The primary structure of the human retinol-binding protein

β-Lactoglobulin and Glycodelin: Two Sides of the Same Coin?

The two lipocalins, β-lactoglobulin (βLg) and glycodelin (Gd), are possibly the most closely related members of the large and widely distributed lipocalin family, yet their functions appear to be substantially different. Indeed, the function of β-lactoglobulin, a major component of ruminant milk, is still unclear although neonatal nutrition is clearly important. On the other hand, glycodelin has several specific functions in reproduction conferred through distinct, tissue specific glycosylation of the polypeptide backbone. It is also associated with some cancer outcomes. The glycodelin gene, PAEP, reflecting one of its names, progestagen-associated endometrial protein, is expressed in many though not all primates, but the name has now also been adopted for the β-lactoglobulin gene (HGNC, www.genenames.org). After a general overview of the two proteins in the context of the lipocalin family, this review considers the properties of each in the light of their physiological functional significance, supplementing earlier reviews to include studies from the past decade. While the biological function of glycodelin is reasonably well defined, that of β-lactoglobulin remains elusive.

Discovery and Structural Analysis to Improve the Enantioselectivity of Hydroxynitrile Lyase from Parafontaria laminata Millipedes for (R)-2-Chloromandelonitrile Synthesis

Hydroxynitrile lyase (HNL) catalyzes the reversible synthesis and degradation of cyanohydrins, which are important synthetic intermediates for fine chemical and pharmaceutical industries. Here, we report the discovery of HNL from Parafontaria laminata (PlamHNL) millipedes, purification of the HNL to homogeneity, expression of the gene for the enzyme in heterologous expression hosts, and increase in the reaction rate and enantioselectivity in the synthesis of 2-chloromandelonitrile by protein engineering. The recombinant PlamHNL expressed in Pichia pastoris is glycosylated and has a higher thermostability and pH stability than the nonglycosylated HNL expressed in Escherichia coli. PlamHNL showed a unique wide substrate specificity among other millipede HNLs acting on various cyanohydrins, including 2-chloromandelonitrile, a key intermediate for the antithrombotic agent clopidogrel. We solved the X-ray crystal structure of the PlamHNL and found that the catalytic residues were almost identical to those of HNL from Chamberlinius hualienensis, although the forming binding cavity was different. In order to improve the catalytic activity and stereoselectivity, a computational structure-guided directed evolution approach was performed by an enzyme-substrate docking simulation at all of the residues that were exposed on the surface of the active site. The PlamHNL-N85Y mutant showed higher conversion (91% conversion with 98.2% ee of the product) than the wild type (76% conversion with 90% ee of the product) at pH 3.5 and 25 °C for 30 min of incubation. This study shows the diversity of millipede HNLs and reveals the molecular basis for improvement of the activity and stereoselectivity of the wild-type HNL to increase the reaction rate and enantioselectivity in the synthesis of 2-chloromandelonitrile.

Human plasma lipocalins and serum albumin: Plasma alternative carriers?

Lipocalins are an evolutionarily conserved family of proteins that bind and transport a variety of exogenous and endogenous ligands. Lipocalins share a conserved eight anti-parallel β-sheet structure. Among the different lipocalins identified in humans, α-1-acid glycoprotein (AGP), apolipoprotein D (apoD), apolipoprotein M (apoM), α1-microglobulin (α1-m) and retinol-binding protein (RBP) are plasma proteins. In particular, AGP is the most important transporter for basic and neutral drugs, apoD, apoM, and RBP mainly bind endogenous molecules such as progesterone, pregnenolone, bilirubin, sphingosine-1-phosphate, and retinol, while α1-m binds the heme. Human serum albumin (HSA) is a monomeric all-α protein that binds endogenous and exogenous molecules like fatty acids, heme, and acidic drugs. Changes in the plasmatic levels of lipocalins and HSA are responsible for the onset of pathological conditions associated with an altered drug transport and delivery. This, however, does not necessary result in potential adverse effects in patients because many drugs can bind both HSA and lipocalins, and therefore mutual compensatory binding mechanisms can be hypothesized. Here, molecular and clinical aspects of ligand transport by plasma lipocalins and HSA are reviewed, with special attention to their role as alterative carriers in health and disease.

Cis-retinoids and the chemistry of vision

We discuss here principal biochemical transformations of retinoid molecules in the visual cycle. We focus our analysis on the accumulating evidence of alternate pathways and functional redundancies in the cycle. The efficiency of the visual cycle depends, on one hand, on fast regeneration of the photo-bleached chromophores. On the other hand, it is crucial that the cyclic process should be highly selective to avoid accumulation of byproducts. The state-of-the-art knowledge indicates that single enzymatically active components of the cycle are not strictly selective and may require chaperones to enhance their rates. It appears that protein-protein interactions significantly improve the biological stability of the visual cycle. In particular, synthesis of thermodynamically less stable 11-cis-retinoid conformers is favored by physical interactions of the isomerases present in the retina with cellular retinaldehyde binding protein.

