The characterisation of protein-bound citrulline

Peptidylarginine deiminase isoforms are differentially expressed in the anagen hair follicles and other human skin appendages

Peptidylarginine deiminases (PAD) catalyze the conversion of arginine residues to citrullines. Five isoforms are known that present distinct tissue locations. In the epidermis, like in the skin, only PAD1, 2, and 3 are expressed. Their pattern of expression in skin appendages is not known. Here, confocal microscopy analysis using highly specific antibodies demonstrated that PAD1 and 3 are expressed in human anagen hair follicles, PAD1 and 2, in arrector pili muscles and sweat glands, whereas no PAD were detected in sebaceous glands. PAD1 was detected in the cuticle and the Huxley layer of the inner root sheath (IRS), and in the companion layer. PAD3 was localized in the medulla, and in the three layers of the IRS. Using anti-modified citrulline antibodies, we also showed that deiminated proteins appeared in the lower part of the IRS, first in the Henle layer, then in the cuticle, and finally in the Huxley layer. Our data demonstrate that PAD3 is the enzyme that deiminates trichohyalin in the medulla and the Henle layer, indicate that PAD1 and 3 are involved in the hair follicle program of differentiation, and suggest a role for PAD1 and 2 in the physiology of sweat glands and arrector pili muscles.

Molecular characterization of peptidylarginine deiminase in HL-60 cells induced by retinoic acid and 1alpha,25-dihydroxyvitamin D(3)

Three types of peptidylarginine deiminase (PAD), which converts a protein arginine residue to a citrulline residue, are widely distributed in animal tissues. Little is known about PAD of hemopoietic cells. We found that PAD activity in human myeloid leukemia HL-60 cells was induced with the granulocyte-inducing agents retinoic acid and dimethyl sulfoxide and with the monocyte-inducing agent 1alpha,25-dihydroxyvitamin D(3). We cloned and characterized a PAD cDNA from retinoic acid-induced cells. The cDNA was 2,238 base pairs long and encoded a 663-amino acid polypeptide. The HL-60 PAD had 50-55% amino acid sequence identities with the three known enzymes and 73% identity with the recently cloned keratinocyte PAD. The recombinant enzyme differs in kinetic properties from the known enzymes. Immunoblotting and Northern blotting with an antiserum against the enzyme and the cDNA, respectively, showed that a protein of approximately 67 kDa increased concomitantly with increase of mRNA of approximately 2.6 kilobases during granulocyte differentiation. During monocyte differentiation the same mRNA and protein increased as in granulocyte differentiation. Neither the enzyme activity nor the protein was found in macrophage-induced cells. These results suggested that expression of the PAD gene is tightly linked to myeloid differentiation.

Colorimetric determination of citrulline residues in proteins

A method is described for the direct colorimetric determination of citrulline residues in proteins based on the reaction with diacetylmonoxime in the presence of lower concentrations of sulfuric acid. The reduced sensitivity due to the lower acid concentration was overcome by the addition of ferric chloride which also contributed to the color stabilization. Insoluble proteins or proteins resulting in turbidity can be analyzed following partial hydrolysis of those with enzyme or acid. The molar absorption coefficient (epsilon) for citrulline at 464 nm was 2.8 x 10(4). The method enabled us to determine low levels of protein-bound citrulline which are beyond the limitations of conventional methods using an amino acid analyzer. We determined the citrulline contents in the cornified cells of the epidermis of newborn rats, as well as soybean trypsin inhibitor.

Biochemical evidence that small proline-rich proteins and trichohyalin function in epithelia by modulation of the biomechanical properties of their cornified cell envelopes

The cornified cell envelope (CE) is a specialized structure involved in barrier function in stratified squamous epithelia, and is assembled by transglutaminase cross-linking of several proteins. Murine forestomach epithelium undergoes particularly rigorous mechanical trauma, and these CEs contain the highest known content of small proline-rich proteins (SPRs). Sequencing analyses of these CEs revealed that SPRs function as cross-bridgers by joining other proteins by use of multiple adjacent glutamines and lysines on only the amino and carboxyl termini and in functionally non-polar ways. Forestomach CEs also use trichohyalin as a novel cross-bridging protein. We performed mathematical modeling of amino acid compositions of the CEs of mouse and human epidermis of different body sites. Although the sum of loricrin + SPRs was conserved, the amount of SPRs varied in relation to the presumed physical requirements of the tissues. Our data suggest that SPRs could serve as modifiers of a composite CE material composed of mostly loricrin; we propose that increasing amounts of cross-bridging SPRs modify the structure of the CE, just as cross-linking proteins strengthen other types of tissues. In this way, different epithelia may use varying amounts of the cross-bridging SPRs to alter the biomechanical properties of the tissue in accordance with specific physical requirements and functions.

