The primary structure of bovine cellular retinoic acid-binding protein

Cellular Retinoic Acid Binding Proteins: Genomic and Non-genomic Functions and their Regulation

Cellular retinoic acid binding proteins (CRABPs) are high-affinity retinoic acid (RA) binding proteins that mainly reside in the cytoplasm. In mammals, this family has two members, CRABPI and II, both highly conserved during evolution. The two proteins share a very similar structure that is characteristic of a "β-clam" motif built up from10-strands. The proteins are encoded by two different genes that share a very similar genomic structure. CRABPI is widely distributed and CRABPII has restricted expression in only certain tissues. The CrabpI gene is driven by a housekeeping promoter, but can be regulated by numerous factors, including thyroid hormones and RA, which engage a specific chromatin-remodeling complex containing either TRAP220 or RIP140 as coactivator and corepressor, respectively. The chromatin-remodeling complex binds the DR4 element in the CrabpI gene promoter to activate or repress this gene in different cellular backgrounds. The CrabpII gene promoter contains a TATA-box and is rapidly activated by RA through an RA response element. Biochemical and cell culture studies carried out in vitro show the two proteins have distinct biological functions. CRABPII mainly functions to deliver RA to the nuclear RA receptors for gene regulation, although recent studies suggest that CRABPII may also be involved in other cellular events, such as RNA stability. In contrast, biochemical and cell culture studies suggest that CRABPI functions mainly in the cytoplasm to modulate intracellular RA availability/concentration and to engage other signaling components such as ERK activity. However, these functional studies remain inconclusive because knocking out one or both genes in mice does not produce definitive phenotypes. Further studies are needed to unambiguously decipher the exact physiological activities of these two proteins.

Cellular retinoic acid binding protein is associated with mitochondria

We report that immunohistochemical staining for cellular retinoic acid-binding protein (CRABP) was restricted to the cytoplasm of cortical cells in bovine adrenal. In contrast, staining for the similar protein, cellular retinol-binding protein (CRBP), was found throughout these cells. After transfections of CRABP and CRBP into cultured cells, immunofluorescence analyses again revealed cytoplasmic restriction only for CRABP, with a pronounced punctate appearance. Use of organelle-specific fluorochromes indicated that CRABP immunofluorescence overlaid exactly with the pattern of the mitochondrial-specific fluorochrome. Confirmation of this association came with subcellular fractionation of the adrenal cortex. CRABP, but not CRBP, co-sedimented with the mitochondria, a novel finding for a member of this superfamily of cellular lipid-binding proteins.

The fatty acid transport function of fatty acid-binding proteins

The intracellular fatty acid-binding proteins (FABPs) comprise a family of 14-15 kDa proteins which bind long-chain fatty acids. A role for FABPs in fatty acid transport has been hypothesized for several decades, and the accumulated indirect and correlative evidence is largely supportive of this proposed function. In recent years, a number of experimental approaches which more directly examine the transport function of FABPs have been taken. These include molecular level in vitro modeling of fatty acid transfer mechanisms, whole cell studies of fatty acid uptake and intracellular transfer following genetic manipulation of FABP type and amount, and an examination of cells and tissues from animals engineered to lack expression of specific FABPs. Collectively, data from these studies have provided strong support for defining the FABPs as fatty acid transport proteins. Further studies are necessary to elucidate the fundamental mechanisms by which cellular fatty acid trafficking is modulated by the FABPs.

Nuclear import of cellular retinoic acid-binding protein type I in mouse embryonic cells

Using confocal microscopy we show that cellular retinoic acid-binding protein type I (CRABP I), expressed in several embryonic cell types, displays a compartmentalized subcellular distribution. The protein was excluded from the nucleus in some cells, while in others it accumulated in the nucleus. In the rat cerebellar cell line ST15A, which expresses CRABP I, the protein was found in the cytoplasm with a prominent nuclear exclusion. Addition of retinoic acid to embryos in vivo and to ST15 A cells in vitro did not affect the localization of the protein. Localization of CRABP I and CRABP I fused to a nuclear localization signal expressed in transfected cells, suggested that cell-specific factors may regulate nuclear import of CRABP I. The potential role of a CRABP I-controlled nuclear import of retinoic acid is discussed.