Acute phase reactants as novel predictors of cardiovascular disease

Acute phase reaction is a systemic response which usually follows a physiological condition that takes place in the beginning of an inflammatory process. This physiological change usually lasts 1-2 days. However, the systemic acute phase response usually lasts longer. The aim of this systemic response is to restore homeostasis. These events are accompanied by upregulation of some proteins (positive acute phase reactants) and downregulation of others (negative acute phase reactants) during inflammatory reactions. Cardiovascular diseases are accompanied by the elevation of several positive acute phase reactants such as C-reactive protein (CRP), serum amyloid A (SAA), fibrinogen, white blood cell count, secretory nonpancreatic phospholipase 2-II (sPLA2-II), ferritin, and ceruloplasmin. Cardiovascular disease is also accompanied by the reduction of negative acute phase reactants such as albumin, transferrin, transthyretin, retinol-binding protein, antithrombin, and transcortin. In this paper, we will be discussing the biological activity and diagnostic and prognostic values of acute phase reactants with cardiovascular importance. The potential therapeutic targets of these reactants will be also discussed.

Clinical indications for plasma protein assays: transthyretin (prealbumin) in inflammation and malnutrition

A large number of circumstances are associated with reduced serum concentrations of transthyretin (TTR), or prealbumin. The most common of these is the acute phase response, which may be due to inflammation, malignancy, trauma, or many other disorders. Some studies have shown a decrease in hospital stay with nutritional therapy based on TTR concentrations, but many recent studies have shown that concentrations of albumin, transferrin, and transthyretin correlate with severity of the underlying disease rather than with anthropometric indicators of hypo- or malnutrition. There are few if any conditions in which the concentration of this protein by itself is more helpful in diagnosis, prognosis, or follow up than are other clinical findings. In the majority of cases, the serum concentration of C-reactive protein is adequate for detection and monitoring of acute phase responses and for prognosis. Although over diagnosis and treatment of presumed protein energy malnutrition is probably not detrimental to most patients, the failure to detect other causes of decreased concentrations (such as serious bacterial infections or malignancy) of the so-called visceral or hepatic proteins could possibly result in increased morbidity or even mortality. In addition to these caveats, assays for TTR have a relatively high level of uncertainty ("imprecision"). Clinical evaluation--history and physical examination--should remain the mainstay of nutritional assessment.

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.

Role of conserved residues in structure and stability: tryptophans of human serum retinol-binding protein, a model for the lipocalin superfamily

Serum retinol binding protein (RBP) is a member of the lipocalin family, proteins with up-and-down beta-barrel folds, low levels of sequence identity, and diverse functions. Although tryptophan 24 of RBP is highly conserved among lipocalins, it does not play a direct role in activity. To determine if Trp24 and other conserved residues have roles in stability and/or folding, we investigated the effects of conservative substitutions for the four tryptophans and some adjacent residues on the structure, stability, and spectroscopic properties of apo-RBP. Crystal structures of recombinant human apo-RBP and of a mutant with substitutions for tryptophans 67 and 91 at 1.7 A and 2.0 A resolution, respectively, as well as stability measurements, indicate that these relatively exposed tryptophans have little influence on structure or stability. Although Trp105 is largely buried in the wall of the beta-barrel, it can be replaced with minor effects on stability to thermal and chemical unfolding. In contrast, substitutions of three different amino acids for Trp24 or replacement of Arg139, a conserved residue that interacts with Trp24, lead to similar large losses in stability and lower yields of native protein generated by in vitro folding. The results and the coordinated nature of natural substitutions at these sites support the idea that conserved residues in functionally divergent homologs have roles in stabilizing the native relative to misfolded structures. They also establish conditions for studies of the kinetics of folding and unfolding by identifying spectroscopic signals for monitoring the formation of different substructures.

Retinal anatomy and function of the transthyretin null mouse

Vitamin A (retinol) is vital for the normal development and function of many tissues in the body including the eye. The purpose of this project was to characterize the retinal anatomy and function of the transthyretin (TTR) null mouse. Mice lacking TTR have been constructed by homologous recombination. Immunocytochemistry was performed to localize short and mid-long wavelength cone opsins as well as morphological examination of the entire retina in wild-type and TTR null mice. Visual function was assessed using the electroretinogram (ERG) and resulting waveforms were analysed in terms of receptoral and postreceptoral components. Retinal morphology of the TTR null mouse was normal. In addition, short and mid-long wavelength cone opsins were localized normally in both TTR null and wild-type retinae. Consistent with these findings, TTR null mice show no anomalies of receptoral (P3) nor post-receptoral (b-wave) ERG components compared with wild-type mice. The results suggest that although circulating plasma levels of retinol and retinol binding protein (RBP) are extremely low, this reduction has little effect on the retinal structure or function of the TTR null mouse. These data are consistent with the existence of mechanisms for the transport of retinol to the retina independent of the classical retinol-RBP-TTR complex.

