Synthesis of collagen: chemical regulation of post-translational events

Collagen biosynthesis involves many unique post-translational events. Inhibition of some of these will lead either to decreased formation of the extracellular collagen fibres or to an accumulation of fibres with altered functional properties. The events that would seem most suitable targets for chemical regulation are triple helix formation, the cleavage of propeptides from the procollagen molecules and cross-link formation. Attempts have recently been made to develop inhibitors of prolyl 4-hydroxylase in particular, as inhibition of this enzyme will prevent triple helix formation and thus lead to a non-functional protein. Prolyl 4-hydroxylase is inhibited competitively with respect to ferrous ion by several bivalent cations, especially zinc, with respect to 2-oxoglutarate by pyridine 2,5-dicarboxylate, pyridine 2,4-dicarboxylate, 3,4-dihydroxybenzoate and many related compounds, with respect to oxygen by superoxide dismutase-active copper chelates and with respect to the peptide substrate by a number of peptides. Triple helix formation can also be inhibited by administering certain proline analogues such as cis-4-hydroxyproline and L-azetidine-2-carboxylic acid, which are incorporated into proteins in place of proline. Only preliminary data are available on the possibilities for using any of these substances to inhibit collagen accumulation in fibrotic processes.

Cloning and characterization of a fifth human lysyl oxidase isoenzyme: the third member of the lysyl oxidase-related subfamily with four scavenger receptor cysteine-rich domains

We report the complete cDNA sequence of the human lysyl oxidase-like 4 (LOXL4) gene, a new member of the lysyl oxidase (LO) gene family. The predicted polypeptide is 756 amino acids long, including a 24-residue signal peptide. The C-terminal region contains a LO domain similar to those of LOX, LOXL, LOXL2 and LOXL3. The N-terminal region has four subregions similar to scavenger receptor cysteine-rich domains that are highly conserved with LOXL2 and LOXL3. The LOXL4 mRNA is approximately 4 kb in size and is expressed in many tissues, the highest levels among the tissues studied being in the skeletal muscle, testis and pancreas. Recombinant LOXL4 expressed in HT-1080 cells was secreted into the culture medium with no evident proteolytic processing.

Prolyl 4-hydroxylase isoenzymes I and II have different expression patterns in several human tissues

Prolyl 4-hydroxylase plays a central role in the synthesis of all collagens. We have previously reported that the recently identified Type II isoenzyme is its main form in chondrocytes and possibly in capillary endothelial cells, while Type I is the main form in many other cell types. We report here that the Type II isoenzyme is clearly the main form in capillary endothelial cells and also in cultured umbilical vein endothelial cells, whereas no Type I isoenzyme could be detected in these cells by immunostaining or Western blotting. The Type II isoenzyme was also the main form in cells of the developing glomeruli in the fetal kidney and tubular structures of collecting duct caliber in both fetal and adult kidney, in occasional sinusoidal structures and epithelia of the bile ducts in the liver, and in some cells of the decidual membrane that probably represented invasive cytotrophoblasts in the placenta. Osteoblasts in a fetal calvaria, i.e., a bone developing by intramembranous ossification, stained strongly for both types of isoenzyme. The Type I isoenzyme was the main form in undifferentiated interstitial mesenchymal cells of the developing kidney, for example, and in fibroblasts and fibroblastic cells in many tissues. Skeletal myocytes and smooth muscle cells appeared to have the Type I isoenzyme as their only prolyl 4-hydroxylase form. Hepatocytes expressed small amounts of the Type I enzyme and very little if any Type II, the Type I expression being increased in malignant hepatocytes and cultured hepatoblastoma cells. The data suggest that the Type I isoenzyme is expressed especially by cells of mesenchymal origin and in developing and malignant tissues, whereas the Type II isoenzyme is expressed, in addition to chondrocytes and osteoblasts, by more differentiated cells, such as endothelial cells and cells of epithelial structures. (J Histochem Cytochem 49:1143-1153, 2001)

High-level production of human type I collagen in the yeast Pichia pastoris

Four human genes, two of them encoding the proalpha1 and proalpha2 chains of type I procollagen and two of them the two types of subunit of prolyl 4-hydroxylase (4-PH), were integrated into the genome of Pichia pastoris. The proalpha1 and proalpha2 chains expressed formed type I procollagen molecules with the correct 2:1 chain ratio, and the 4-PH subunits formed an active enzyme tetramer that fully hydroxylated the proalpha chains. Chains lacking their N but not C propeptides formed pCcollagen molecules with the 2:1 chain ratio and, surprisingly, the expression levels of pCcollagen were 1.5-3-fold relative to those of procollagen. Both types of molecule could be converted by pepsin treatment to collagen molecules that formed native-type fibrils in vitro. The expression levels obtained for the pCcollagen using only single copies of each of the four genes and a 2 l fermenter ranged up to 0.5 g/l, indicating that it should be possible to optimize this system for high-level production of recombinant human type I collagen for numerous medical applications.

Cloning and characterization of a fourth human lysyl oxidase isoenzyme

We report here the complete cDNA sequence and exon-intron organization of the human lysyl oxidase-like (LOXL)3 gene, a new member of the lysyl oxidase (LO) gene family. The predicted polypeptide is 753 amino acids in length, including a signal peptide of 25 residues. The C-terminal region, residues 529-729, contains a LO domain similar to those in the LOX (the first characterized LO isoenzyme), LOXL and LOXL2 polypeptides. It possesses the putative copper binding sequence, and the lysine and tyrosine residues that form the lysyltyrosyl quinone cofactor. The N-terminal region, which is similar to that in LOXL2 but not those in LOX and LOXL, contains four subregions similar to scavenger receptor cysteine-rich domains and a putative nuclear localization signal. Recombinant LOXL3, expressed in HT-1080 cells, was secreted into the culture medium but was not detected by immunofluorescence staining in nuclei. The LOXL3 mRNA is 3.1 kb in size and is expressed in many tissues, the highest levels among the tissues studied being seen in the placenta, heart, ovary, testis, small intestine and spleen.

Domains b' and a' of protein disulfide isomerase fulfill the minimum requirement for function as a subunit of prolyl 4-hydroxylase. The N-terminal domains a and b enhances this function and can be substituted in part by those of ERp57

Protein disulfide isomerase (PDI) is a modular polypeptide consisting of four domains, a, b, b', and a', plus an acidic C-terminal extension, c. PDI carries out multiple functions, acting as the beta subunit in the animal prolyl 4-hydroxylases and in the microsomal triglyceride transfer protein and independently acting as a protein folding catalyst. We report here that the minimum sequence requirement for the assembly of an active prolyl 4-hydroxylase alpha(2)beta(2) tetramer in insect cell coexpression experiments is fulfilled by the PDI domain construct b'a' but that the sequential addition of the b and a domains greatly increases the level of enzyme activity obtained. In the assembly of active prolyl 4-hydroxylase tetramers, the a and b domains of PDI, but not b' and a', can in part be substituted by the corresponding domains of ERp57, a PDI isoform that functions naturally in association with the lectins calnexin and calreticulin. The a' domain of PDI could not be substituted by the PDI a domain, suggesting that both b' and a' domains contain regions critical for prolyl 4-hydroxylase assembly. All PDI domain constructs and PDI/ERp57 hybrids that contain the b' domain can bind the 14-amino acid peptide Delta-somatostatin, as measured by cross-linking; however, binding of the misfolded protein "scrambled" RNase required the addition of domains ab or a' of PDI. The human prolyl 4-hydroxylase alpha subunit has at least two isoforms, alpha(I) and alpha(II), which form with the PDI polypeptide the (alpha(I))(2)beta(2) and (alpha(II))(2)beta(2) tetramers. We report here that all the PDI domain constructs and PDI/ERp57 hybrid polypeptides tested were more effectively associated with the alpha(II) subunit than the alpha(I) subunit.