Expression of transglutaminase 1 in human hair follicles, sebaceous glands and sweat glands

To explore the function of the enzyme transglutaminase 1 (TGase 1), its distribution was analysed by immunofluorescence microscopy, postembedding immunoelectron microscopy and in situ hybridization. TGase 1 was expressed in the outer root sheath (ORS) cells in the distal portion of the isthmus and infundibulum of human hair follicles. In the level of the proximal and middle portion of the isthmus, TGase 1 was observed in the keratinized area of the inner root sheath (IRS) and ORS cells. In the bulbar and suprabulbar portions, TGase 1 was present in the ORS and IRS cells. The cortex and medulla cells in these regions also contained TGase 1. A high level of fluorescence was observed at the cuticle of the cortex. Sebaceous and sweat gland cells contained abundant TGase 1. Possible functions of TGase 1 in these epidermal appendages are discussed.

The fate of trichohyalin. Sequential post-translational modifications by peptidyl-arginine deiminase and transglutaminases

Trichohyalin (THH) is a major structural protein of the inner root sheath cells and medulla layer of the hair follicle and, to a lesser extent, of other specialized epithelia. THH is a high molecular weight insoluble alpha-helix-rich protein that forms rigid structures as a result of postsynthetic modifications by two Ca2+-dependent enzymes, transglutaminases (TGases) (protein cross-linking) and peptidyl-arginine deiminase (conversion of arginines to citrullines with loss of organized structure). The modified THH is thought to serve as a keratin intermediate filament matrix protein and/or as a constituent of the cell envelope. In this paper, we have explored in vitro the order of processing of THH to fulfill these functions, using an expressed truncated, more soluble form THH-8. THH-8 is a complete substrate for three known TGases expressed in epithelia, but the kinetic efficiency with TGase 3 is by far the greatest. Following maximal conversion of its arginines to citrullines, THH-8 is cross-linked even more efficiently by TGase 3, using most glutamines partially and all lysines. In addition, we show that insoluble aggregates of THH-8 or native pig tongue THH can be solubilized following peptidyl-arginine deiminase modification. Together, these data suggest an in vivo model in which THH located in insoluble cytoplasmic droplets is first modified by peptidyl-arginine deiminase which denatures it and makes it more soluble. This renders it available for efficient cross-linking by TGase 3 to form highly cross-linked rigid structures in the cells. This temporal order of reaction is supported by the observation that THH is expressed in hair follicle cells before the TGase 3 enzyme.

Protein unfolding by peptidylarginine deiminase. Substrate specificity and structural relationships of the natural substrates trichohyalin and filaggrin

Peptidylarginine deiminases, which are commonly found in mammalian cells, catalyze the deimination of protein-bound arginine residues to citrullines. However, very little is known about their substrate requirements and the significance or consequences of this postsynthetic modification. We have explored this reaction in vitro with two known substrates filaggrin and trichohyalin. First, the degree and rate of modification of arginines to citrullines directly correlates with the structural order of the substrate. In filaggrin, which has little structural order, the reaction proceeded rapidly to >95% completion. However, in the highly alpha-helical protein trichohyalin, the reaction proceeded slowly to about 25% and could be forced to a maximum of about 65%. Second, the rate and degree of modification depends on the sequence location of the target arginines. Third, we show by gel electrophoresis, circular dichroism, and fluorescence spectroscopy that the reaction interferes with organized protein structure: the net formation of >/=10% citrulline results in protein denaturation. Cyanate modification of the lysines in model alpha-helix-rich proteins to homocitrullines also results in loss of organized structure. These data suggest that the ureido group on the citrulline formed by the peptidylarginine deiminase enzyme modification functions to unfold proteins due to decrease in net charge, loss of potential ionic bonds, and interference with H bonds.