Developmental expression, tissue distribution and hormonal regulation of fish (Sparus aurata) serum retinol-binding protein

Retinol-binding protein (RBP) is the specific carrier of retinol in vertebrates and forms a 1:1 complex with transthyretin (TTR). A cDNA encoding serum RBP was cloned from liver and 7-day larvae of the marine fish Sparus aurata. The mature protein is 176 amino acids long and shows sequence identity of 77-78%, 56%, 63% and 62% with rainbow trout, Xenopus, chicken and human RBP, respectively. Northern blot analysis of hepatic RBP revealed two transcripts: a major one of approximately 1.4-1.5 kb and a minor of approximately 0.7 kb. Distribution of RBP mRNA in various tissues was studied by RT-PCR and showed high expression in liver and skin, and low expression in brain, kidney and gill filament (20-35% of the level in liver). RBP expression in intestine, pyloric caeca, muscle and pituitary was estimated to be approximately 7-14% of the level in liver. The ontogeny of RBP expression in S. aurata was examined in unfertilized eggs, embryos and larvae by using RT-PCR followed by hybridization with a specific probe. RBP transcript was found in all larval stages studied. Very low levels of RBP mRNA were detected in unfertilized eggs and in embryos 8 h after fertilization with a gradual increase at 12 h and 15-16 h post-fertilization. A single injection of estradiol-17beta to S. aurata immature, bisexual fish or to adult males reduced steady-state levels of hepatic RBP by 37 and 25%, respectively. The same treatment induced vitellogenin expression. The present data suggest that in fish, liver is the main site of RBP synthesis, but that RBP may have an important function in fish skin. RBP is expressed early in embryonic development and in fish its expression can be down regulated by estrogen.

The transthyretin-retinol-binding protein complex

Transthyretin (TTR, formerly called prealbumin), one of the transporters of the hormone thyroxine and the lipocalin retinol-binding protein (RBP), the specific carrier of the vitamin, are known to form, under physiological conditions, a macromolecular complex that is believed to play an important physiological role: prevention of glomerular filtration of the low molecular weight RBP in the kidneys. The physiological significance of complex formation is discussed first, followed by a brief description of the three-dimensional structure of the two participating proteins. The two X-ray models of the complex available are subsequently discussed and compared and finally the non-crystallographic evidence that supports these models is reviewed.

Biologically active factors in bovine milk and dairy byproducts: influence on cell culture

Substantial progress has been made in our knowledge of the biological properties of mammal milks. Many nutritional, biochemical, immunological, or other biological properties have been studied in mature or industrially processed bovine milk as well as in human milk and colostrum. This article is a critical review of selected publications covering (1) the use of bovine milk or dairy byproducts (processed acid and enzymatic whey fractions) as a serum substitute for cell cultures, (2) specific factors in bovine milk and industrially processed milk the affect cell proliferation, and (3) the known functional and biological roles of two whey proteins: beta-lactoglobulin and the PP3 component.

Localization of cellular retinoid-binding proteins suggests specific roles for retinoids in the adult central nervous system

Retinoic acid, the active metabolite of retinoids (vitamin A compounds), is thought to act as a gene regulator via ligand-activated transcription factors. In order to investigate possible roles of retinoids and retinoid-controlled gene expression in brain function, we have used immunohistochemistry to localize the possible presence of two intracellular retinoid-binding proteins, cellular retinol-binding protein type I and cellular retinoic acid-binding protein type I, in the adult rat central nervous system. We find a widespread, yet distinct, presence of these two binding proteins in the brain and spinal cord. Most of the immunoreactivity is neuronal, including cell somata, as well as dendritic and axonal processes and axon terminals. Cellular retinol-binding protein type I-immunoreactivity is also found in the walls of cerebral blood vessels, the meninges, the choroid plexus, certain ependymal cells, tanocytes and certain other glial elements. The cellular retinol-binding protein type I- and cellular retinoic acid-binding protein type I-immunoreactivity patterns appear to be almost exclusively non-overlapping. Very strong cellular retinol-binding protein type I-immunoreactivity is found in the dendritic layers of the hippocampal formation and dentate gyrus. Cellular retinol-binding protein type I-immunoreactivity is also present in layer 5 cortical pyramidal neurons and neurons in the glomerular layer of the olfactory bulb. Many other areas, e.g. hypothalamic nuclei and amygdala areas, contain networks of varicose cellular retinol-binding protein type I-immunoreactive nerve fibers. The medial amygdaloid nucleus contains strongly cellular retinol-binding protein type I-positive neurons. Cellular retinoic acid-binding protein type I-immunoreactivity is more restricted in the adult brain. Strong cellular retinoic acid-binding protein type I-immunoreactivity is, however, found in a population of medium-sized neurons scattered throughout the striatum, in neurons in the glomerular layer of the olfactory bulb, the olfactory nerve and in a group of nerve cells close to the third ventricle in hypothalamus. The remarkably selective patterns of cellular retinol-binding protein type I- and cellular retinoic acid-binding protein type I-immunoreactivity discovered in the adult rat brain suggest that retinoids have important roles as regulators of gene expression in normal brain function. The high levels of cellular retinol-binding protein type I-immunoreactivity found in hippocampus suggest that one such role might relate to brain plasticity.