Expression of recombinant human type I-III collagens in the yeast pichia pastoris

An efficient expression system for recombinant human collagens will have numerous scientific and medical applications. However, most recombinant systems are unsuitable for this purpose, as they do not have sufficient prolyl 4-hydroxylase activity. We have developed methods for producing the three major fibril-forming human collagens, types I, II and III, in the methylotrophic yeast Pichia pastoris. These methods are based on co-expression of procollagen polypeptide chains with the alpha- and beta-subunits of prolyl 4-hydroxylase. The triple-helical type-I, -II and-III procollagens were found to accumulate predominantly within the endoplasmic reticulum of the yeast cells and could be purified from the cell lysates by a procedure that included a pepsin treatment to convert the procollagens into collagens and to digest most of the non-collagenous proteins. All the purified recombinant collagens were identical in 4-hydroxyproline content with the corresponding non-recombinant human proteins, and all the recombinant collagens formed native-type fibrils. The expression levels using single-copy integrants and a 2 litre bioreactor ranged from 0.2 to 0.6 g/l depending on the collagen type.

Collagens and collagen-related diseases

The collagen superfamily of proteins plays a dominant role in maintaining the integrity of various tissues and also has a number of other important functions. The superfamily now includes more than 20 collagen types with altogether at least 38 distinct polypeptide chains, and more than 15 additional proteins that have collagen-like domains. Most collagens form polymeric assemblies, such as fibrils, networks and filaments, and the superfamily can be divided into several families based on these assemblies and other features. All collagens also contain noncollagenous domains, and many of these have important functions that are distinct from those of the collagen domains. Major interest has been focused on endostatin, a fragment released from type XVIII collagen, which potently inhibits angiogenesis and tumour growth. Collagen synthesis requires eight specific post-translational enzymes, some of which are attractive targets for the development of drugs to inhibit collagen accumulation in fibrotic diseases. The critical roles of collagens have been clearly illustrated by the wide spectrum of diseases caused by the more than 1,000 mutations that have thus far been identified in 22 genes for 12 out of the more than 20 collagen types. These diseases include osteogenesis imperfecta, many chondrodysplasias, several subtypes of the Ehlers-Danlos syndrome, Alport syndrome, Bethlem myopathy, certain subtypes of epidermolysis bullosa, Knobloch syndrome and also some cases of osteoporosis, arterial aneurysms, osteoarthrosis, and intervertebral disc disease. The characterization of mutations in additional collagen genes will probably add further diseases to this list. Mice with genetically engineered collagen mutations have proved valuable for defining the functions of various collagens and for studying many aspects of the related diseases.

Mechanistic Studies on Prolyl-4-Hydroxylase: Demonstration That the Ferryl Intermediate Does Not Exchange with Water

Prolyl-4-hydroxylase catalyzes the formation of 4-hydroxyproline in collagens. In contrast to deacetoxy/deacetylcephalosporin C synthase, p-hydroxyphenylpyruvate hydroxylase, lysyl hydroxylase and alpha-ketoisocaproate oxygenase, no incorporation of (18)O-labeled water into the hydroxylated product was found for the human type I prolyl-4-hydroxylase when N-Cbz-Gly-L-Phe-L-Pro-Gly-OEt was used as a substrate. This suggests that the ferryl intermediate for this enzyme is not solvent accessible. Copyright 2000 Academic Press.

Mechanistic studies on prolyl-4-hydroxylase: the vitamin C requiring uncoupled oxidation

A deuterated substrate for the human type I prolyl-4-hydroxylase was synthesized and its V/K deuterium isotope effect was determined to be 3.4 +/- 0.2. This isotope effect was attributed to the uncoupled oxidation. A dehydroproline containing tetrapeptide was also found to stimulate the uncoupled oxidation.

A compound heterozygote patient with Ehlers-Danlos syndrome type VI has a deletion in one allele and a splicing defect in the other allele of the lysyl hydroxylase gene

We report the first deletion mutation and the first splicing defect in the lysyl hydroxylase gene in a compound heterozygote patient with Ehlers-Danlos syndrome type VI with markedly reduced lysyl hydroxylase activity. Northern analysis of the RNA isolated from skin fibroblasts of the patient demonstrated the presence of a truncated lysyl hydroxylase mRNA. PCR and sequence analysis confirmed the truncation and indicated that the cells contain two types of shortened mRNAs, one lacking the sequences corresponding to exon 16 and the other lacking that corresponding to exon 17 of the lysyl hydroxylase gene. Analysis of genomic DNA revealed deletion of the penultimate adenosine from the 3' end of intron 15 from one allele. This defect was probably responsible for the skipping of exon 16 sequences from the transcript. The other allele, inherited from the mother, contains an Alu-Alu recombination with a deletion of about 3,000 nucleotides from the gene; this abnormality explains the lack of exon 17 sequences. The identified mutations in exon 16 and exon 17 do not alter the reading frame of the transcripts.

Mutations that destabilize the a' domain of human protein-disulfide isomerase indirectly affect peptide binding

Protein-disulfide isomerase (PDI) is a catalyst of folding of disulfide-bonded proteins and also a multifunctional polypeptide that acts as the beta-subunit in the prolyl 4-hydroxylase alpha(2)beta(2)-tetramer (P4H) and the microsomal triglyceride transfer protein alphabeta-dimer. The principal peptide-binding site of PDI is located in the b' domain, but all domains contribute to the binding of misfolded proteins. Mutations in the C-terminal part of the a' domain have significant effects on the assembly of the P4H tetramer and other functions of PDI. In this study we have addressed the question of whether these mutations in the C-terminal part of the a' domain, which affect P4H assembly, also affect peptide binding to PDI. We observed a strong correlation between P4H assembly competence and peptide binding; mutants of PDI that failed to form a functional P4H tetramer were also inactive in peptide binding. However, there was also a correlation between inactivity in these assays and indicators of conformational disruption, such as protease sensitivity. Peptide binding activity could be restored in inactive, protease-sensitive mutants by selective proteolytic removal of the mutated a' domain. Hence we propose that structural changes in the a' domain indirectly affect peptide binding to the b' domain.

Type XIII collagen forms homotrimers with three triple helical collagenous domains and its association into disulfide-bonded trimers is enhanced by prolyl 4-hydroxylase

Type XIII collagen is a type II transmembrane protein predicted to consist of a short cytosolic domain, a single transmembrane domain, and three collagenous domains flanked by noncollagenous sequences. Previous studies on mRNAs indicate that the structures of the collagenous domain closest to the cell membrane, COL1, the adjacent noncollagenous domain, NC2, and the C-terminal domains COL3 and NC4 are subject to alternative splicing. In order to extend studies of type XIII collagen from cDNAs to the protein level we have produced it in insect cells by means of baculoviruses. Type XIII collagen alpha chains were found to associate into disulfide-bonded trimers, and hydroxylation of proline residues dramatically enhanced this association. This protein contains altogether eight cysteine residues, and interchain disulfide bonds could be located in the NC1 domain and possibly at the junction of COL1 and NC2, while the two cysteine residues in NC4 are likely to form intrachain bonds. Pepsin and trypsin/chymotrypsin digestions indicated that the type XIII collagen alpha chains form homotrimers whose three collagenous domains are in triple helical conformation. The thermal stabilities (T(m)) of the COL1, COL2, and COL3 domains were 38, 49 and 40 degrees C, respectively. The T(m) of the central collagenous domain is unusually high, which in the light of this domain being invariant in terms of alternative splicing suggests that the central portion of the molecule may have an important role in the stability of the molecule. All in all, most of the type XIII collagen ectodomain appears to be present in triple helical conformation, which is in clear contrast to the short or highly interrupted triple helical domains of the other known collagenous transmembrane proteins.