From structure to function: possible biological roles of a new widespread protein family binding hydrophobic ligands and displaying a nucleotide binding site

A cytosolic 21-23 kDa protein isolated from bovine brain was demonstrated to bind hydrophobic ligands, particularly phosphatidylethanolamine. The protein was encountered in numerous tissues of several species. High expression of the mRNA encoding the 21-23 kDa protein was found in rat testes. Immunohistochemical studies showed the presence of the 21-23 kDa protein in the elongated spermatids and epididymal fluid of rat testis and in brain oligodendrocytes of developing rats. As the bovine, human and rat brain 21-23 kDa proteins had only few sequence homologies with already know proteins, ti was concluded that they belong to a new protein family. In order to get additional information on the structural features of the 21-23 kDa protein, we built a molecular model which displayed a nucleotide binding site. The affinity of the bovine brain 21-23 kDa protein towards nucleotides as well as its association with cytosolic proteins and small GTP-binding proteins were demonstrated. Recently, significant sequence homologies were found with an antigen from Onchocerca volvulus, a fruit fly odorant-binding protein and the yeast protein TFS1 which is a dosage-dependent suppressor of CDC25 mutations. A positive regulation of RAS is carried out by CDC25 product which facilitates the GDP/GTP exchange on RAS proteins. These results imply that 21-23 kDa proteins function in oxidoreduction reactions and signal mechanisms during cell growth and maturation.

Induction of retinoid resistance by all-trans retinoic acid in acute promyelocytic leukemia after remission

Acute promyelocytic leukemia (APL) results from a malignant process that leads to the accumulation in the blood and the bone marrow of myeloid precursor cells characterized by an abnormal behavior and a differentiation arrest. It aroused considerable interest well beyond the hematologic field during the last five years since APL has two unique features i) the remission of the disease obtained with all-trans retinoic acid (ATRA) treatment ii) the presence in APL blasts of an abnormal protein, the promyelocytic myeloid leukemia/retinoic acid receptor (PML/RAR alpha) protein. APL is characterized cytogenetically by a t(15;17) translocation which involves both the PML gene on chromosome 15 and the RAR alpha gene on chromosome 17 and gives rise to the PML/RAR alpha fusion protein.

The cellular retinoic acid binding proteins

The two cellular retinoic acid binding proteins, CRABP I and CRABP II, belong to a family of small cytosolic lipid binding proteins and are highly conserved during evolution. Both proteins are expressed during embryogenesis, particularly in the developing nervous system, craniofacial region and limb bud. CRABP I is also expressed in several adult tissues, however, in contrast, CRABP II expression appears to be limited to the skin. It is likely that these proteins serve as regulators in the transport and metabolism of retinoic acid in the developing embryo and throughout adult life. It has been proposed that CRABP I sequesters retinoic acid in the cytoplasm and prevents nuclear uptake of retinoic acid. A role in catabolism of retinoic acid has also been proposed. Recent gene targeting experiments have shown that neither of the two CRABPs are essential for normal embryonic development or adult life. Examination of CRABP I expression at subcellular resolution reveals a differential cytoplasmic and/or nuclear localization of the protein. A regulated nuclear uptake of CRABP I implies a role for this protein in the intracellular transport of retinoic acid. A protein mediated mechanism which controls the nuclear uptake of retinoic acid may play an important role in the transactivation of the nuclear retinoic acid receptors.

Crystal structures of cellular retinoic acid binding proteins I and II in complex with all-trans-retinoic acid and a synthetic retinoid

BACKGROUND

Retinoic acid (RA) plays a fundamental role in diverse cellular activities. Cellular RA binding proteins (CRABPs) are thought to act by modulating the amount of RA available to nuclear RA receptors. CRABPs and cellular retinol-binding proteins (CRBPs) share a unique fold of two orthogonal beta-sheets that encapsulate their ligands. It has been suggested that a trio of residues are the prime determinants defining the high specificity of CRBPs and CRABPs for their physiological ligands.

RESULTS

Bovine/murine CRABP I and human CRABP II have been crystallized in complex with their natural ligand, all-trans-RA. Human CRABP II has also been crystallized in complex with a synthetic retinoid, 'compound 19'. Their structures have been determined and refined at resolutions of 2.9 A, 1.8 A and 2.2 A, respectively.