The development of an assay for human neutrophil lipocalin (HNL)--to be used as a specific marker of neutrophil activity in vivo and vitro

Human neutrophil lipocalin (HNL) is a newly discovered protein from human neutrophil secretory granules. A double-antibody radioimmunoassay (RIA) was developed for the measurement of HNL in various body fluids and its high specificity was confirmed by the absence of cross-reaction with other granulocyte granule proteins. The RIA measures HNL within the range of 4-256 micrograms/l. The intra- and interassay coefficients of variation were less than 6% and 10%, respectively. When HNL was added to serum samples full recovery was obtained. Sera and plasma from 100 apparently healthy individuals revealed a mean level of 78.40 micrograms/l (range 37.95-190.87 micrograms/l) in serum and a mean level of 50.65 micrograms/l (range 30.51-105.8 micrograms/l) in EDTA-plasma. The distribution of HNL after gel filtration indicated that HNL exists mainly in two major forms, dimer and monomer. This, in addition to the excellent recovery, suggests that these major forms of HNL do not bind to compounds in serum or plasma that would interfere with the assay. The high specificity, sensitivity, reproducibility and accuracy of the present assay should facilitate the measurement of HNL in blood and other body fluids.

Rapid validation of molecular structures of biological samples by electrospray-mass spectrometry

A short account is presented of the method of measuring molecular masses (M(r)) of pure biological samples by electrospray ionisation mass spectrometry. It is demonstrated that the technique yields M(r) values with an effective accuracy equal to or better than 0.008% of the calculated M(r), provided that the correct molecular structure is employed in the calculation. It is therefore recommended that this method of measuring M(r)'s should be considered to form an essential part of all studies aimed at elucidating the molecular structure of purified biological macromolecules or for confirming the identity of labelled samples of such molecules.

Antigenic determinants of human serum retinol binding protein as probed with monoclonal antibodies

Monoclonal antibodies raised against human serum retinol-binding protein (hRBP) were used as probes for the study of the antigenic determinants of hRBP and those shared with the same protein from other species. The antibodies could be classified into four distinct groups and react with the homologous proteins from the rat as well as the rabbit sera. Three of these antibodies recognize sequential or continuous epitopes while the remaining antibody is directed against a discontinuous or conformational epitope. By chemical cleavage with cyanogen bromide, the domains recognized by the monoclonal antibodies could be delineated. By solid-phase synthetic approach, the core sequences recognized by two of these monoclonal antibodies were identified to amino acid sequences 45-51 and 128-131 of the primary amino acid sequence of hRBP.

Mechanism of pH-induced release of retinol from retinol-binding protein

A hypothesis is proposed explaining the mechanism of pH-induced release of retinol from retinol-binding protein (RBP). A number of conservative positively charged side chains located on the retinol-binding face of the RBP molecule are involved in salt bridges with conservative negatively charged groups. At low pH these salt bridges are broken and the retinol-binding face of RBP holds from 8 to 12 positively charged groups, which can ensure a proper orientation of the RBP molecule relative to a negatively charged membrane, facilitating the release of retinol. The disruption of salt bridges and the electrostatic repulsion of positive charges can soften the structure of the molecule near the entrance to the retinol-binding pocket, which can trigger both the release of retinol and the transition of RBP to the molten globule state.