Deimination of human myelin basic protein by a peptidylarginine deiminase from bovine brain

A peptidylarginine deiminase (PAD; EC 3.5.3.15) has been isolated from bovine brain and some of its characteristics have been studied. The enzyme showed an absolute requirement for Ca2+, a temperature optimum at approximately 50 degrees C, and two Km values when benzoylarginine ethyl ester was used as substrate, 0.78 mM and 11.2 mM. The higher Km has not been reported previously. Protein substrates for the enzyme included polyarginine and myelin basic protein but not histones. Because one of the components of MBP contains six citrullinyl residues per mole, enzymic deimination appeared to be a likely mechanism. When the most cationic component (C-1) was subjected to PAD in solution, 17 of the 19 arginyl residues were modified. From sequence analyses we concluded that the nature of the amino acid residues adjacent to the deiminated arginine were not modifiers of the reaction as arginyl residues in a variety of environments were deiminated. This deimination was reflected in a large increase in random structure, as measured by [theta]200. At 5 degrees C, the [theta]200 of the deiminated protein was -70 x 10(3) compared with -30 x 10(3) deg.cm2/dmol for the native protein. When the temperature was increased to 70 degrees C, the [theta]200 was -44 x 10(3) for the deiminated protein and -20 x 10(7) deg.cm2/dmol for the native C-1. When plotted as a function of temperature, [theta]200 decreased linearly from 5 degrees C to 50 degrees C for both proteins and did not change from 50 degrees C to 70 degrees C. PAD provides a mechanism for deimination of arginyl residues of myelin basic protein. The selective deimination of the six arginyl residues that are consistently found deiminated in C-8 may be determined by the orientation of the protein in the membrane and/or the more complex lipid composition of myelin may affect the selectivity of the deimination.

Analysis of the sheep trichohyalin gene: potential structural and calcium-binding roles of trichohyalin in the hair follicle

Trichohyalin is a structural protein that is produced and retained in the cells of the inner root sheath and medulla of the hair follicle. The gene for sheep trichohyalin has been purified and the complete amino acid sequence of trichohyalin determined in an attempt to increase the understanding of the structure and function of this protein in the filamentous network of the hardened inner root sheath cells. Sheep trichohyalin has a molecular weight of 201,172 and is characterized by the presence of a high proportion of glutamate, arginine, glutamine, and leucine residues, together totaling more than 75% of the amino acids. Over 65% of trichohyalin consists of two sets of tandem peptide repeats which are based on two different consensus sequences. Trichohyalin is predicted to form an elongated alpha-helical rod structure but does not contain the sequences required for the formation of intermediate filaments. The amino terminus of trichohyalin contains two EF hand calcium-binding domains indicating that trichohyalin plays more than a structural role within the hair follicle. In situ hybridization studies have shown that trichohyalin is expressed in the epithelia of the tongue, hoof, and rumen as well as in the inner root sheath and medulla of the hair follicle.

Human trichohyalin gene is clustered with the genes for other epidermal structural proteins and calcium-binding proteins at chromosomal locus 1q21

Trichohyalin is a major differentiation product of hard keratinizing tissues such as the inner root sheath and medullary cells of the hair follicle and the filiform papillae of the tongue, as well as terminally differentiating epidermal cells. It consists largely of quasi-repeating peptide repeats and functions primarily as an intermediate filament-associated protein in these tissues. By mapping with human-rodent somatic cell hybrids and fluorescent in situ hybridization, we demonstrate that its gene maps to chromosomal region 1q21. Interestingly, genes encoding several other structural proteins expressed during terminal differentiation in the epidermis map to this region, as do also several members of the S-100 class of small calcium-binding proteins.