CONCLUSIONS

The retinoid-binding site in CRABPs differs significantly from that observed in CRBP. Structural changes in three juxtaposed areas of the protein create a new, displaced binding site for RA. The carboxylate of the ligand interacts with the expected trio of residues (Arg132, Tyr134 and Arg111; CRABP II numbering). The RA ligand is almost flat with the beta-ionone ring showing a significant deviation (-33 degrees) from a cis conformation relative to the isoprene tail. The edge atoms of the beta-ionone ring are accessible to solvent in a suitable orientation for presentation to metabolizing enzymes. The bulkier synthetic retinoid causes small conformational changes in the protein structure.

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.

Two cytosolic protein families implicated in lipid-binding: main structural and functional features

1. According to the important biological role of fatty acids and phospholipids in cell membranes, two cytosolic proteins implicated in their binding and transport in brain were considered, namely: Fatty Acid-Binding Protein and basic 21 kDa protein. 2. They were reviewed as well as their related protein families. 3. Although the two protein groups do not present significant sequence homologies, they share several similar properties and might thus be implicated in common physiological functions.

Test of a model for the effects of retinoic acid on urodele limb regeneration

Previous studies have shown that in axolotls (Ambystoma mexicanum), retinoic acid (RA) treatment evokes pattern completion in limb regenerates derived from anterior and dorsal half zeugopodia (lower arms and legs), but causes regenerative failure in posterior and ventral half zeugopodia. Pattern completion in anterior and dorsal half limbs may be explained by postulating that intercalary regeneration occurs in the antero-posterior (AP) and dorsoventral (DV) axes between blastema cells that are posteriorized (anterior half limb) or ventralized (dorsal half limb) by RA, and circumferential anterodorsal cells that remain unaffected by RA and thus maintain their original positional identities. The contrasting regenerative failure of RA-treated posterior and ventral half zeugopodia may likewise be explained by postulating that all the blastema cells in the posterior half are posteriorized, and all the cells in the ventral half are ventralized by RA, thus eliminating differentials in transverse positional identity essential for blastema formation and outgrowth. To test these postulates we grafted blastemas derived from limbs halved in the AP and DV axes of control and RA-treated animals to untreated whole limb stumps and analyzed the patterns of supernumerary (SN) regeneration. The site or location of SN formation will demonstrate (1) whether RA has posteriorized and ventralized the positional identity of the blastema cells and (2) if blastema cells in the periphery of the anterodorsal quadrant of the limb are resistant to these RA-induced changes in positional identity.(ABSTRACT TRUNCATED AT 250 WORDS)

Primary structure of a photoactive yellow protein from the phototrophic bacterium Ectothiorhodospira halophila, with evidence for the mass and the binding site of the chromophore

The complete amino acid sequence of the 125-residue photoactive yellow protein (PYP) from Ectothiorhodospira halophila has been determined to be MEHVAFGSEDIENTLAKMDDGQLDGLAFGAIQLDGDGNILQYNAAEGDITGRDPKEVIGKNFFKDVAP+ ++ CTDSPEFYGKFKEGVASGNLNTMFEYTFDYQMTPTKVKVHMKKALSGDSYWVFVKRV. This is the first sequence to be reported for this class of proteins. There is no obvious sequence homology to any other protein, although the crystal structure, known at 2.4 A resolution (McRee, D.E., et al., 1989, Proc. Natl. Acad. Sci. USA 86, 6533-6537), indicates a relationship to the similarly sized fatty acid binding protein (FABP), a representative of a family of eukaryotic proteins that bind hydrophobic molecules. The amino acid sequence exhibits no greater similarity between PYP and FABP than for proteins chosen at random (8%). The photoactive yellow protein contains an unidentified chromophore that is bleached by light but recovers within a second. Here we demonstrate that the chromophore is bound covalently to Cys 69 instead of Lys 111 as deduced from the crystal structure analysis. The partially exposed side chains of Tyr 76, 94, and 118, plus Trp 119 appear to be arranged in a cluster and probably become more exposed due to a conformational change of the protein resulting from light-induced chromophore bleaching. The charged residues are not uniformly distributed on the protein surface but are arranged in positive and negative clusters on opposite sides of the protein. The exact chemical nature of the chromophore remains undetermined, but we here propose a possible structure based on precise mass analysis of a chromophore-binding peptide by electrospray ionization mass spectrometry and on the fact that the chromophore can be cleaved off the apoprotein upon reduction with a thiol reagent. The molecular mass of the chromophore, including an SH group, is 147.6 Da (+/- 0.5 Da); the cysteine residue to which it is bound is at sequence position 69.