The primary structure of piscine (Oncorhynchus mykiss) retinol-binding protein and a comparison with the three-dimensional structure of mammalian retinol-binding protein

1. The primary structures of two variants of rainbow trout (Oncorhynchus mykiss) plasma retinol-binding protein (RBP) were determined and found to be approximately 60% identical with those of both human and Xenopus laevis RBPs. The comparable sequence similarities that we have found agree with the estimate of similar divergence times between bony fishes and mammals and between bony fishes and amphibians. The two piscine RBP variants differ by six amino acid substitutions at positions that are not crucial for the interaction with retinol, on the basis of the human RBP three-dimensional structure [Cowan, S. W., Newcomer, M. E. & Jones, T. A. (1990) Proteins Struct. Func. Genet. 8, 44-61]. 2. Models were developed for the three-dimensional structures of rainbow trout and X. laevis RBPs, based on that of human RBP. The overall three-dimensional structure appears to be very well preserved for RBPs isolated from vertebrate species for which the divergence time is 350-400 million years. At variance with an almost absolute conservation for the residues that participate in the formation of the retinol binding site in mammalian RBPs, several amino acid replacements are found for this part of the RBP molecule when the comparison is extended to piscine and amphibian RBPs. However, the only allowed amino acid replacements are either conservative or more than 0.4 nm distant from retinol. Besides the retinol binding site, a few regions at the protein surface appear to be rather conserved during phylogenetic development of vertebrates and, therefore, might be involved in molecular interactions.

Alpha 2u-globulin is the only member of the lipocalin protein superfamily that binds to hyaline droplet inducing agents

The rate-limiting step in chemically induced, male rat-specific hyaline droplet nephropathy is the reversible binding of a xenobiotic to alpha 2u-globulin. In this study, equilibrium saturation binding experiments were conducted to evaluate the in vitro binding of d-limonene-1,2-oxide (dLO) and 2,4,4-trimethyl-2-pentanol (TMP-OH) to alpha 2u-globulin and members of the alpha 2u-globulin protein superfamily. Both dLO and TMP-OH bound to alpha 2u-globulin, with Scatchard analysis yielding dissociation constants of 5.6 and 6.4 x 10(-7) M, respectively. The Bmax for binding (nmol bound/mg protein) was 50.7 and 61.1 for dLO and TMP-OH, respectively, yielding a molar ratio of approximately 1 for both ligands. The ability of dLO and TMP-OH to bind to human-derived alpha 1-acid glycoprotein, rat-derived retinol-binding protein, human protein-1, and bovine beta-lactoglobulin was also studied. These superfamily proteins are generally abundant in plasma, are freely filtered across the glomerulus, and can bind a wide range of ligands. However, neither dLO nor TMP-OH bound to any of the superfamily proteins. In contrast, under identical experimental conditions, alpha 1-acid glycoprotein did bind progesterone (Kd = 10(-6) M), whereas both beta-lactoglobulin and retinol-binding protein bound retinol (Kd = 10(-8) M for both proteins). These results indicate that, under conditions where alpha 2u-globulin superfamily proteins bind to established ligands, the proteins do not interact with hyaline droplet inducing agents. Thus, the interaction between male rat-specific nephrotoxicants and alpha 2u-globulin is unique to this protein. More importantly, these results provide direct evidence that the presence of the alpha 2u-globulin superfamily proteins does not predispose humans to develop hyaline droplet nephropathy and renal cancer from this class of chemicals.

Retinol-binding protein is in the molten globule state at low pH

Using far- and near-UV circular dichroism, viscosity, tryptophan fluorescence, NMR spectra, binding of a hydrophobic probe, and microcalorimetry, we have shown that the apo form of human retinol-binding protein (RBP) at neutral pH is in a rigid state with properties similar to those of holo-RBP. On the contrary, at acidic pH apo-RBP is in the molten globule state which has been earlier revealed for a number of proteins under mild denaturing conditions. We have also shown that, at equilibrium, the pH-induced retinol release from holo-RBP parallels denaturation of the apoprotein. These findings are consistent with our hypothesis that the transformation of RBP into the molten globule state is involved in the mechanism whereby retinol is delivered to target cells. In particular, a local acidic pH near the membrane surface of target cells might cause the transition of RBP to the molten globule state as well as the release of retinol.

Two distinct genes encode two major isoelectric forms of insecticyanin in the tobacco hornworm, Manduca sexta

Insecticyanin is a blue pigment found in the epidermis and hemolymph of the tobacco hornworm, Manduca sexta. Two distinct full-length cDNA species were isolated and shown to encode the two major isoelectric forms of insecticyanin, the a form (INS-a) and b form (INS-b). Sequence analysis of the two cDNA clones, pE1-I1 and Pe3-I2, reveals that both contain an 618-bp open reading frame which predicts an 189-amino-acid protein and an 17-amino-acid signal peptide. Comparison of the deduced INS-a and INS-b proteins show 13 amino acid differences, of which six are conserved. Three amino acid substitutions are also found between the deduced INS-b sequence and the sequenced INS-b protein isolated from the hemolymph. Isolation and characterization of five genomic clones revealed that pE1-I1 and pE3-I2 come from two different genes. Both INS-a and INS-b genes have four exons interrupted by three introns at the same positions. The two genes share 93% nucleotide similarity in the coding region. Moreover, the 'TATA box' and 'CAAT box' are completely conserved in the putative promoter regions of the two genes. Primer extension revealed that both INS-a and INS-b genes begin their transcription at position -52 relative to their translation initiation codon, ATG. We conclude that the two genes are the result of gene duplication.