Trichohyalin: purification from porcine tongue epithelium and characterization of the native protein

Trichohyalin, a protein of Mr between 190 and 220 kDa in different species, was first demonstrated in large granules of the inner root sheath and medulla of hair follicles and may provide a matrix for keratin filaments. We have purified trichohyalin in milligram quantities from a citric acid-insoluble fraction derived from pig tongue epithelium. Trichohyalin was extracted under conditions of low ionic strength from the citric acid-insoluble fraction, separated by gel-filtration chromatography in buffer containing 1 M NaBr, and concentrated by ion-exchange chromatography in buffer containing 4 M urea. The purified material, which is soluble in buffers containing 1 M NaBr, was considered to be trichohyalin because of its characteristic molecular weight and amino acid composition and its localization to hair follicle inner root sheath and medulla by indirect immunofluorescence using antibodies against the purified protein. Immunofluorescence showed that trichohyalin is a major protein of filiform papillae of the tongue. Unlike trichohyalin from other animals examined, the porcine protein is a doublet on SDS polyacrylamide gels of 195 and 210 kDa; both bands are recognized by different antibodies, their two-dimensional peptide maps are nearly identical, and they have nearly identical isoelectric points of about 6.6. Trichohyalin has a Stokes radius of 124 A on gel filtration and a Svedberg constant of 6, consistent with an extended structure. The protein probably associates reversibly in solution, and the native protein we have isolated may be dimeric, because crosslinking of the iodinated purified protein with disuccinimidyl suberate demonstrated the presence of a dimer, which could be dissociated in the presence of high concentrations of urea. Rotary shadowing electron microscopy of the native protein showed a filamentous structure averaging 85 nm in length with a single globular-appearing end-domain. The purification of native trichohyalin provides a basis for future functional studies.

Interaction of trichohyalin with intermediate filaments: three immunologically defined stages of trichohyalin maturation

"Trichohyalin" is a 220-kD protein found in trichohyalin granules that are present as major differentiation products in the medulla and inner root sheath cells of human hair follicles. It was unclear whether this protein served as an intermediate filament precursor in the inner root sheath or as an intermediate-filament-associated (matrix) protein. We have produced a panel of monoclonal antibodies (AE15-17) to this protein and used them to trace its fate during inner root sheath differentiation. These studies have allowed us to define three immunologically distinct forms of this trichohyalin protein. They are 1) the AE15-positive form, which is found throughout all trichohyalin granules; 2) the AE16-positive form, which is localized as discrete punctae on the surface of trichohyalin granules; and 3) the AE17-positive, intermediate-filament-bound form, which associates with the inner root sheath filaments with a regular, 400-nm periodicity. From these results we suggest that the 220-kD trichohyalin protein is an intermediate-filament-associated protein that may play a role in the lateral aggregation, precise alignment, and stabilization of inner root sheath filament bundles.

Trichohyalin, an intermediate filament-associated protein of the hair follicle

A precursor protein associated with the formation of the citrulline-containing intermediate filaments of the hair follicle has been isolated and characterized. The protein, with a molecular weight of 190,000, was isolated from sheep wool follicles and purified until it yielded a single band on a SDS polyacrylamide gel. The Mr 190,000 protein has a high content of lysine and glutamic acid/glutamine residues and is rich in arginine residues, some of which, it is postulated, undergo a side chain conversion in situ into citrulline residues. Polyclonal antibodies were raised to the purified protein, and these cross-react with similar proteins from extracts of guinea pig and human follicles and rat vibrissae inner root sheaths. Tissue immunochemical methods have localized the Mr 190,000 protein to the trichohyalin granules of the developing inner root sheath of the wool follicle. We propose that the old term trichohyalin be retained to describe this Mr 190,000 protein. Immunoelectron microscopy has located the Mr 190,000 protein to the trichohyalin granules but not to the newly synthesized filaments. This technique has revealed that trichohyalin becomes associated with the filaments at later stages of development. These results indicate a possible matrix role for trichohyalin.

Epidermal and hair follicle transglutaminases and crosslinking in skin

Epidermal and hair follicle transglutaminases crosslink structural proteins in the skin by epsilon-(gamma-glutamyl)-lysine bonds. This crosslinking produces protein polymers that are extremely insoluble and, until recently, difficult to characterize. Epidermal transglutaminase is localized to the granular layer of the epidermis. It catalyzes the crosslinking of a soluble cytoplasmic precursor to form the cornified envelope that lines the inner membrane of the mature keratinocyte in the stratum corneum. Hair follicle transglutaminase is localized to the inner root sheath and medulla of the hair follicle. It crosslinks a poorly characterized citrulline-rich protein. The enzymes and their substrates have been shown to be important markers of normal differentiation. Regulation of these processes is currently under investigation.

