulates the conformation of the loop AB for high- and low-affinity binding.

Tear lipocalin captures exogenous lipid from abnormal corneal surfaces

Purpose. The cornea is protected by apical hydrophilic transmembrane mucins and tears. In pathologic states the mucin barrier is disrupted, creating potential for meibomian lipids to adhere more strongly. Undisplaced lipids create an unwettable surface. The hypothesis that pathologic ocular surfaces alter lipid binding and the ability of tear proteins to remove lipids was tested. Methods. Corneas with pathologic surfaces were studied for lipid adhesion and removal by tears. Capture of fluorescence-labeled phospholipids by human tears was assessed by steady state fluorometry. Tear proteins were separated by gel filtration chromatography and analyzed for bound lipids. Results. Contact angle measurements revealed strong lipid adherence to corneas submerged in buffer. Lower contact angles are observed for lipids on completely de-epithelialized corneas compared with intact corneas (P = 0.04). Lipid removal from these surfaces is greater with whole tears than with tears depleted of tear lipocalin (P < 0.0005). Significantly fewer lipids are captured by tears from Bowman's layer than from epithelial-bearing surfaces (P < 0.025). The only tear component to bind the fluorescence-tagged lipid is tear lipocalin. The histology of a rare case of dry eye disease demonstrates the dominant features of contemporaneous bullous keratopathy. Lipid sequestration from this cornea by tear lipocalin was robust. Conclusions. Lipid is captured by tear lipocalin from corneas with bullous keratopathy and dry eye. Lipid removal is slightly abrogated by greater lipid adhesion to Bowman's layer. Reduced secretion of tear lipocalin documented in dry eye disease could hamper lipid removal and exacerbate ocular surface pathology.

pH-Dependent conformational changes in tear lipocalin by site-directed tryptophan fluorescence

Tear lipocalin (TL), a major protein of human tears, binds a broad array of endogenous ligands. pH-dependent ligand binding in TL may have functional implications in tears. Previously, conformational selections of the AB and GH loops have been implicated in ligand binding by site-directed tryptophan fluorescence (SDTF). In this study, SDTF was applied to the AB and GH loops to investigate pH-driven conformational changes relevant to ligand binding. Both loops demonstrate significant but distinct conformational rearrangements over a wide pH range. In the low-pH transition, from 7.3 to 3.0, residues of the GH loop exhibit decreased solvent accessibilities. In acrylamide quenching experiments, the average quenching rate constant (k(q), accessibility parameter) of the residues in the GH loop is decreased approximately 38%, from 2.1 x 10(9) to 1.3 x 10(9) M(-1) s(-1). However, despite the significant changes in accessibilities for some residues in the AB loop, the average accessibility per residue remained unchanged (average k(q) = 1.2 M(-1) s(-1)). Accordingly, the low-pH transition induces conformational changes that reshuffle the accessibility profiles of the residues in the AB loop. A significant difference in the titration curves between the holo and apo forms of the W28 mutant suggests that the protonation states of the residues around position 28 modulate conformational switches of the AB loop relevant to ligand binding.

Intracavitary ligand distribution in tear lipocalin by site-directed tryptophan fluorescence

Site-directed tryptophan fluorescence has been successfully used to determine the solution structure of tear lipocalin. Here, the technique is extended to measure the binding energy landscape. Single Trp mutants of tear lipocalin are bound to the native ligand and an analogue tagged with a quencher group to both populate and discriminate the excited protein states. Steady-state and time-resolved fluorescence quenching data reveal the intracavitary state of the ligand. The static components of fluorescence quenching identify the residues where nonfluorescence complexes form. An asymmetric distribution of the ligand within the cavity reflects the complex energy landscape of the excited protein states. These findings suggest that the excited protein states are not unique but consist of many substates. The roughness of the binding energy landscape is about 2.5kBT. The excited protein states originate primarily from conformational selections of loops AB and GH, a portal region. In contrast to static quenching, the dynamic components of fluorescence quenching by the ligand are relevant to both local side chain and ligand dynamics. Apparent bimolecular rate constants for collisional quenching of Trp by the nitroxide moiety are approximately 1 / 5 x 10(12) M(-1) s(-1). Estimations made for effective ligand concentrations establish actual rate constants on the order of 12 x 10(9) M(-1) s(-1). Prior to exit from the cavity of the protein, ligands explore binding sites in nanoseconds. Although microsecond fluctuations are rate-limiting processes in ligand binding for many proteins, accompanying nanosecond motion may be necessary for propagation of ligand binding.