A developmentally regulated gene of Echinococcus granulosus codes for a 15.5-kilodalton polypeptide related to fatty acid binding proteins

A stage-specific expressed gene has been isolated from a cDNA expression library of Echinococcus granulosus protoscolices. The isolated clone contains the complete coding sequence. The corresponding protein (EgDf1) has a molecular weight of 15.5 kDa and is expressed at the tegumental level in the protoscolices, being undetectable in the germinal layer of the metacestode. This protein shares an important homology with a family of low-molecular weight proteins involved in the binding of hydrophobic ligands. This family includes a protein of Schistosoma mansoni (Sm 14) that has immunoprotective activity in rodents. Histochemical and metabolic data already reported for E. granulosus suggest that EgDf1 could be a molecular marker for early events in the process of protoscolex differentiation.

A comparative study of the conformational properties of Escherichia coli-derived rat intestinal and liver fatty acid binding proteins

Fourier transform infrared spectroscopy has been used to examine the conformation in aqueous solution of Escherichia coli-expressed rat intestinal and liver fatty-acid binding proteins (I-FABP and L-FABP, respectively). While I-FABP is known from X-ray analysis to have a predominantly beta-structure with 10 antiparallel beta-strands forming two orthogonal sheets that surround the ligand binding pocket, no structural data are available for L-FABP. As expected for homologous proteins with related functions, the secondary structures of I-FABP and L-FABP are very similar. In both proteins, the conformation-sensitive amide-I band shows the maximum absorption at around 1630 cm-1, proving that beta-sheet is the major structural element. However, there are three critical differences between I-FABP and L-FABP; (i), a different solvent accessibility of the protein backbone; (ii), a different pH sensitivity and (iii), a different thermostability, with L-FABP being thermally more stable than I-FABP. These results suggest that, in spite of having a similar overall conformation, the architecture of these proteins is stabilized by slightly different interactions. Such dissimilarities, well-paralleled by fatty-acid binding studies, may provide a structural basis for their functional diversification.

Expression of cellular retinoid-binding proteins during normal and abnormal epidermal differentiation

Retinoids have important roles in growth and differentiation of epidermal cells. We have analyzed the expression of two intracellular retinoid-binding proteins, the cellular retinol-binding protein type I and the cellular retinoic acid-binding protein type I, during normal and abnormal epidermal differentiation. Both proteins were found to be expressed in normal epidermis with increasing expression from basal layer towards superficial layers. In psoriatic lesions, a hyperproliferative condition of the skin, the epidermal expression of cellular retinol-binding protein I was induced, whereas expression of cellular retinoic acid-binding protein I was sharply down-regulated. This and other features of psoriatic lesions indicate that down-regulation of cellular retinoic acid-binding protein I expression might cause aberrant retinoid-regulated gene expression in skin. In basal and squamous cell carcinomas, cellular retinoic acid-binding protein I expression was down-regulated, whereas cellular retinol-binding protein I was expressed. Apart from epidermal cells, a mesenchymal, dendritic cell-type, strongly expressing cellular retinoic acid-binding protein I, was identified in the dermis. In several hyperproliferative conditions of the skin, including psoriasis, and squamous and basal cell carcinomas, this cell type was abundant. These results have implications for the role of retinoids in normal and abnormal epidermal differentiation and suggest that part of the phenotype of psoriasis is due to inappropriate metabolism of retinoic acid in skin.