Complete exon-intron organization of the gene for human lysyl hydroxylase 3 (LH3)

Lysyl hydroxylase (LH) catalyzes the formation of hydroxylysine in collagens and related proteins by the hydroxylation of lysine residues in peptide linkages. Three isoenzymes of LH have so far been characterized. We report here that the human LH3 gene is 11.6 kb in size and consists of 19 exons. Transcription is initiated at one major site and several minor sites, the first exon containing 249-335 bp of untranslated sequences and 109 bp of a translated sequence. Exons 2-18 are similar in size to those of the human LH1 gene, whereas the introns are markedly shorter. The LH3 gene contains a total of 15 full length Alu retroposons or partial Alu fragments of more than 100 bp, in introns 5, 6, 12, 15 and 17. These generate a potential for genomic rearrangements, as has been shown for the LH1 gene in Ehlers-Danlos syndrome type VI. The 5'-flanking region of the LH3 gene was found to be entirely different from that of the LH1 gene, suggesting different regulation of these two genes.

Accumulation of properly folded human type III procollagen molecules in specific intracellular membranous compartments in the yeast Pichia pastoris

It was recently reported that co-expression of the proalpha1(III) chain of human type III procollagen with the subunits of human prolyl 4-hydroxylase in Pichia pastoris produces fully hydroxylated and properly folded recombinant type III procollagen molecules (Vuorela, A., Myllyharju, J., Nissi, R., Pihlajaniemi, T., Kivirikko, K.I., 1997. Assembly of human prolyl 4-hydroxylase and type III collagen in the yeast Pichia pastoris: formation of a stable enzyme tetramer requires coexpression with collagen and assembly of a stable collagen requires coexpression with prolyl 4-hydroxylase. EMBO J. 16, 6702-6712). These properly folded molecules accumulated inside the yeast cell, however, only approximately 10% were found in the culture medium. We report here that replacement of the authentic signal sequence of the human proalpha1(III) with the Saccharomyces cerevisiae alpha mating factor prepro sequence led only to a minor increase in the amount secreted. Immunoelectron microscopy studies indicated that the procollagen molecules accumulate in specific membranous vesicular compartments that are closely associated with the nuclear membrane. Prolyl 4-hydroxylase, an endoplasmic reticulum (ER) lumenal enzyme, was found to be located in the same compartments. Non-helical proalpha1(III) chains produced by expression without recombinant prolyl 4-hydroxylase likewise accumulated within these compartments. The data indicate that properly folded recombinant procollagen molecules accumulate within the ER and do not proceed further in the secretory pathway. This may be related to the large size of the procollagen molecule.

Coexpression with collagen markedly increases the half-life of the recombinant human prolyl 4-hydroxylase tetramer in the yeast Pichia pastoris

Recent coexpression studies of the subunits of human prolyl 4-hydroxylase (4-PH) in the yeast Pichia pastoris have indicated that only a minor fraction of them were present in the alpha2beta2 tetramer, while coexpression with type III procollagen markedly increased their assembly level. We report here that the half-life of the recombinant 4-PH tetramer in Pichia when studied by pulse-chase experiments was only 50 min. Coexpression with the pro alpha1(III) chains increased this half-life to 12.5 h. Coexpression with the pro alpha1(I) chains, which were produced at half the level of the pro alpha1(III) chains, gave a half-life of 6.5 h. Coexpression with collagen thus markedly increases the half-life of the 4-PH tetramer, and the half-life may be related to the level of collagen expression.

Evidence for 4-hydroxyproline in viral proteins. Characterization of a viral prolyl 4-hydroxylase and its peptide substrates

4-Hydroxyproline, the characteristic amino acid of collagens and collagen-like proteins in animals, is also found in certain proline-rich proteins in plants but has been believed to be absent from viral and bacterial proteins. We report here on the cloning and characterization from a eukaryotic algal virus, Paramecium bursaria Chlorella virus-1, of a 242-residue polypeptide, which shows distinct sequence similarity to the C-terminal half of the catalytic alpha subunits of animal prolyl 4-hydroxylases. The recombinant polypeptide, expressed in Escherichia coli, was found to be a soluble monomer and to hydroxylate both (Pro-Pro-Gly)(10) and poly(L-proline), the standard substrates of animal and plant prolyl 4-hydroxylases, respectively. Synthetic peptides such as (Pro-Ala-Pro-Lys)(n), (Ser-Pro-Lys-Pro-Pro)(5), and (Pro-Glu-Pro-Pro-Ala)(5) corresponding to proline-rich repeats coded by the viral genome also served as substrates. (Pro-Ala-Pro-Lys)(10) was a particularly good substrate, with a K(m) of 20 microM. The prolines in both positions in this repeat were hydroxylated, those preceding the alanines being hydroxylated more efficiently. The data strongly suggest that P. bursaria Chlorella virus-1 expresses proteins in which many prolines become hydroxylated to 4-hydroxyproline by a novel viral prolyl 4-hydroxylase.

Recombinant human type II collagens with low and high levels of hydroxylysine and its glycosylated forms show marked differences in fibrillogenesis in vitro

Type II collagen is the main structural component of hyaline cartilages where it forms networks of thin fibrils that differ in morphology from the much thicker fibrils of type I collagen. We studied here in vitro the formation of fibrils of pepsin-treated recombinant human type II collagen produced in insect cells. Two kinds of type II collagen preparation were used: low hydroxylysine collagen having 2.0 hydroxylysine residues/1,000 amino acids, including 1.3 glycosylated hydroxylysines; and high hydroxylysine collagen having 19 hydroxylysines/1,000 amino acids, including 8.9 glycosylated hydroxylysines. A marked difference in fibril formation was found between these two kinds of collagen preparation, in that the maximal turbidity of the former was reached within 5 min under the standard assay conditions, whereas the absorbance of the latter increased until about 600 min. The critical concentration with the latter was about 10-fold, and the absorbance/microgram collagen incorporated into the fibrils was about one-sixth. The morphology of the fibrils was also different, in that the high hydroxylysine collagen formed thin fibrils with essentially no interfibril interaction or aggregation, whereas the low hydroxylysine collagen formed thick fibrils on a background of thin ones. The data thus indicate that regulation of the extents of lysine hydroxylation and hydroxylysine glycosylation may play a major role in the regulation of collagen fibril formation and the morphology of the fibrils.

Cloning of the alpha subunit of prolyl 4-hydroxylase from Drosophila and expression and characterization of the corresponding enzyme tetramer with some unique properties

Prolyl 4-hydroxylase catalyzes the formation of 4-hydroxyproline in collagens. The vertebrate enzymes are alpha2beta2 tetramers, whereas the Caenorhabditis elegans enzyme is an alphabeta dimer, the beta subunit being identical to protein-disulfide isomerase (PDI). We report here that the processed Drosophila melanogaster alpha subunit is 516 amino acid residues in length and shows 34 and 35% sequence identities to the two types of human alpha subunit and 31% identity to the C. elegans alpha subunit. Its coexpression in insect cells with the Drosophila PDI polypeptide produced an active enzyme tetramer, and small amounts of a hybrid tetramer were also obtained upon coexpression with human PDI. Four of the five recently identified critical residues at the catalytic site were conserved, but a histidine that probably helps the binding of 2-oxoglutarate to the Fe2+ and its decarboxylation was replaced by arginine 490. The enzyme had a higher Km for 2-oxoglutarate, a lower reaction velocity, and a higher percentage of uncoupled decarboxylation than the human enzymes. The mutation R490H reduced the percentage of uncoupled decarboxylation, whereas R490S increased the Km for 2-oxoglutarate, reduced the reaction velocity, and increased the percentage of uncoupled decarboxylation. The recently identified peptide-binding domain showed a relatively low identity to those from other species, and the Km of the Drosophila enzyme for (Pro-Pro-Gly)10 was higher than that of any other animal prolyl 4-hydroxylase studied. A 1. 9-kilobase mRNA coding for this alpha subunit was present in Drosophila larvae.