Ligand binding site of tear lipocalin: contribution of a trigonal cluster of charged residues probed by 8-anilino-1-naphthalenesulfonic acid

Human tear lipocalin (TL) exhibits diverse functions, most of which are linked to ligand binding. To map the binding site of TL for some amphiphilic ligands, we capitalized on the hydrophobic and hydrophilic properties of 8-anilino-1-naphthalenesulfonic acid (ANS). In single Trp mutants, resonance energy transfer from Trp to ANS indicates that the naphthalene group of ANS is proximate to Leu105 in the cavity. Binding energies of TL to ANS and its analogues reveal contributions from electrostatic interactions. The sulfonate group of ANS interacts strongly with the nonconserved intracavitary residue Lys114 and less with neighboring residues His84 and Glu34. This trigonal cluster of residues may play a role in the ligand recognition site for some negatively charged ligands. Because many drugs possess sulfonate groups, the trigonal cluster-sulfonate interaction can also be exploited as a lipocalin-based drug delivery mechanism. The binding of lauric acid and its analogues shows that fatty acids assume heterogeneous orientations in the cavity of TL. Predominantly, the hydrocarbon tail is buried in the cavity of TL and the carboxyl group is oriented toward the mouth. However, TL can also interact, albeit relatively weakly, with fatty acids oriented in the opposite direction. As the major lipid binding protein of tears, the ability to accommodate fatty acids in two opposing orientations may have functional implications for TL. At the aqueous-lipid interface, fatty acids whose carboxyl groups are positioned toward the aqueous phase are available for interaction with TL that could augment stability of the tear film.

Site-directed circular dichroism of proteins: 1Lb bands of Trp resolve position-specific features in tear lipocalin

The absorption spectra of N-acetyl-L-tryptophanamide in various solvents were resolved into the sums of the (1)L(a) and (1)L(b) components. The relative intensities of the 0-0 transitions of the (1)L(b) bands correlate linearly with the solvent polarity values (E(T)(N)). A novel strategy that uses a set of the experimental (1)L(b) bands was employed to resolve the near-UV circular dichroism (CD) spectra of tryptophanyl residues. Resolved spectral parameters from the single-tryptophan mutants of tear lipocalin (TL), F99W and Y87W, corroborate the fluorescence and structural data of TL. Analysis of the (1)L(b) bands of the Trp CD spectra in proteins is a valuable tool to obtain the local features. The dimethyl sulfoxide (DMSO)-like (1)L(b) band of Trp CD spectra may be used as a "fingerprint" to identify the tryptophanyl side chains in situations where the benzene rings of Trp have van der Waals interactions with the side chains of its nearest neighbor. In addition, the signs and intensities of the components hold information about the side chain conformations and dynamics in proteins. Combined with Trp mutagenesis, this method, which we call site-directed circular dichroism, is broadly applicable to various proteins to obtain the position-specific data.

Evidence for internal and external binding sites on human tear lipocalin

8-anilino-1-naphthalenesulfonic acid (ANS) is widely used as a probe for locating binding sites of proteins. To characterize the binding sites of tear lipocalin (TL), we studied ANS binding to apoTL by steady-state and time-resolved fluorescence. Deconvolution of ANS binding revealed that two lifetime components, 16.99ns and 2.76ns at pH 7.3, have dissociation constants of 0.58muM and 5.7muM, respectively. At pH 3.0, the lifetime components show decreased affinities with dissociation constants of 2.42muM and approximately 21muM, respectively. Selective displacement of ANS molecules from the ANS-apoTL complex by stearic acid discriminates the internal and external binding sites. Dependence of the binding affinity on ionic strength under various conditions provides strong evidence that an electrostatic interaction is involved. Time-resolved fluorescence is a promising tool to segregate multiple binding sites of proteins.