The primary structure of fatty-acid-binding protein from nurse shark liver. Structural and evolutionary relationship to the mammalian fatty-acid-binding protein family

The primary structure of a fatty-acid-binding protein (FABP) isolated from the liver of the nurse shark (Ginglymostoma cirratum) was determined by high-performance tandem mass spectrometry (employing multichannel array detection) and Edman degradation. Shark liver FABP consists of 132 amino acids with an acetylated N-terminal valine. The chemical molecular mass of the intact protein determined by electrospray ionization mass spectrometry (Mr = 15124 +/- 2.5) was in good agreement with that calculated from the amino acid sequence (Mr = 15121.3). The amino acid sequence of shark liver FABP displays significantly greater similarity to the FABP expressed in mammalian heart, peripheral nerve myelin and adipose tissue (61-53% sequence similarity) than to the FABP expressed in mammalian liver (22% similarity). Phylogenetic trees derived from the comparison of the shark liver FABP amino acid sequence with the members of the mammalian fatty-acid/retinoid-binding protein gene family indicate the initial divergence of an ancestral gene into two major subfamilies: one comprising the genes for mammalian liver FABP and gastrotropin, the other comprising the genes for mammalian cellular retinol-binding proteins I and II, cellular retinoic-acid-binding protein myelin P2 protein, adipocyte FABP, heart FABP and shark liver FABP, the latter having diverged from the ancestral gene that ultimately gave rise to the present day mammalian heart-FABP, adipocyte FABP and myelin P2 protein sequences. The sequence for intestinal FABP from the rat could be assigned to either subfamily, depending on the approach used for phylogenetic tree construction, but clearly diverged at a relatively early evolutionary time point. Indeed, sequences proximately ancestral or closely related to mammalian intestinal FABP, liver FABP, gastrotropin and the retinoid-binding group of proteins appear to have arisen prior to the divergence of shark liver FABP and should therefore also be present in elasmobranchs. The presence in shark liver of an FABP which differs substantially in primary structure from mammalian liver FABP, while being closely related to the FABP expressed in mammalian heart muscle, peripheral nerve myelin and adipocytes, opens a further dimension regarding the question of the existence of structure-dependent and tissue-specific specialization of FABP function in lipid metabolism.

Involvement of cellular retinoic acid-binding proteins I and II (CRABPI and CRABPII) and of the cellular retinol-binding protein I (CRBPI) in odontogenesis in the mouse

The coordination of the activities of individual cells during development is regulated in part by epigenetic signals either encoded in the insoluble extracellular matrix or provided by small diffusible factors such as growth factors peptides and retinoids. Odontogenesis offers a suitable model to correlate the temporospatial distributions of such molecules, and of their cell receptors and ligands, with particular developmental processes. We have analyzed, by in situ hybridization, the distribution patterns of CRABPI, CRABPII and CRBPI transcripts during odontogenesis in the mouse. CRABPI transcripts were restricted to the mitogenic regions of the dental mesenchyme during late bell stages and were absent from post-mitotic odontoblasts. The only epithelial site of CRABPI transcription was the labial epithelial loop of the continuously growing incisor. CRABPII transcription was preponderant in the mitogenic zones of the dental epithelium: differential labeling of the dental epithelium occurred as early as the dental bud stage and during subsequent molar morphogenesis, this labeling became confined in the epithelial loops. The graded distribution of CRABPII transcripts along the anteroposterior axis of the continuously growing incisor was superimposed with the gradient of mitoses. CRABPII transcripts were absent from post-mitotic ameloblasts. It is concluded that during odontogenesis the expressions of the CRABPI and CRABPII genes are confined to regions exhibiting the highest rate of cell proliferation whenever differential mitotic activity is required. Moreover, the putative effects of retinoic acid on the regulation of cell proliferation kinetics in the dental epithelium and in the dental mesenchyme imply distinct CRABPs. CRBPI transcripts were restricted to the dental mesenchyme prior to the onset of CRABPI transcription. This observation supports the hypothesis that the two proteins might perform antagonistic functions in some retinoic acid-mediated developmental processes.