A common epitope of β-lactoglobulin and serum retinol-binding proteins: Elucidation of its core sequence using synthetic peptides

One of the monoclonal antibodies raised against bovine beta-lactoglobulin reacted with human serum retinol binding protein. The finding that this monoclonal antibody also reacted with the serum retinol binding proteins isolated from other animals, suggested that this epitopic conformation is conserved among these proteins. Using ELISA and various synthetic peptides of defined sequence, we show in this paper that the epitope defined by this monoclonal antibody comprises of the highly conserved core sequence of DTDY present in beta-lactoglobulin and retinol binding proteins.

Three-dimensional structure and active site of three hydrophobic molecule-binding proteins with significant amino acid sequence similarity

We review our work on bovine and human retinol-binding protein (RBP), bovine beta lactoglobulin (BLG), and bovine odorant-binding protein (OBP). These three proteins share a sequence similarity high enough to justify the proposal that their three-dimensional structure ought to be quite similar, and they also share the function of similar or even identical hydrophobic ligand binding, although with a very different degree of specificity. Thus they constitute an ideal system to exhaustively explore the question of three-dimensional structure prediction from sequence similarity and the related question of binding site prediction for similar ligands. We have used x-ray diffraction techniques on single crystals of human and bovine RBP, bovine milk BLG, and bovine nasal mucosa OBP to investigate this problem. The results of these crystallographic studies indicate that to the level of resolution so far attained, the three-dimensional structure of these three proteins is reasonably predicted from the sequence similarity. The fold is the same and structural differences are rather subtle. Finally, we present experimental evidence that the binding sites of RBP, BLG, and OBP are in different regions of the molecules. Thus, it appears that although sequence alignment has correctly predicted the protein fold, it has incorrectly predicted the hydrophobic ligand-binding sites.

The piscine plasma retinol-binding protein. Purification, partial amino acid sequence and interaction with mammalian transthyretin of rainbow trout (Oncorhynchus mykiss) retinol-binding protein

1. Retinol-binding protein (RBP) has been isolated from the pooled plasma or rainbow trouts (Oncorhinchus mykiss) by gel filtration, hydrophobic interaction chromatography and ion-exchange chromatography. By this procedure two forms of the protein, both with a molecular mass (approximately 20 kDa) similar to that of mammalian RBP, were purified to homogeneity. Five amino acid substitutions have been found in the partial (about 60%) sequences of the two forms of trout RBP, which are presumably acetylated at their N terminus. The apparent participation of six conserved cysteines in the formation of disulphide bridges, as in human RBP, and the similarity (about 60%) of the amino acid sequence of trout and mammalian RBPs, indicate the existence of a similar overall structure organization in evolutionary distant RBPs. 2. Although the two forms of trout RBP are not physiologically involved in the formation of any protein--protein complex in plasma, they are capable of interacting with mammalian transthyretin, albeit with a binding affinity (K'd = 15-40 microM) considerably lower than that of mammalian RBP. Our data indicate that the two forms of trout RBP also possess the region that in mammalian RBP has the functional role of binding transthyretin. It is suggested that transthyretin (or a homologous protein) was modified, during phylogenetic development of the non mammalian vertebrates, to acquire a binding site for such a region of the RBP molecule.

The bovine plasma retinol-binding protein. Amino acid sequence, interaction with transthyretin, crystallization and preliminary X-ray data

1. The primary structure of bovine plasma retinol-binding protein (RBP) has been determined and found to be more than 90% identical to human and rabbit RBPs, and more than 80% identical to rat RBP. Main changes in amino acid sequence are observed in two regions on the surface of the protein molecule (residues 138-148 and 169-183). 2. The interactions of bovine RBP with bovine and human transthyretins were investigated using the technique of fluorescence polarization. Bovine RBP was able to form high affinity complexes (K'd = 0.34 +/- 0.02 microM) with both bovine and human transthyretins, displaying a stoichiometry of approximately 2 molecules RBP/molecule transthyretin in both cases. The sites that participate in protein-protein interactions are thus very similar, and this tends to exclude the involvement of the superficial regions more significantly substituted in mammalian RBPs (residues 138-151 and 167-183) in the protein-protein recognition. 3. Bovine RBP has been crystallized (space group P2(1)2(1)2(1), with a = 4.61 nm, b = 4.91 nm, c = 7.61 nm) and the crystals are suitable for high-resolution X-ray diffraction studies.