Identification of a novel proline-rich peptide-binding domain in prolyl 4-hydroxylase

Prolyl 4-hydroxylase (EC 1.14.11.2) catalyzes the hydroxylation of -X-Pro-Gly- sequences and plays a central role in the synthesis of all collagens. The [alpha(I)]2beta2 type I enzyme is effectively inhibited by poly(L-proline), whereas the [alpha(II)]2beta2 type II enzyme is not. We report here that the poly(L-proline) and (Pro-Pro-Gly)10 peptide substrate-binding domain of prolyl 4-hydroxylase is distinct from the catalytic domain and consists of approximately 100 amino acids. Peptides of 10-19 kDa beginning around residue 140 in the 517 residue alpha(I) subunit remained bound to poly(L-proline) agarose after limited proteolysis of the human type I enzyme tetramer. A recombinant polypeptide corresponding to the alpha(I) subunit residues 138-244 and expressed in Escherichia coli was soluble, became effectively bound to poly(L-proline) agarose and could be eluted with (Pro-Pro-Gly)10. This polypeptide is distinct from the SH3 and WW domains, and from profilin, and thus represents a new type of proline-rich peptide-binding module. Studies with enzyme tetramers containing mutated alpha subunits demonstrated that the presence of a glutamate and a glutamine in the alpha(II) subunit in the positions corresponding to Ile182 and Tyr233 in the alpha(I) subunit explains most of the lack of poly(L-proline) binding of the type II prolyl 4-hydroxylase.

KEYWORDS

collagen/dioxygenases/peptide-binding domain/ proline-rich/prolyl hydroxylase

The acidic C-terminal domain of protein disulfide isomerase is not critical for the enzyme subunit function or for the chaperone or disulfide isomerase activities of the polypeptide

Protein disulfide isomerase (PDI) is a multifunctional polypeptide that acts as a subunit in the animal prolyl 4-hydroxylases and the microsomal triglyceride transfer protein, and as a chaperone that binds various peptides and assists their folding. We report here that deletion of PDI sequences corresponding to the entire C-terminal domain c, previously thought to be critical for chaperone activity, had no inhibitory effect on the assembly of recombinant prolyl 4-hydroxylase in insect cells or on the in vitro chaperone activity or disulfide isomerase activity of purified PDI. However, partially overlapping critical regions for all these functions were identified at the C-terminal end of the preceding thioredoxin-like domain a'. Point mutations introduced into this region identified several residues as critical for prolyl 4-hydroxylase assembly. Circular dichroism spectra of three mutants suggested that two of these mutations may have caused only local alterations, whereas one of them may have led to more extensive structural changes. The critical region identified here corresponds to the C-terminal alpha helix of domain a', but this is not the only critical region for any of these functions.

Cloning and characterization of a third human lysyl hydroxylase isoform

Lysyl hydroxylase (EC 1.14.11.4), a homodimer, catalyzes the formation of hydroxylysine in collagens. Recently, an isoenzyme termed lysyl hydroxylase 2 has been cloned from human sources [M. Valtavaara, H. Papponen, A.-M. Pirttilä, K. Hiltunen, H. Helander and R. Myllylä (1997) J. Biol. Chem. 272, 6831-6834]. We report here on the cloning of a third human lysyl hydroxylase isoenzyme, termed lysyl hydroxylase 3. The cDNA clones encode a 738 amino acid polypeptide, including a signal peptide of 24 residues. The overall amino acid sequence identity between the processed human lysyl hydroxylase 3 and 1 polypeptides is 59%, and that between the processed lysyl hydroxylase 3 and 2 polypeptides is 57%, whereas the identity to the processed Caenorhabditis elegans polypeptide is only 45%. All four recently identified critical residues at the catalytic site, two histidines, one aspartate, and one arginine, are conserved in all these polypeptides. The mRNA for lysyl hydroxylase 3 was found to be expressed in a variety of tissues, but distinct differences appear to exist in the expression patterns of the three isoenzyme mRNAs. Recombinant lysyl hydroxylase 3 expressed in insect cells by means of a baculovirus vector was found to be more soluble than lysyl hydroxylase 1 expressed in the same cell type. No differences in catalytic properties were found between the recombinant lysyl hydroxylase 3 and 1 isoenzymes. Deficiency in lysyl hydroxylase 1 activity is known to cause the type VI variant of the Ehlers-Danlos syndrome, and it is therefore possible that deficiency in lysyl hydroxylase 3 activity may lead to some other variant of this syndrome or to some other heritable connective tissue disorder.

Identification of arginine-700 as the residue that binds the C-5 carboxyl group of 2-oxoglutarate in human lysyl hydroxylase 1

Lysyl hydroxylase catalyzes the formation of hydroxylysine in collagens by a reaction that involves oxidative decarboxylation of 2-oxoglutarate. Its binding site can be divided into two main subsites: subsite I consists of a positively charged side-chain which binds the C-5 carboxyl group, while subsite II consists of two coordination sites of the enzyme-bound Fe2+ and is chelated by the C-1-C-2 moiety. In order to identify subsite I, we converted Arg-697, Arg-700 and Ser-705 individually to alanine and Arg-700 also to lysine, and expressed the mutant enzymes in insect cells. Arg-700-Ala inactivated lysyl hydroxylase completely, whereas Arg-697-Ala and Ser-723-Ala had only a relatively minor effect. Arg-700-Lys produced 93% inactivation under standard assay conditions, the main effect being a 10-fold increase in the Km for 2-oxoglutarate, whereas the Vmax was unchanged. Arg-700 thus provides the positively charged residue that binds the C-5 carboxyl group of 2-oxoglutarate, whereas Ser-705 appears to be of no functional significance in this binding.

Collagen hydroxylases and the protein disulfide isomerase subunit of prolyl 4-hydroxylases

Prolyl 4-hydroxylases catalyze the formation of 4-hydroxyproline in collagens and other proteins with an appropriate collagen-like stretch of amino acid residues. The enzyme requires Fe(II), 2-oxoglutarate, molecular oxygen, and ascorbate. This review concentrates on recent progress toward understanding the detailed mechanism of 4-hydroxylase action, including: (a) occurrence and function of the enzyme in animals; (b) general molecular properties; (c) intracellular sites of hydroxylation; (d) peptide substrates and mechanistic roles of the cosubstrates; (e) insights into the development of antifibrotic drugs; (f) studies of the enzyme's subunits and their catalytic function; and (g) mutations that lead to Ehlers-Danlos Syndrome. An account of the regulation of collagen hydroxylase activities is also provided.