Comparative biochemistry of the guinea-pig: a partial checklist

A great deal is known about guinea-pig biochemistry, but the information is scattered and difficult to assemble. The guinea-pig also possesses a number of unusual biochemical features which add to its interest. For these reasons we have compiled a list of biochemical characteristics of the guinea-pig, organized in a series of tables, with brief discussions of some of the entries.

Biochemistry of transglutaminases and cross-linking in the skin

Transglutaminase is a calcium-dependent enzyme found widely in nature. It catalyzes the formation of epsilon-(gamma-glutamyl)lysine bonds that participate in processes varying from fibrin clot formation to epidermal cell envelope formation. Epidermal transglutaminase is localized to the granular layer of the epidermis. It catalyzes the covalent cross-linking of a soluble cytoplasmic substrate into large polymers to form the cornified envelope that lines the inner membrane of keratinocytes in the stratum corneum. The soluble precursor from epidermis has been named keratolinin, and from keratinocyte culture, it has been named involucrin. Hair follicle transglutaminase is biochemically and immunochemically distinct from its epidermal counterpart. It has been localized to the inner root sheath and medulla of the hair follicle. The substrate of hair follicle transglutaminase has been poorly defined but appears to be rich in the amino acid citrulline. Transglutaminase has been shown to be an important marker of normal differentiation. There is a rise in its activity at the time of keratinization, and transglutaminase activity has been shown to be greatly decreased in basal cell epithelioma and in psoriasis. Keratinocyte cell culture has proven most helpful in delineating the processes of normal differentiation and keratinization, since the formation of the cell envelope in culture appears to parallel the formation in vivo.

Purification and properties of a brain enzyme which deiminates proteins

The deimination of guanidyl groups of peptides, proteins and other arginine-containing compounds is catalyzed by enzymes found in mammalian brain and epidermis. In cow, the brain and epidermal enzymes differ kinetically and physically, but both may be quantitated by measuring the production of benzoyl citrulline ethyl ester from benzoyl-arginine ethyl ester. The brain enzyme has been purified to apparent homogeneity, as judged by the presence of only one 85,000 dalton band in purified preparations when examined by SDS-polyacrylamide gel electrophoresis. An antibody raised to this band precipitates pure and partially purified brain enzyme but not partially purified epidermal enzyme, using the Ouchterlony technique. The antibody bound to an insoluble matrix removes brain enzyme activity from solution but not epidermal enzyme activity. The Km of the brain enzyme for benzoyl-arginine ethyl ester is about 0.33 mM.

Partial purification and specificity of an arginine-converting enzyme from bovine epidermis

An enzyme which catalyzes the conversion of intraprotein arginine residue to intraprotein citrulline residue is present in bovine snout epidermis. This arginine-converting enzyme has been partially purified by (NH4)2SO4 preciptation chromatography on DEAE-cellulose and chromatography on Sephadex G-200. The enzyme is active at neutral pH, requires Ca2+ and a reducing agent and has an apparent molecular weight of 69 000. Its substrates include histone, polyarginine, S-carboxymethyl cysteine-hair keratin, S-carboxymethyl cysteine epidermal keratin and prekeratin and S-carboxymethyl cysteine-trichohyalin. A large number of proteins, synthetic and naturally occurring peptides, and other guanidine-containing compounds were substrates of the arginine-converting enzyme.

The presence of citrulline in epidermal proteins

Citrulline is present in the stratum corneum proteins of human, cow snout, pig snout and guinea pig epidermis but is absent from the stratum corneum proteins of frog, mouse, turtle, rat and hamster epidermis. The amino acid is released by acid hydrolysis and ranges from 1.7 to 5.5 residues per thousand residues of protein amino acid. Protein derived citrulline co-chromatographs with authentic L-citrulline on an amino acid analyzer, on Dowex-50, on Dowex-2 and on thin-layer chromatography. Dansylated material co-chromatographed with authentic dansyl-L-citrulline in two thin-layer chromatography systems. Labelling experiments have shown that the protein bound citrulline is derived from protein bound arginine and probably results from enzymatic conversion of the guanido group to the ureido group.