Monoclonal antibody studies of the antigenic determinants of human plasma retinol-binding protein

A battery of monoclonal antibodies (MoAbs) against human retinol-binding protein (RBP) was produced to obtain useful probes for the study of the antigenic determinants of RBP. The 12 antibodies all reacted with human RBP by immunoblotting. Based on antibody cross-competition radioimmunoassays, four distinct and different groups of antibodies were identified: group I, 1A4 and 2F4; group II, 1G10, 5C5, 6F4, and 7G3; group III, 5H6, 6C7, 10G5, and 14E3; and group IV, 5H9 and 13A1. Information about the epitopes of RBP recognized by these MoAbs was obtained by testing the reactivity of each antibody with human, rabbit, and rat RBPs by immunoblotting. Group I and group IV antibodies reacted to a similar extent with human, rabbit, and rat RBPs. Group II antibodies reacted strongly with human and rabbit RBPs, but reacted very weakly with rat RBP. Group III antibodies reacted strongly with human RBP, but did not react with rabbit or rat RBP. Thus, the epitopes for group I and group IV antibodies appear to be regions of the RBP molecule that are conserved across the three species, whereas group III antibodies recognized only human RBP. In a preliminary study, the reactivity of each antibody with purified cyanogen bromide fragments of RBP was tested by slot immunoblotting. None of the MoAbs reacted with any of the cyanogen bromide fragments. This study shows that MoAbs specific for at least four different regions of the RBP molecule can be produced; hence, RBP contains at least four major antigenic domains.

Crystallographic refinement of human serum retinol binding protein at 2A resolution

Human serum retinol binding protein (RBP) in complex with retinol has been crystallographically refined to an R-factor of 18.1% with 2A resolution data. The protein topology results in an anti-parallel beta-barrel that encapsulates the retinol ligand. A detailed description of the protein and the binding site is provided. Our structural work has helped to define a family of proteins, many of which are carrier proteins for smaller ligand molecules. We describe the structural basis for the conservation of sequence within the family.

Retinol-binding protein is synthesized in the mammalian eye

As the chromophoric component of the visual pigment, retinol plays an essential role in vision. In the plasma, retinol is transported by retinol-binding protein (RBP) in complex with transthyretin (TTR, prealbumin). In previous work we demonstrated intraocular synthesis of TTR. To determine whether RBP is also synthesized in the eye, we performed Northern and Western blot analysis of rat eye, and detected both RBP mRNA and immunoreactive RBP. Regional Northern analysis of bovine eye localized RBP mRNA to ciliary body/iris and retina/RPE. Preliminary immunohistochemical studies revealed a widespread but heterogeneous distribution of RBP in rat eye. We postulate that ocular RBP and TTR are involved in the intraocular translocation of retinol.

Molecular structure of the bilin binding protein (BBP) from Pieris brassicae after refinement at 2.0 A resolution

The bilin binding protein (BBP) from the insect Pieris brassicae has been analysed for amino acid sequence, spectral properties and three-dimensional structure. The crystal structure that had been determined by isomorphous replacement has been refined at 2.0 A (1 A = 0.1 nm) resolution to an R-value of 0.20. The asymmetric unit contains four independent subunits of BBP. The co-ordinate differences are 0.25 A, in accord with the estimated error in co-ordinates. The polypeptide chain fold is characterized by an eight-stranded barrel. The connecting loops splay out at the upper end of the barrel and open it, whilst the lower end is closed. The overall shape resembles a calyx. The biliverdin IX gamma chromophore is located in a central cleft at the upper end of the barrel. The bilatriene moiety is in cyclic helical geometry with configuration Z,Z,Z and conformation syn,syn,syn. The geometry is in accord with the spectral properties and permits a correlation between sign of the circular dichroism bands and sense of the bilatriene helices. The fold of BBP is related to retinol binding protein (RBP), as had been recognized in the preliminary analysis, although the amino acid sequences of RBP and BBP show only 10% homology. There are large differences in the loops at the upper end of the barrel, whilst the segments of the centre and the lower end of the barrel superimpose closely. The ligands of BBP and RBP, biliverdin and retinol, respectively, are also similarly located.

A survey on cytosolic non-enzymic proteins involved in the metabolism of lipophilic compounds: from organic anion binders to new protein families

This review deals with recent advances in the research of cytosolic non-enzymic proteins involved in the metabolism of lipophilic compounds. Emphasis is given to the important contribution of structural data in the understanding of the functional properties of these proteins and in the emergence of new protein families. The possibility that many of the 'cytosolic' proteins might be structure-bound and structure-forming in the living cell is discussed, with references to so far available structural data and to recent investigations on the architecture and biochemical composition of the cytoplasm. The aim of this review is to present in a condensed form (227 references) the evolution in the study of cytosolic proteins binding and transferring lipophilic compounds and to enable interested investigators to become aware of current concepts and perspectives in this active and steadily growing area of research.