Oligomeric state of lipocalin-1 (LCN1) by multiangle laser light scattering and fluorescence anisotropy decay

Multiangle laser light scattering and fluorescence anisotropy decay measurements clarified the oligomeric states of native and recombinant tear lipocalin (lipocalin-1, TL). Native TL is monomeric. Recombinant TL (5-68 microM) with or without the histidine tag shows less than 7% dimer formation that is not in equilibrium with the monomeric form. Fluorescence anisotropy decay showed a correlation time of 9-10 ns for TL (10 microM-1 mM). Hydrodynamic calculations based on the crystallographic structure of a monomeric TL mutant closely concur with the observed correlation time. The solution properties calculated with HYDROPRO and SOLPRO programs from the available crystallographic structure of a monomeric TL mutant concur closely with the observed fluorescence anisotropy decay. The resulting model shows that protein topology is the major determinant of rotational correlation time and accounts for deviation from the Stokes-Einstein relation. The data challenge previous gel filtration studies to show that native TL exists predominantly as a monomer in solution rather than as a dimer. Delipidation of TL results in a formation of a complex oligomeric state (up to 25%). These findings are important as the dynamic processes in the tear film are limited by diffusional, translational as well as rotational, properties of the protein.

Lipocalins - a family portrait

Lipocalins are a widely distributed group of proteins whose common feature is the presence of six-or eight-stranded beta-barrel in their tertiary structure and highly conservative motifs short conserved region, (SCR) in their amino acid sequences. The presence of three SCRs is typical for kernel lipocalins, while outlier lipocalins have only one or two such regions. Owing to their ability to bind and transport small, hydrophobic molecules, lipocalins participate in the distribution of such substances. However, the physiological significance of lipocalins is not limited to transfer processes. They play an important role in the regulation of immunological and developmental processes, and are also involved in the reactions of organisms to various stress factors and in the pathways of signal transduction. Of special interest is the enzymatic activity found in a few members of the lipocalin family, as well as the interaction with natural membranes, both directly with lipids and through membrane-localized protein receptors.

The 1.8-Å Crystal Structure of Human Tear Lipocalin Reveals an Extended Branched Cavity with Capacity for Multiple Ligands

In contrast with earlier assumptions, which classified human tear lipocalin (Tlc) as an outlier member of the lipocalin protein family, the 1.8-A resolution crystal structure of the recombinant apoprotein confirms the typical eight-stranded antiparallel beta-barrel architecture with an alpha-helix attached to it. The fold of Tlc most closely resembles the bovine dander allergen Bos d 2, a well characterized prototypic lipocalin, but also reveals similarity with beta-lactoglobulin. However, compared with other lipocalin structures Tlc exhibits an extremely wide ligand pocket, whose entrance is formed by four partially disordered loops. The cavity deeply extends into the beta-barrel structure, where it ends in two distinct lobes. This unusual structural feature explains the known promiscuity of Tlc for various ligands, with chemical structures ranging from lipids and retinoids to the macrocyclic antibiotic rifampin and even to microbial siderophores. Notably, earlier findings of biological activity as a thiol protease inhibitor have no correspondence in the three-dimensional structure of Tlc, rather it appears that its proteolytic fragments could be responsible for this phenomenon. Hence, the present structural analysis sheds new light on the ligand binding activity of this functionally obscure but abundant human lipocalin.

Presence of tear lipocalin and other major proteins in lacrimal fluid of rabbits

The lipocalins are a highly divergent, ubiquitous family of proteins that commonly function in binding lipophilic molecules. Although a specific tear lipocalin is a major component of lacrimal fluid and tears in many mammals, there has been no definitive identification of such a protein in rabbit tears. The goals of this project were to identify the major proteins in rabbit (Oryctolagus cuniculus) lacrimal fluid, so as to determine if they include a lipocalin and, if such a protein is present, to determine its source. Lacrimal fluid was collected from NZW sexually mature female rabbits, and culture medium from rabbit lacrimal gland epithelial (acinar) and interstitial cells was isolated. Proteins from these fluids were separated by SDS-PAGE electrophoresis and analyzed by sequencing the intact proteins and sequencing or mass analysis of fragments derived by trypsin digestion. Proteins of approximately 85 and 67 kDa were identified as rabbit transferrin and serum albumin, respectively, while components of 17 and 7 kDa had N-terminal sequences identical to those of lipophilin CL and AL, respectively. BLAST searches of the nr database with the N-terminal sequence of a protein of 18 kDa did not identify any homologues. However, when used to scan the PROSITE database, it was found to contain a lipocalin signature sequence. It is closely related to two lipocalins previously isolated from rabbit saliva and nasal mucus. Further studies with the N-terminal and internal sequences confirmed that the lacrimal protein is a lipocalin that is truncated at the N-terminus as compared with other tear lipocalins and is more similar to odorant binding proteins from rodents.