The novel type II prolyl 4-hydroxylase is the main enzyme form in chondrocytes and capillary endothelial cells, whereas the type I enzyme predominates in most cells

Procollagen-proline dioxygenase (EC 1.14.11.2), an alpha2beta2 tetramer in vertebrates, plays a central role in the synthesis of all collagens. Recently an isoform of the alpha subunit, the alpha(II) subunit, was characterized in man and mouse and found to form a tetramer with the same beta subunit as the previously known alpha(I) subunit. We report here that the (alpha(I))2beta2 type I tetramer is the main enzyme form in most cell types and tissues and that its contribution to total prolyl 4-hydroxylase activity in cultured cells increases in confluence. Surprisingly, however, the (alpha(II))2beta2 type II enzyme was found to represent at least about 70% of the total prolyl 4-hydroxylase activity in cultured mouse chondrocytes and about 80% in mouse cartilage, the corresponding percentage in mouse bone being about 45% and that in many other mouse tissues about 10% or less. Immunofluorescence studies on samples from a fetal human foot confirmed these data and additionally indicated that the type II enzyme represents the main or only enzyme form in capillary endothelial cells. Thus the type II prolyl 4-hydroxylase is likely to play a major role in the development of cartilages and cartilaginous bones and also of capillaries.

Prolyl 4-hydroxylases and their protein disulfide isomerase subunit

Prolyl 4-hydroxylases (EC 1.14,11.2) catalyze the formation of 4-hydroxyproline in collagens and other proteins with collagen-like sequences. The vertebrate type I and type II enzymes are [alpha (I)]2 beta 2 and [alpha (II)]2 beta 2 tetramers, respectively, whereas the enzyme from the nematode Caenorhabditis elegans is an alpha beta dimer. The type I enzyme is the major form in most but not all vertebrate tissues. The catalytic properties of the various enzyme forms are highly similar, but there are distinct, although small, differences in K(m) values for various peptide substrates between the enzyme forms and major differences in Ki values for the competitive inhibitor, poly(L-proline). Prolyl 4-hydroxylase requires Fe2+, 2-oxoglutarate, O2 and ascorbate. Kinetic studies and theoretical considerations have led to elucidation of the reaction mechanism, and recent extensive site-directed mutagenesis studies have identified five critical residues at the cosubstrate binding sites. A number of compounds have been characterized that inhibit it competitively with respect to some of the cosubstrates, and three groups of suicide inactivators have also been identified. The beta subunit in all forms of prolyl 4-hydroxylase is identical to protein disulfide isomerase (PDI), a multifunctional polypeptide that also serves as a subunit in the microsomal triglyceride transfer protein, as a chaperone-like polypeptide that probably assists folding of a number of newly synthesized proteins, and in several other functions. The main role of the PDI polypeptide as a protein subunit is probably related to its chaperone function. Recent expression studies of recombinant human prolyl 4-hydroxylase subunits in a yeast have indicated that the formation of a stable enzyme tetramer in vivo requires coexpression of collagen polypeptide chains.

A splice-site mutation that induces exon skipping and reduction in lysyl hydroxylase mRNA levels but does not create a nonsense codon in Ehlers-Danlos syndrome type VI

The type VI variant of Ehlers-Danlos syndrome (EDS) is a heritable connective tissue disorder caused by a deficiency in the activity of lysyl hydroxylase, an enzyme required for the post-translational processing of collagens. We have characterized a novel type of mutation in a young female patient with type VI EDS, in which cells possess only 12% of the lysyl hydroxylase activity that is detected in unaffected cells. The syndrome was found to be caused by a homozygous insertion of two thymidines at the 5' splice site consensus sequence of intron 9 in the lysyl hydroxylase gene. The insertion interfered with normal splicing of the primary RNA transcript and resulted in an inframe deletion of the 132 nucleotides coded by exon 9 from the lysyl hydroxylase mRNA. In addition, the mutation caused a marked reduction in the steady-state level of the truncated mRNA, which was less than 15% of the level found in unaffected cells. The mutation also reduced the amount of the enzyme protein produced, which was estimated to be about 20% of that in control cells. However, the mutation did not affect the stability of the abnormally spliced mRNA nor the normal localization of the enzyme protein in the endoplasmic reticulum. According to our results, the reduction in enzymatic activity observed in this patient is caused by low levels of both lysyl hydroxylase mRNA and enzyme protein. The primary cellular defect associated with this mutation, therefore, appears to be at the level of nuclear mRNA metabolism even though the mutation did not create a premature translation termination codon.

Expression and characterization of recombinant human type II collagens with low and high contents of hydroxylysine and its glycosylated forms

Insect cells coinfected with two baculoviruses, one coding for the pro alpha chains of human type II procollagen and the other for both the alpha and beta subunits of human prolyl 4-hydroxylase, produced the cartilage-specific type II collagen with a stable triple helix. The highest expression levels, up to 50 mg/l of type II collagen, were obtained in suspension culture using a modified construct in which sequences coding for the signal peptide and N propeptide of type II procollagen had been replaced by those for type III procollagen. The type III N propeptide artificially generated into type II procollagen was found to be cleaved at a much higher rate than the wild-type type II N propeptide, probably because the former interacted poorly with the triple-helical domain of type II procollagen. The amino acid composition of the recombinant type II collagen was very similar to that of the non-recombinant protein, but the hydroxylysine content was only 17% and that of glycosylated hydroxylysines was equally low. The hydroxylysine content was increased to the level found in the non-recombinant collagen by using an additional baculovirus coding for lysyl hydroxylase, and a substantial increase was also found in the glycosylated hydroxylysine content. No difference in thermal stability was found between the low- and high-hydroxylysine collagens.

Assembly of human prolyl 4-hydroxylase and type III collagen in the yeast pichia pastoris: formation of a stable enzyme tetramer requires coexpression with collagen and assembly of a stable collagen requires coexpression with prolyl 4-hydroxylase

Prolyl 4-hydroxylase, the key enzyme of collagen synthesis, is an alpha2beta2 tetramer, the beta subunit of which is protein disulfide isomerase (PDI). Coexpression of the human alpha subunit and PDI in Pichia produced trace amounts of an active tetramer. A much higher, although still low, assembly level was obtained using a Saccharomyces pre-pro sequence in PDI. Coexpression with human type III procollagen unexpectedly increased the assembly level 10-fold, with no increase in the total amounts of the subunits. The recombinant enzyme was active not only in Pichia extracts but also inside the yeast cell, indicating that Pichia must have a system for transporting all the cosubstrates needed by the enzyme into the lumen of the endoplasmic reticulum. The 4-hydroxyproline-containing procollagen polypeptide chains were of full length and formed molecules with stable triple helices even though Pichia probably has no Hsp47-like protein. The data indicate that collagen synthesis in Pichia, and probably also in other cells, involves a highly unusual control mechanism, in that production of a stable prolyl 4-hydroxylase requires collagen expression while assembly of a stable collagen requires enzyme expression. This Pichia system seems ideal for the high-level production of various recombinant collagens for numerous scientific and medical purposes.

Expression of wild-type and modified proalpha chains of human type I procollagen in insect cells leads to the formation of stable [alpha1(I)]2alpha2(I) collagen heterotrimers and [alpha1(I)]3 homotrimers but not [alpha2(I)]3 homotrimers

Insect cells coinfected with a baculovirus coding for the proalpha1(I) chain of human type I procollagen and a double promoter virus coding for the alpha and beta subunits of human prolyl 4-hydroxylase produced homotrimeric [proalpha1(I)]3 procollagen molecules. The use of an additional virus coding for the proalpha2(I) chain led to the formation of a heterotrimeric molecule with the correct 2:1 ratio of proalpha1 to proalpha2 chains of type I procollagen (proalpha1(I) and proalpha2(I) chains, respectively), unless the proalpha1(I) chain was expressed in a relatively large excess. Replacement of the sequences coding for the signal peptide and the N propeptide of the proalpha1(I) chain with those of the proalpha1(III) chain increased level of expression of the proalpha1(I) chain, whereas no similar effect was found when the corresponding modification was made to the virus coding for the proalpha2(I) chain. Molecules containing such modified N propeptides were found to be processed at their N terminus more rapidly than those containing the wild-type propeptides. The Tm of the type I collagen homotrimer was similar to that of the heterotrimer, both values being about 42-43 degrees C when determined by circular dichroism. The wild-type proalpha2(I) chain formed no homotrimers. Replacement of the C propeptide of the proalpha2(I) chain with that of the proalpha1(I) chain or proalpha1 chain of type III procollagen (proalpha1(III) chain) led to the formation of homotrimers, but the alpha2(I) chains in such molecules were completely digested by pepsin in 1 h at 22 degrees C. The data thus suggest that, in addition to control at the level of the C propeptide, other restrictions may exist at the level of the collagen domain that prevent the formation of stable homotrimeric [proalpha2(I)]3 molecules in insect cells.

