Hydroxyproline biosynthesis in plant cells. Peptidyl proline hydroxylase from carrot disks

Arabinosylation Plays a Crucial Role in Extensin Cross-linking In Vitro

Extensins (EXTs) are hydroxyproline-rich glycoproteins (HRGPs) that are structural components of the plant primary cell wall. They are basic proteins and are highly glycosylated with carbohydrate accounting for >50% of their dry weight. Carbohydrate occurs as monogalactosyl serine and arabinosyl hydroxyproline, with arabinosides ranging in size from ~1 to 4 or 5 residues. Proposed functions of EXT arabinosylation include stabilizing the polyproline II helix structure and facilitating EXT cross-linking. Here, the involvement of arabinosylation in EXT cross-linking was investigated by assaying the initial cross-linking rate and degree of cross-linking of partially or fully de-arabinosylated EXTs using an in vitro cross-linking assay followed by gel permeation chromatography. Our results indicate that EXT arabinosylation is required for EXT cross-linking in vitro and the fourth arabinosyl residue in the tetraarabinoside chain, which is uniquely α-linked, may determine the initial cross-linking rate. Our results also confirm the conserved structure of the oligoarabinosides across species, indicating an evolutionary significance for EXT arabinosylation.

Chlamydomonas agglutinin is a hydroxyproline-rich glycoprotein

The mt(+) sexual agglutinin from Chlamydomonas reinhardi is shown to contain approximately 12% hydroxyproline, and two inhibitors of hydroxyproline formation, alpha,alpha'-dipyridyl and 3,4-dehydroproline, are shown to block the production of agglutinin activity in an in vivo bioassay system. These results indicate that the agglutinin glycoprotein may be related to a class of hydroxyproline-rich glycoproteins found in the extracellular matrix of higher plants, several of which have been shown to have lectin activity.

Occurrence and stress response of N-methylproline compounds in Tamarix species

A number of N-methylproline analogues have been found to accumulate in different species of Tamarix. These include N-methyl-L-proline (MP), trans-4-hydroxy-N-methyl-L-proline (M4HP) and trans-3-hydroxy-N-methyl-L-proline (M3HP). The three compounds appeared in all species but their relative and absolute levels depend upon species, ecotype and level of applied salt stress. A salt-conditioned ecotype of T. jordanis (Sodom) dramatically increased its accumulation of all proline analogues when subject to salt stress whereas a non-saline ecotype (Gilboa) showed little effect. The levels of M4HP and M3HP in T. meyeri increased with increasing salt stress whereas MP levels remained almost constant.

Cloning and characterization of a low molecular weight prolyl 4-hydroxylase from Arabidopsis thaliana. Effective hydroxylation of proline-rich, collagen-like, and hypoxia-inducible transcription factor alpha-like peptides

4-Hydroxyproline is found in collagens and collagen-like proteins in animals and in many glycoproteins in plants. Animal prolyl 4-hydroxylases (P4Hs) have been cloned and characterized from many sources, but no plant P4H has been cloned so far. We report here that the genome of Arabidopsis thaliana encodes six P4H-like polypeptides, one of which, a 283-residue soluble monomer, was cloned and characterized here as a recombinant protein. Catalytically critical residues identified in animal P4Hs are conserved in this P4H, and their mutagenesis led to complete or almost complete inactivation. The recombinant P4H effectively hydroxylated poly(l-proline) and many synthetic peptides corresponding to proline-rich repeats present in plant glycoproteins and other proteins. Surprisingly, collagen-like peptides were also good substrates, the V(max) with (Pro-Pro-Gly)(10) being similar to that with poly(l-proline). The enzyme acted in this peptide preferentially on prolines in Y positions in the X-Y-Gly triplets. Correspondingly, (Gly-Pro-4Hyp)(5) and (Pro-Ala-Gly)(5) were poor substrates, with V(max) values less than 5 and 20% of that obtained with (Pro-Pro-Gly)(10), respectively, the K(m) for the latter also being high. Peptides representing the N- and C-terminal hydroxylation sites present in hypoxia-inducible transcription factor alpha also served as substrates. As these peptides contain only one proline residue, a poly(l-proline) type II conformation was clearly not required for hydroxylation.