Primary structure of a 15-kDa protein from rat intestinal epithelium. Sequence similarity to fatty-acid-binding proteins

An abundant and novel cytosolic protein was purified from the rat intestinal epithelium by gel filtration, ion-exchange and hydroxylapatite chromatography. The protein was eluted into two different positions (fractions 1 and 2) on DEAE-cellulose chromatography. We have completed the primary structure of the protein of fraction 1 by Edman degradation. The protein (144565 Da) contains 127 amino acid residues and has an acetylated alanine at its NH2-terminus. Comparison of the primary structure of the protein with porcine gastrotropin [Walz, A. D., Wider, M. D., Snow, J. W., Dass, C. & Desiderio, D. M. (1988) J. Biol. Chem. 263, 14189-14195] and rat hepatic fatty-acid-binding protein revealed that identical residues within these proteins are found in 90 and 54 out of a total of 127 positions, respectively. Bioactivity studies demonstrated that neither the protein nor liver and intestinal fatty-acid-binding proteins influence gastric acid secretory activity in rats with gastric fistulas compared to pentagastrin. The protein showed very low affinity for palmitic-acid-binding in vitro assay system and only trace amounts of endogenous fatty acids were detected from the protein. The protein, rat intestinal 15-kDa protein is considered to be a new member of the fatty-acid-binding protein family based on its structural features.

Molecular cloning and transcriptional mapping of the mouse cellular retinoic acid-binding protein gene

The gene encoding the mouse cellular retinoic acid-binding protein (CRABP) has been isolated from a mouse genomic library and its structure has been determined. This gene spans approximately 10.5 kb and consists of four exons encoding 24, 59, 38, and 16 amino acid residues, respectively. This gene structure is very similar to the structures of other related genes belonging to the same protein family such as the human cellular retinol-binding protein, the rat cellular retinol-binding protein II, the rat fatty acid-binding protein, and the mouse adipocyte P2 protein. The site for transcription initiation has been mapped to the 93rd nucleotide upstream from the translation initiation codon ATG using both primer extension and RNase protection assays. From the DNA sequence, the promoter of the CRABP gene resembles those found in the "housekeeping" genes in that it is very G/C rich, lacks a TATA box, and contains multiple copies of the sequence GGGCGG. The deduced amino acid sequence of the translated region is identical to the amino acid sequence of the known bovine CRABP, and the DNA sequence of the transcribed region from the mouse gene shows approximately 78% homology to that of the bovine cDNA.

New approaches and concepts in the study of differentiation of oral epithelia

Epithelial structural proteins, the keratins and keratin-associated proteins, are useful as markers of differentiation because their expression is both region-specific and differentiation-specific. In general, basal cells in all stratified oral epithelia express similar keratins, while the suprabasal cells express a specific set of markers indicating commitment to a distinct program of differentiation. Critical factors in the regulation of epithelial protein expression are now under investigation. The promoter regions of keratin genes are being characterized to determine what sequences within the genes are responsible for differential expression. One important extracellular factor that influences epithelial protein expression is retinol (vitamin A), which exerts its effects via a group of nuclear receptor proteins that may also be expressed in a region-specific manner. These molecular biological approaches enhance our understanding of the mechanisms regulating differentiation of oral epithelia and its regional complexity.

Expression of cellular retinoic acid binding protein in the developing nervous system of mouse embryo

The expression of cellular retinoic acid binding protein, CRABP, in developing mouse embryos was immunohistochemically analyzed. Differentiating young neurons and neuronal fibers in the myelencephalon, metencephalon, mesencephalon and spinal cord in 10.5- and 12.5-day embryos showed intense expression of CRABP, but undifferentiated cells in the neural tube did not. Neural tissue in 16.5-day embryos expressed less amount of binding protein than the tissues of the younger stages. These expressions of CRABP suggest that retinoic acid participates in neurogenesis at early developmental stages via expression of cellular retinoic acid binding protein.

Functions of fatty acid binding proteins

Cytosolic fatty acid binding proteins (FABP) belong to a gene family of which eight members have been conclusively identified. These 14-15 kDa proteins are abundantly expressed in a highly tissue-specific manner. Although the functions of the cytosolic FABP are not clearly established, they appear to enhance the transfer of long-chain fatty acids between artificial and native lipid membranes, and also to have a stimulatory effect on a number of enzymes of fatty acid metabolism in vitro. These findings, as well as the tissue expression, ligand binding properties, ontogeny and regulation of these proteins provide a considerable body of indirect evidence supporting a broad role for the FABP in the intracellular transport and metabolism of long-chain fatty acids. The available data also support the existence of structure- and tissue-specific specialization of function among different members of the FABP gene family. Moreover, FABP may also have a possible role in the modulation of cell growth and proliferation, possibly by virtue of their affinity for ligands such as prostaglandins, leukotrienes and fatty acids, which are known to influence cell growth activity. FABP structurally unrelated to the cytosolic gene family have also been identified in the plasma membranes of several tissues (FABPpm). These proteins have not been fully characterized to date, but strong evidence suggest that they function in the transport of long-chain fatty acids across the plasma membrane.