Cloning of the human prolyl 4-hydroxylase alpha subunit isoform alpha(II) and characterization of the type II enzyme tetramer. The alpha(I) and alpha(II) subunits do not form a mixed alpha(I)alpha(II)beta2 tetramer

Prolyl 4-hydroxylase (proline hydroxylase, EC 1.14.11.2) catalyzes the formation of 4-hydroxyproline in collagens. The vertebrate enzyme is an alpha2beta2 tetramer, the beta subunit of which is identical to protein disulfide-isomerase (PDI, EC 5.3.4.1). We report here on cloning of the recently discovered alpha(II) subunit from human sources. The mRNA for the alpha(II) subunit was found to be expressed in a variety of human tissues, and the presence of the corresponding polypeptide and the (alpha(II))2beta2 tetramer was demonstrated in cultured human WI-38 and HT-1080 cells. The type II tetramer was found to represent about 30% of the total prolyl 4-hydroxylase in these cells and about 5-15% in various chick embryo tissues. The results of coexpression in insect cells argued strongly against the formation of a mixed alpha(I)alpha(II)beta2 tetramer. PDI/beta polypeptide containing a histidine tag in its N terminus was found to form prolyl 4-hydroxylase tetramers as readily as the wild-type PDI/beta polypeptide, and histidine-tagged forms of prolyl 4-hydroxylase appear to offer an excellent source for a simple large scale purification of the recombinant enzyme. The properties of the purified human type II enzyme were very similar to those of the type I enzyme, but the Ki of the former for poly(L-proline) was about 200-1000 times that of the latter. In agreement with this, a minor difference, about 3-6-fold, was found between the two enzymes in the Km values for three peptide substrates. The existence of two forms of prolyl 4-hydroxylase in human cells raises the possibility that mutations in one enzyme form may not be lethal despite the central role of this enzyme in the synthesis of all collagens.

Structures of the human gene for the protein disulfide isomerase-related polypeptide ERp60 and a processed gene and assignment of these genes to 15q15 and 1q21

ERp60 (also known as ERp61 or GRP58) is an isoform of protein disulfide isomerase (PDI) that has two thioredoxin-like domains a and a' in positions corresponding to those of domains a and a' in the PDI polypeptide and shows a significant amino acid sequence similarity to PDI in almost all parts. We report here that the human ERp60 gene is about 18 kb in size and consists of 13 exons. No distinct correlation was found between its exon-intron organization and the modular structure of the ERp60 polypeptide, nor were any similarities in exon-intron organization found between the human ERp60, PDI, and thioredoxin genes. The 5' flanking region of the ERp60 gene has no TATAA box or CCAAT motif but contains several potential binding sites for transcription factors. The highest levels of expression of the ERp60 mRNA were found by Northern blotting in the liver, placenta, lung, pancreas, and kidney, and the lowest in the heart, skeletal muscle, and brain. We also isolated an intronless ERp60 gene that probably represents a pseudogene. The ERp60 gene was mapped by fluorescence in situ hybridization to 15q15 and the processed gene to 1q21, so that neither was located on the same chromosome as the human PDI and thioredoxin genes.

Characterization of the iron- and 2-oxoglutarate-binding sites of human prolyl 4-hydroxylase

Prolyl 4-hydroxylase (EC 1.14.11.2), an alpha2beta2 tetramer, catalyzes the formation of 4-hydroxyproline in collagens. We converted 16 residues in the human alpha subunit individually to other amino acids, and expressed the mutant polypeptides together with the wild-type beta subunit in insect cells. Asp414Ala and Asp414Asn inactivated the enzyme completely, whereas Asp414Glu increased the K(m) for Fe2+ 15-fold and that for 2-oxoglutarate 5-fold. His412Glu, His483Glu and His483Arg inactivated the tetramer completely, as did Lys493Ala and Lys493His, whereas Lys493Arg increased the K(m) for 2-oxoglutarate 15-fold. His501Arg, His501Lys, His501Asn and His501Gln reduced the enzyme activity by 85-95%; all these mutations increased the K(m) for 2-oxoglutarate 2- to 3-fold and enhanced the rate of uncoupled decarboxylation of 2-oxoglutarate as a percentage of the rate of the complete reaction up to 12-fold. These and other data indicate that His412, Asp414 and His483 provide the three ligands required for the binding of Fe2+ to a catalytic site, while Lys493 provides the residue required for binding of the C-5 carboxyl group of 2-oxoglutarate. His501 is an additional critical residue at the catalytic site, probably being involved in both the binding of the C-1 carboxyl group of 2-oxoglutarate and the decarboxylation of this cosubstrate.

Duplication of seven exons in the lysyl hydroxylase gene is associated with longer forms of a repetitive sequence within the gene and is a common cause for the type VI variant of Ehlers-Danlos syndrome

The type VI variant of the Ehlers-Danlos syndrome (EDS) is a recessively inherited connective tissue disorder which, in most families, is due to a deficiency in lysyl hydroxylase activity. We have recently characterized a homozygous duplication of 8.9 kb in the lysyl hydroxylase gene (PLOD) in two EDS VI families. The duplication is caused by a homologous recombination of Alu sequences in introns 9 and 16. Using PCR, we have analyzed 26 additional EDS VI families from various countries and found that 7 of them have this duplication. Our data has shown a frequency of 19.1% for this mutant allele among 35 EDS VI families studied by us so far. Our haplotype analysis shows a variation in the sequence of DNA region surrounding the duplication. There is an association between a particular allele size class, the long form, at the dinucleotide repeat within intron 16 and the duplication mutation in PLOD. Screening of a general population revealed one positive finding among 582 alleles tested. An abnormal sequence in exon 17 of the gene, which generated a stop codon in the exon sequence and aberrant mRNA processing, was responsible for the nonfunctionality of the other allele in one of the compound heterozygous patients.

Involvement of prolyl 4-hydroxylase in the assembly of trimeric minicollagen XII. Study in a baculovirus expression system

We have shown previously that hydroxylation played a critical role in the trimer assembly and disulfide bonding of the three constituent alpha chains of a minicollagen composed of the extreme C-terminal collagenous (COL1) and noncollagenous (NC1) domains of type XII collagen in HeLa cells (Mazzorana, M., Gruffat, H., Sergeant, A., and van der Rest, M. (1993) J. Biol. Chem. 268, 3029-3032). We have further characterized the involvement of prolyl 4-hydroxylase in the assembly of the three alpha chains to form trimeric disulfide-bonded type XII minicollagen in an insect cell expression system. For this purpose, type XII minicollagen was produced in insect cells from baculovirus vectors, alone or together with wild-type human prolyl 4-hydroxylase or with the human enzyme mutated in the catalytic site of its alpha or beta subunits or with the individual alpha or beta subunits. When type XII minicollagen was produced alone, negligible amounts of disulfide-bonded trimers were found to be produced by the cells. However, coproduction of the collagen with the two subunits of the wild-type human enzyme dramatically increased the amount of disulfide-bonded trimeric type XII minicollagen molecules. In contrast, coproduction of the collagen with alpha subunits that had a mutation completely inactivating the human enzyme failed to enhance the trimer assembly. These results directly show that an active prolyl 4-hydroxylase is required for the assembly of disulfide-bonded trimers of type XII minicollagen.