Human apolipoprotein D gene: gene sequence, chromosome localization, and homology to the alpha 2u-globulin superfamily

The exons and bordering intron nucleotides of the human apolipoprotein D (apo D) gene have been sequenced. The protein-coding portion of the gene is divided into five exons which span approximately 12,000 bp. At least one intron interrupts the 5' untranslated region. The gene has been localized to the p14.2----qter region of human chromosome 3. Apo D shares homology with the alpha 2u-globulin superfamily of genes, including approximately 25% amino acid homology with human retinol-binding protein (RBP). Similarity of intron locations in both apo D and RBP suggests that these two genes derived from a common ancestor.

Crystallization, crystal structure analysis and preliminary molecular model of the bilin binding protein from the insect Pieris brassicae

The bilin binding protein of the butterfly Pieris brassicae has been prepared, crystallized and its crystal structure determined at high resolution using film and FAST area detector intensity data. The crystallographic asymmetric unit contains a tetramer of identical subunits with a molecular weight of about 90,000. The crystal structure was determined by isomorphous replacement. Use was made of the molecular symmetry to improve phases. A molecular interpretation of the electron density distribution and partial tracing of the polypeptide chain was possible without amino acid sequence information, as the fold is very similar to retinol binding protein. It is characterized by a beta-barrel formed by two orthogonal beta-sheets and an alpha-helix. The bilin pigment seems to be bound within the beta-barrel analogously to retinol in retinol binding protein. The tetramer in the crystal has C2 symmetry and is a dimer of dimers of quasi-equivalent subunits.

The complete amino acid sequence of human serum retinol-binding protein

The complete amino acid sequence of human serum Retinol-binding protein (RBP) including the distribution of its three disulfide bridges, has been determined. The protein consists of 182 amino acid residues, the order of which was determined following the isolation of five CNBr-fragments. Direct amino acid sequence analysis in an automatic liquid phase sequencer provided almost the entire sequences of the five CNBr-fragments. Several sets of enzymatically derived peptides of RBP were also used to elucidate the primary structure. RBP displays significant homology to bovine beta-lactoglobulin, human alpha 1-microglobulin and rat alpha 1-microglobulin. RBP contains an internal homology. Thus, residues 36 to 83 display statistically significant homology with residues 96 to 141.

The primary structure of the nonspecific lipid transfer protein (sterol carrier protein 2) from bovine liver

The primary structure of the nonspecific lipid transfer protein (sterol carrier protein 2) from bovine liver has been determined. The protein consists of a single polypeptide chain of 121 amino acid residues with serine as the amino-terminal and alanine as the carboxy-terminal residue. The protein contains one single cysteine and tryptophan residue and lacks tyrosine, histidine and arginine.

Factors affecting the concentration of free holo retinol-binding protein in human plasma

The total concentrations of retinol, retinol-binding protein and prealbumin were determined in plasma from twenty healthy men and sixty patients with various inflammatory conditions. These concentrations were all strongly correlated to each other and lower in the patient group. The concentration of free (not prealbumin-bound) holo retinol-binding protein, the presumed 'active' supplier of retinol to the tissues, was calculated. It was found not to be decreased in the patient group. Of the measured total concentrations and their possible ratios in the whole material, the retinol/prealbumin ratio showed the strongest correlation to the concentration of free holo retinol-binding protein. The importance of the concentration of free holo retinol-binding protein for the vitamin A supply to the cells was supported by calculations on data from the literature showing that this concentration better than the above-mentioned total concentrations distinguished between patients with normal and abnormal dark adaptation ability.

Effect of starvation and elective surgery on hand dynamometry and circulating concentration of various proteins

The effect of 4 days total starvation (water only) in five normal subjects on the circulating concentrations of various proteins was studied. Changes in plasma albumin and total protein concentrations were compared with those of six patients undergoing elective abdominal surgery with partial starvation and six patients undergoing orthopaedic surgery with adequate feeding - (0.126-0.146 MJ/kg/day and 1.2-1.4 g protein/kg/day). In a companion study hand grip strength was measured daily in ten normal subjects during starvation and in 18 patients undergoing surgery for hernia repair (n = 6), cholecystectomy (n = 6) and major abdominal surgery (n = 6). Starvation produced marked reductions (approximately 30%) in the circulating concentrations of retinol binding protein and prealbumin but did not significantly affect the plasma concentration of immunoglobulins (IgG, IgA, IgM) acute phase reactants (orosomucoid, haptoglobin, alpha(1) antitrypsin), albumin and total protein. On the other hand both types of elective surgery produced significant reductions in plasma albumin and total protein concentrations irrespective of feeding. Grip strength was not significantly altered by four days of starvation but surgery produced a temporary reduction in grip strength, the extent and duration of which was related to the severity of operation. This study helps to separate the effect of surgery and starvation on hand dynamometry and circulating protein concentrations and indicates their limitations as indicators of nutritional state.