Localization of cellular retinoic acid-binding protein to amacrine cells of rat retina

Monoclonal antibodies to performic acid-oxidized cellular retinoic acid-binding protein (CRABP) from bovine retina were prepared by fusion of spleen cells from immunized mice with mouse myeloma cells. Five antibodies were studied in detail. It was established by ELISA that the antibodies react with CRABP and oxidized CRABP, but not with other oxidized or unmodified retinoid-binding proteins. Competitive ELISA demonstrated that the antibodies react with heat-denatured antigen but not with native protein. Western blotting and immunostaining, following sodium dodecyl sulfate gel electrophoresis, provided evidence for recognition of a single component in retinal supernatants whose staining is prevented by preabsorption of the antibody with heat-denatured CRABP. The insoluble fraction from a retinal homogenate contains residual CRABP and two weakly-reacting components, whose staining is not affected by preabsorption of the antibody with antigen. Each antibody produces the same staining pattern on cryostat sections of rat retina by indirect immunofluorescence. Amacrine somata on both sides of the inner plexiform layer are labeled, as well as processes forming laminae within this layer. These results suggest that retinoic acid may play a functional role in the inner retina.

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.

Biologically active aromatic retinoids bearing azido photoaffinity-labeling groups and their binding to cellular retinoic acid-binding protein

Retinoids bearing azido photoaffinity-labeling groups (azidoretinoids) have potential as probes for investigating the molecular mechanisms of action of all-trans-retinoic acid (RA) as mediated by its cellular retinoic acid-binding protein (CRABP) and nuclear receptor proteins. Two new azidoretinoids, 3-azido-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1E- propen-1-yl]-benzonic acid and 4-(4-azido-5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-anthracenyl)be nzoic acid were synthesized, and evaluated for their in vitro biological potency, and binding affinity for CRABP. Like RA, these aromatic azides had significant activity in modulating cell differentiation in retinoid-deficient hamster tracheal organ culture (ED500.02 nM and 0.03 nM, respectively) and in the inhibition of the induction of ornithine decarboxylase in mouse epidermis (ED50 7.0 nmol and 0.5 nmol, respectively). They also possessed high binding affinity for CRABP (ID50 0.9 microM and 0.85 microM, respectively). The tritiated aromatic azides were further evaluated for their ability to bind covalently to CRABP after photolysis. On photolysis at -78 degrees C, the two radiolabeled azidoretinoids formed stable adducts with CRABP. Treatment of the adducts with either RA or p-chloromercuriphenylsulfonic acid (CMPS) and subsequent dialysis did not cause any dissociation, indicating the formation of a covalent bond. In contrast, treatment of the unirradiated complexes with RA or CMPS led to dissociation of the complex. Synthesis of affinity labels and characterization of CRABP-retinoid complexes should provide useful information on the ligand-binding regions and insights into the mechanism of action of RA.

Genomic organization of the region encoding guinea pig lipoprotein lipase; evidence for exon fusion and unconventional splicing

The coding sequence of guinea pig lipoprotein lipase (LPL) is organized into nine exons and spans a region of approximately 14 kb of the guinea pig genome. A non-conforming 5'-splice site is located on the first intron, which exhibits a 12-nucleotide perfect match with the 5'-end of the second exon. A previously described tryptic cleavage site is located on exon V, close to the 3' end of this exon. A similarity to vitellogenin resides on exons IV and V, and a putative active site is found on exon IV. A novel similarity to a fatty-acid-binding protein is noted on exon VI, adjacent to the postulated heparin-binding region. We suggest that free fatty acids (FFA) and heparin to some extent share the same site of interaction on the LPL molecule; and that a high local concentration of FFA can displace LPL from its site of action--the vascular endothelium--by competing for binding to heparan sulfate.