Baculovirus expression of two protein disulphide isomerase isoforms from Caenorhabditis elegans and characterization of prolyl 4-hydroxylases containing one of these polypeptides as their beta subunit

Protein disulphide isomerase (PDI; EC 5.3.4.1) is a multifunctional polypeptide that is identical to the beta subunit of prolyl 4-hydroxylases. We report here on the cloning and expression of the Caenorhabditis elegans PDI/beta polypeptide and its isoform. The overall amino acid sequence identity and similarity between the processed human and C. elegans PDI/beta polypeptides are 61% and 85% respectively, and those between the C. elegans PDI/beta polypeptide and the PDI isoform 46% and 73%. The isoform differs from the PDI/beta and ERp60 polypeptides in that its N-terminal thioredoxin-like domain has an unusual catalytic site sequence -CVHC-. Expression studies in insect cells demonstrated that the C. elegans PDI/beta polypeptide forms an active prolyl 4-hydroxylase alpha 2 beta 2 tetramer with the human alpha subunit and an alpha beta dimer with the C. elegans alpha subunit, whereas the C. elegans PDI isoform formed no prolyl 4-hydroxylase with either alpha subunit. Removal of the 32-residue C-terminal extension from the C. elegans alpha subunit totally eliminated alpha beta dimer formation. The C. elegans PDI/beta polypeptide formed less prolyl 4-hydroxylase with both the human and C. elegans alpha subunits than did the human PDI/beta polypeptide, being particularly ineffective with the C. elegans alpha subunit. Experiments with hybrid polypeptides in which the C-terminal regions had been exchanged between the human and C. elegans PDI/beta polypeptides indicated that differences in the C-terminal region are one reason, but not the only one, for the differences in prolyl 4-hydroxylase formation between the human and C. elegans PDI/beta polypeptides. The catalytic properties of the C. elegans prolyl 4-hydroxylase alpha beta dimer were very similar to those of the vertebrate type II prolyl 4-hydroxylase tetramer, including the K(m) for the hydroxylation of long polypeptide substrates.

ERp60 does not substitute for protein disulphide isomerase as the beta-subunit of prolyl 4-hydroxylase

Prolyl 4-hydroxylase (EC 1.14.11.2) catalyses the formation of 4-hydroxyproline in collagens. The vertebrate enzymes are alpha 2 beta 2 tetramers while the Caenorhabditis elegans enzyme is an alpha beta dimer. The beta-subunit is identical to protein disulphide isomerase (PDI), a multifunctional endoplasmic reticulum luminal polypeptide. ERp60 is a PDI isoform that was initially misidentified as a phosphatidylinositol-specific phospholipase C. We report here on the cloning and expression of the human and Drosophila ERp60 polypeptides. The overall amino acid sequence identity and similarity between the processed human ERp60 and PDI polypeptides are 29% and 56% respectively, and those between the Drosophila ERp60 and human PDI polypeptides 29% and 55%. The two ERp60 polypeptides were found to be similar to human PDI within almost all their domains, the only exception being the extreme C-terminal region. Nevertheless, when the human or Drosophila ERp60 was expressed in insect cells together with an alpha-subunit of human prolyl 4-hydroxylase, no tetramer was formed and no prolyl 4-hydroxylase activity was generated in the cells. Additional experiments with hybrid polypeptides in which the C-terminal regions had been exchanged between the human ERp60 and PDI polypeptides demonstrated that the differences in the C-terminal region are not the only reason for the lack of prolyl 4-hydroxylase tetramer formation by ERp60.

Characterization of human type III collagen expressed in a baculovirus system. Production of a protein with a stable triple helix requires coexpression with the two types of recombinant prolyl 4-hydroxylase subunit

An efficient expression system for recombinant collagens would have numerous scientific and practical applications. Nevertheless, most recombinant systems are not suitable for this purpose, as they do not have sufficient amounts of prolyl 4-hydroxylase activity. Pro-alpha 1 chains of human type III collagen expressed in insect cells by a baculovirus vector are reported here to contain significant amounts of 4-hydroxyproline and to form triple-helical molecules, although the Tm of the triple helices was only about 32-34 degrees C. Coexpression of the pro-alpha1(III) chains with the alpha and beta subunits of human prolyl 4-hydroxylase increased the Tm to about 40 degrees C, provided that ascorbate was added to the culture medium. The level of expression of type III procollagen was also increased in the presence of the recombinant prolyl 4-hydroxylase, and the pro-alpha 1(III) chains and alpha1(III) chains were found to be present in disulfide-bonded molecules. Most of the triple-helical collagen produced was retained within the insect cells and could be extracted from the cell pellet. The highest expression levels were obtained in High Five cells, which produced up to about 80 microg of cellular type III collagen (120 microg of procollagen) per 5 X 10(6) cells in monolayer culture and up to 40 mg/liter of cellular type III collagen (60 mg/liter procollagen) in suspension. The 4-hydroxyproline content and Tm of the purified recombinant type III collagen were very similar to those of the nonrecombinant protein, but the hydroxylysine content was slightly lower, being about 3 residues/1000 in the former and 5/1000 in the latter.

Site-directed mutagenesis of human lysyl hydroxylase expressed in insect cells. Identification of histidine residues and an aspartic acid residue critical for catalytic activity

Lysyl hydroxylase (EC 1.14.11.4), an alpha 2 homodimer, catalyzes the formation of hydroxylysine in collagens. We expressed here human lysyl hydroxylase in insect cells by baculovirus vectors. About 90% of the enzyme produced was soluble 32 h after infection, whereas only 10% was soluble at 72 h. Twelve histidines, five aspartates, and all four asparagines that may act as N-glycosylation sites were converted individually to serine, alanine, or glutamine, respectively, and the mutant enzymes were expressed in insect cells. Three histidine mutations and one aspartate mutation appeared to inactivate the enzyme completely. These and other data suggest that histidines 656 and 708 and aspartate 658 provide the three ligands required for the binding of Fe2+ to a catalytic site, whereas the role of the third critical histidine (residue 706) remains to be established. Three additional histidine mutations also had a major effect, although they did not inactivate the enzyme completely, whereas six further histidine mutations and four out of five aspartate mutations had a much more minor effect. Data on the four asparagine mutations suggested that only two of the potential N-glycosylation sites may be fully glycosylated in insect cells and that one of these carbohydrate units may be needed for full enzyme activity.

Co-expression of the alpha subunit of human prolyl 4-hydroxylase with BiP polypeptide in insect cells leads to the formation of soluble and insoluble complexes. Soluble alpha-subunit-BiP complexes have no prolyl 4-hydroxylase activity

Prolyl 4-hydroxylase (EC 1.14.11.2) catalyses the post-translational formation of 4-hydroxyproline in collagens. The vertebrate enzymes are alpha2beta2 tetramers, their beta subunit being identical to protein disulphide isomerase (PDI). The function of the PDI-beta subunit in prolyl 4-hydroxylases is not fully understood, but it seems to be that of keeping the highly insoluble alpha subunits in solution. We report here that expression of the alpha subunit of human type I prolyl 4-hydroxylase in insect cells together with BiP polypeptide leads to the formation of both soluble and insoluble alpha-subunit-BiP complexes. Formation of the soluble complexes was evident from (1) a marked increase in the amount of the alpha subunit in the soluble fraction of the cell homogenates when expressed together with BiP, (2) immunoprecipitation experiments and (3) demonstration of the presence of some of the complexes by polyacrylamide gel electrophoresis under non-denaturing conditions. Formation of the insoluble complexes was suggested by an increase in the amount of BiP in the insoluble fraction when expressed together with the alpha subunit. Nevertheless the soluble alpha-subunit-BiP complexes had no prolyl 4-hydroxylase activity. This indicates that the function of the PDI-beta subunit in the prolyl 4-hydroxylase tetramer is not only that of keeping the alpha subunits in solution but appears to be more specific, probably that of keeping them in a catalytically active, non-aggregated conformation.