Tear lipocalin: potential for selective delivery of rifampin

The potential of ligand binding proteins as drug carriers and delivery systems has recently sparked great interest. We investigated the potential of tear lipocalin (TL) to bind the antibiotic, rifampin, and the environmental conditions for controlled release. To determine if TL binds rifampin, gel filtration was used to isolate protein fractions of tears. Rifampin was detected by absorbance spectroscopy in the elution fractions containing TL. The bound complex of rifampin-TL generates optical activity at about 360 nm, indicating a unique conformation at the binding site. Rifampin has a higher affinity for TL (Kd=128 microM) than albumin. Rifampin is released from the TL calyx in acidic conditions and is displaced by palmitic acid. Autooxidation of free rifampin begins in minutes but is delayed by at least 3 h in the presence of TL. These properties are conducive to stabilization and delivery of rifampin to tubercles that are acidic and rich in fatty acids. These studies show the potential of TL as a carrier for rifampin with controlled release to a targeted environment.

Resolving near-ultraviolet circular dichroism spectra of single trp mutants in tear lipocalin

Near-ultraviolet circular dichroism (near-UV CD) spectra of tryptophan residues in proteins are complicated because the line shapes are derived from the overlap of both the 1L(a) and the 1L(b) electronic bands that vary independently. Contributing to this complexity, tryptophan near-UV CD spectra differ in the relative amplitude of the 0-0 vibronic band compared to the rest of the 1L(b) spectrum, an inherent feature that may result in poor fitting. To resolve this problem, a computer program that incorporated the separation of the 0-0 transition of 1L(b) component from the rest of the 1L(b) was written in LabVIEW and its amplitude was allowed to vary independently. This method showed dramatically improved fitting of 1L(a) and 1L(b) components in the near-UV CD tryptophan spectra in tear lipocalin mutants featuring low intensity of the 0-0 1L(b) component. Side chain dynamic characteristics (mobility and accessibility to the solvent) identified from different spectroscopic techniques were related to differences in Trp near-UV CD spectra. This method is broadly applicable to different types of Trp near-UV CD spectra.

Functional cavity dimensions of tear lipocalin

PURPOSE

We calibrated the cavity of tear lipocalin with a series of fluorescent labeled lipids of increasing chain length and varying diameter.

METHODS

Cavity length was assessed with competitive fluorescent assays in which DAUDA was displaced from apo-tear lipocalin with ligands of increasing carbon chain lengths from C12-C24. The concentrations of competitors that inhibit 50% of the binding of DAUDA (IC(50)) were compared. Functional diameters of tear lipocalin and beta-lactoglobulin were estimated with fatty acids bearing fluorescent labels of various diameters. The cavity dimensions of other lipocalins were derived from their published crystal structure coordinates.

RESULTS

In tear lipocalin, the binding affinities of fatty acids increased up to a carbon chain length of 18 (22.5 A) but remained constant from C18-C24. The cavity length of other lipocalins in crystal form were similar to tear lipocalin in solution. Tear lipocalin showed decreased binding affinities with progressively increasing ring dimensions of the ligand. In contrast to beta-lactoglobulin and retinol binding protein, tear lipocalin bound DAUDA and cholesterol in the calyx. Neither tear lipocalin nor beta-lactoglobulin bound P646 in their respective cavities. The calculated inter-sheet distances at the mouth of the crystallized lipocalins ranged from 16-22A.

CONCLUSIONS

Tear lipocalin is more promiscuous than beta-lactoglobulin or retinol binding protein because of a greater functional diameter. Differences in ligand specificity of the various lipocalins can not be explained simply by variation in cavity length or the intersheet distances at the calyx mouths as determined by crystal structure. Other factors may influence ligand specificity such as size and/or dynamic motion of loops between the beta strands.