The origin of citrulline-containing proteins in the hair follicle and the chemical nature of trichohyalin, an intracellular precursor

The present studies have demonstrated that the medulla and inner root sheath cells develop within their cytoplasm a protein that is unique in composition and is present in the trichohyalin granules. The protein is rich in arginine residues, some of which undergo a side-chain conversion in situ into citrulline residues. An unusual Ca2+-dependent enzyme activity distinguishable from cross-linking transamidase has been detected in the hair follicle and will act in vitro on trichohyalin protein as the natural substrate. The conversion in vivo must occur during the time that the medullary and inner root sheath cells move up the follicle and their cytoplasm fills with cross-linked protein containing citrulline. The function of citrulline in these proteins is not understood but its formation is a major process during hair growth.

The enzymic derivation of citrulline residues from arginine residues in situ during the biosynthesis of hair proteins that are cross-linked by isopeptide bonds

An enzymic activity present in hair follicles is described that can convert arginine residues to citrulline residues in proteins in situ. The Ca2+ dependent enzyme activity has been detected in hair follicle extracts but not in similar extracts of serum, liver or brain. The enzyme appears to act on proteins other than hair proteins and the citrulline produced can be quantitated in acid hydrolysates by a colorimetric procedure. The formation of citrulline has been confirmed by amino acid analysis and does not appear to be related to the formation of isopeptide linkages which is catalysed by the transamidase present in hair follicles.

Arginine transferase activity in homogenates from guinea-pig hair follicles

The transfer of arginine from tRNA to the amino terminal region of acceptor proteins has been demonstrated in postribosomal supernatant from guinea-pig hair follicle homogenate. The reaction has no requirement for template nucleic acids, ATP, GTP, or Mg++, and is unaffected by high concentrations of cycloheximide. It is concluded that the arginine transfer activity of the guinea-pig hair follicle is due to the action of a soluble enzyme similar to the arginine transferases previously described for other mammalian tissues.

Isolation of peptides containing citrulline and the cross-link, epsilon-(gamma-glutamyl)lysine, from hair medulla protein

The epsilon-(gamma-glutamyl)lysine cross-link has been unequivocally localized in the citrulline-containing protein of hair medulla tissue. This was achieved by the isolation of cross-linked peptides that contain citrulline. The peptides were purified by molecular sieving, cation-exchange and paper chromatography and paper electrophoresis and their amino acid compositions are reported. They were detected by analyzing a large number obtained from a tryptic digest of thermolytic peptides of the medulla protein. There is no evidence at present as to whether the cross-link is in fact inter- or intrachain, or both.

Characterization of the proteins of guinea-pig hair and hair-follicle tissue

This work forms a part of a study of the mechanism and control of protein synthesis in the hair follicle and concerns the characterization of the proteins of hair-follicle tissue and for comparative reasons those of the hair itself. 1. Five different groups of reduced carboxymethylated proteins were delineated from both tissues; these were: group 1A proteins, which appeared to be aggregates of the group 2 proteins; group 1B proteins, soluble at pH4.4, which were thought to originate from the medulla and inner-rootsheath layers; group 2 proteins, which were defined as the main low-sulphur keratin proteins insoluble at pH4.4; group 3 proteins, the precise origin of which is not known; and the group 4 proteins, which were defined as the main high-sulphur keratin proteins soluble at pH4.4. 2. With the single exception of the group 1B proteins, the types and properties of all hair and hair-follicle proteins were identical as far as could be determined by use of such criteria as multiplicity of components, molecular charge, molecular weight and amino acid composition. 3. Two significant quantitative differences were noted: in follicle extracts there were more group 2 proteins but less group 3 and group 4 proteins than in hair extracts; and secondly, in the follicle group 4 proteins, there were more proteins of lowest molecular weight and S-carboxymethylcysteine content, but fewer proteins of the highest molecular weight and S-carboxymethylcysteine conent than in the hair group 4 proteins. 4. These quantitative differences are discussed in terms of the mechanism of synthesis of the keratin proteins. 5. Follicle group 1B proteins are postulated to have arisen from the trichohyalin droplets of the developing medulla and inner-root-sheath layers of the follicle and may be precursors of the proteins of the mature medulla and inner root sheath.