The role of protein disulphide isomerase in the microsomal triacylglycerol transfer protein does not reside in its isomerase activity

The microsomal triacylglycerol transfer protein (MTP), an alpha beta dimer, is obligatory for the assembly of apoB-containing lipoproteins in liver and intestinal cells. The beta subunit is identical with protein disulphide isomerase, a 58 kDa endoplasmic reticulum luminal protein involved in ensuring correct disulphide bond formation of newly synthesized proteins. We report here the expression of the human MTP subunits in Spodoptera frugiperda cells. When the alpha subunit was expressed alone, the polypeptide formed insoluble aggregates that were devoid of triacylglycerol transfer activity. In contrast, when the alpha and beta subunits were co-expressed, soluble alpha beta dimers were formed with significant triacylglycerol transfer activity. Expression of the alpha subunit with a mutant protein disulphide isomerase polypeptide in which both -CGHC- catalytic sites had been inactivated also yielded alpha beta dimers that had comparable levels of lipid transfer activity relative to wild-type dimers. The results indicate that the role of the beta subunit in MTP seems to be to keep the alpha subunit in a catalytically active, non-aggregated conformation and that disulphide isomerase activity of the beta subunit is not required for this function.

Expression of mRNAs for lysyl oxidase and type III procollagen in cultured fibroblasts from patients with the Menkes and occipital horn syndromes as determined by quantitative polymerase chain reaction

The Menkes syndrome and the occipital horn syndrome are two X-linked recessively inherited disorders characterized by abnormalities in copper metabolism. These abnormalities are associated with a reduction in the activity of lysyl oxidase (EC 1.4.3.13), an extracellular copper enzyme that initiates the crosslinking of collagens and elastin. We report here that the amount of lysyl oxidase mRNA, as studied by Northern blotting, and the number of lysyl oxidase mRNA molecules per picogram of RNA, as determined by a quantitative PCR method, were decreased in three cultured skin fibroblast lines from patients with the Menkes syndrome and two from patients with the occipital horn syndrome compared with four control cell lines. The decreased lysyl oxidase activity found in these disorders thus appears to be a least in part due to a pretranslational mechanism. No decrease was found in the number of the beta-actin mRNA molecules in the Menkes cell lines, but rather a slight increase, whereas a decrease was found in these molecules in the occipital horn cell lines. An additional abnormality found in the Menkes cell lines was a significant increase in the number of mRNA molecules for type III procollagen in two of the three cell lines investigated. The present and previous data indicate that the Menkes syndrome may involve several abnormalities in the expression of genes for connective tissue proteins.

Quantitative polymerase chain reaction of lysyl oxidase mRNA in malignantly transformed human cell lines demonstrates that their low lysyl oxidase activity is due to low quantities of its mRNA and low levels of transcription of the respective gene

Lysyl oxidase (EC 1.4.3.13), an extracellular copper amino oxidase, initiates the cross-linking of collagens and elastin by catalyzing oxidative deamination of the epsilon-amino group in certain lysine and hydroxylysine residues. We developed here a polymerase chain reaction (PCR) method for the quantification of lysyl oxidase mRNA in which a synthetic RNA is used as an internal standard for coamplification with the targeted mRNA. The amount of lysyl oxidase mRNA when studied by Northern blot analysis and the number of lysyl oxidase mRNA molecules when determined by the quantitative PCR method were found to be markedly low in various malignantly transformed cell lines relative to control cell lines, quantitative PCR indicating values of about 2-10% of those in the controls. No difference was found in the number of beta-actin mRNA molecules between the transformed cells and the controls. Nuclear runoff experiments indicated that most if not all of the decrease in the number of lysyl oxidase mRNA molecules can be explained by diminished transcription of the respective gene.

Conservation of the sizes of 53 introns and over 100 intronic sequences for the binding of common transcription factors in the human and mouse genes for type II procollagen (COL2A1)

Over 11,000 bp of previously undefined sequences of the human COL2A1 gene were defined. The results made it possible to compare the intron structures of a highly complex gene from man and mouse. Surprisingly, the sizes of the 53 introns of the two genes were highly conserved with a mean difference of 13%. After alignment of the sequences, 69% of the intron sequences were identical. The introns contained consensus sequences for the binding of over 100 different transcription factors that were conserved in the introns of the two genes. The first intron of the gene contained 80 conserved consensus sequences and the remaining 52 introns of the gene contained 106 conserved sequences for the binding of transcription factors. The 5'-end of intron 2 in both genes had a potential for forming a stem loop in RNA transcripts.

Collagens: molecular biology, diseases, and potentials for therapy

The collagen superfamily of proteins now contains at least 19 proteins formally defined as collagens and an additional ten proteins that have collagen-like domains. The most abundant collagens form extracellular fibrils or network-like structures, but the others fulfill a variety of biological functions. Some of the eight highly specific post-translational enzymes involved in collagen biosynthesis have recently been cloned. Over 400 mutations in 6 different collagens cause a variety of human diseases that include osteogenesis imperfecta, chondrodysplasias, some forms of osteoporosis, some forms of osteoarthritis, and the renal disease known as the Alport syndrome. Many of the disease phenotypes have been produced in transgenic mice with mutated collagen genes. There has been increasing interest in the possibility that the unique post-translational enzymes involved in collagen biosynthesis offer attractive targets for specifically inhibiting excessive fibrotic reactions in a number of diseases. A number of experiments suggest it may be possible to inhibit collagen synthesis with oligo-nucleotides or antisense genes.

Cloning, baculovirus expression, and characterization of a second mouse prolyl 4-hydroxylase alpha-subunit isoform: formation of an alpha 2 beta 2 tetramer with the protein disulfide-isomerase/beta subunit

Prolyl 4-hydroxylase (EC 1.14.11.2) catalyzes the posttranslational formation of 4-hydroxyproline in collagens. The vertebrate enzyme is an alpha 2 beta 2 tetramer, the beta subunit of which is a highly unusual multifunctional polypeptide, being identical to protein disulfide-isomerase (EC 5.3.4.1). We report here the cloning of a second mouse alpha subunit isoform, termed the alpha (II) subunit. This polypeptide consists of 518 aa and a signal peptide of 19 aa. The processed polypeptide is one residue longer than the mouse alpha (I) subunit (the previously known type), the cloning of which is also reported here. The overall amino acid sequence identity between the mouse alpha (II) and alpha (I) subunits is 63%. The mRNA for the alpha (II) subunit was found to be expressed in a variety of mouse tissues. When the alpha (II) subunit was expressed together with the human protein disulfide-isomerase/beta subunit in insect cells by baculovirus vectors, an active prolyl 4-hydroxylase was formed, and this protein appeared to be an alpha (II) 2 beta 2 tetramer. The activity of this enzyme was very similar to that of the human alpha (I) 2 beta 2 tetramer, and most of its catalytic properties were also highly similar, but it differed distinctly from the latter in that it was inhibited by poly(L-proline) only at very high concentrations. This property may explain why the type II enzyme was not recognized earlier, as an early step in the standard purification procedure for prolyl 4-hydroxylase is affinity chromatography on a poly(L-proline) column.