Contiguous hydroxyproline residues direct hydroxyproline arabinosylation in Nicotiana tabacum

Hydroxyproline (Hyp) O-glycosylation characterizes the hydroxyproline-rich glycoprotein (HRGP) superfamily of the plant extracellular matrix. Hyp glycosylation occurs in two modes: Arabinosylation adds short oligoarabinosides (Hyp-arabinosides) while galactosylation leads to the addition of larger arabinogalactan polysaccharides (Hyp-polysaccharides). We hypothesize that sequence-dependent glycosylation of small peptide motifs results in glycomodules. These small functional units in combination with other repetitive peptide modules define the properties of HRGPs. The Hyp contiguity hypothesis predicts arabinosylation of contiguous Hyp residues and galactosylation of clustered noncontiguous Hyp residues. To determine the minimum level of Hyp contiguity that directs arabinosylation, we designed a series of synthetic genes encoding repetitive (Ser-Pro(2))(n), (Ser-Pro(3))(n), and (Ser-Pro(4))(n). A signal sequence targeted these endogenous substrates to the endoplasmic reticulum/Golgi for post-translational proline hydroxylation and glycosylation in transformed Nicotiana tabacum cells. The fusion glycoproteins also contained green fluorescence protein, facilitating their detection and isolation. The (Ser-Pro(2))(n) and (Ser-Hyp(4))(n) fusion glycoproteins yielded Hyp-arabinosides but no Hyp-polysaccharide. The motif (Ser-Pro(3))(n) was incompletely hydroxylated, yielding mixed contiguous/noncontiguous Hyp and a corresponding mixture of Hyp-arabinosides and Hyp-polysaccharides. These results plus circular dichroic spectra of the glycosylated and deglycosylated (Ser-Pro(2))(n), (Ser-Pro(3))(n), and (Ser-Pro(4))(n) modules corroborate the Hyp contiguity hypothesis and indicate that Hyp O-glycosylation is indeed sequence-driven.

Synthetic genes for glycoprotein design and the elucidation of hydroxyproline-O-glycosylation codes

Design of hydroxyproline (Hyp)-rich glycoproteins (HRGPs) offers an approach for the structural and functional analysis of these wall components, which are broadly implicated in plant growth and development. HRGPs consist of multiple small repetitive "glycomodules" extensively O-glycosylated through the Hyp residues. The patterns of Hyp-O-glycosylation are putatively coded by the primary sequence as described by the Hyp contiguity hypothesis, which predicts contiguous Hyp residues to be attachment sites of small arabinooligosaccharides (1-5 Ara residues/Hyp); while clustered, noncontiguous Hyp residues are sites of arabinogalactan polysaccharide attachment. As a test, we designed two simple HRGPs as fusion proteins with green fluorescent protein. The first was a repetitive Ser-Hyp motif that encoded only clustered noncontiguous Hyp residues, predicted polysaccharide addition sites. The resulting glycoprotein had arabinogalactan polysaccharide O-linked to all Hyp residues. The second construct, based on the consensus sequence of a gum arabic HRGP, contained both arabinogalactan and arabinooligosaccharide addition sites and, as predicted, gave a product that contained both saccharide types. These results identify an O-glycosylation code of plants.

Prolyl 4-hydroxylase from Volvox carteri. A low-Mr enzyme antigenically related to the alpha subunit of the vertebrate enzyme

Prolyl 4-hydroxylase was isolated in a highly purified form from a multi-cellular green alga, Volvox carteri, by a procedure consisting of ion-exchange chromatography and affinity chromatography on poly(L-hydroxyproline) coupled to Sepharose. Two other affinity-column procedures were also developed, one involving 3,4-dihydroxyphenylacetate and the other 3,4-dihydroxyphenylpropionate linked to Sepharose. The Km values of the Volvox enzyme for the co-substrates and the peptide substrate, as well as the inhibition constants for selected 2-oxoglutarate analogues, were similar to those of the enzyme from Chlamydomonas reinhardii, except that the Km for 2-oxoglutarate with the Volvox enzyme was 6-fold greater. The temperature optimum of the Volvox enzyme was also 10 degrees C higher. The apparent Mr of the Volvox enzyme by gel filtration was about 40,000, being similar to that reported for the Chlamydomonas enzyme but markedly lower than that of the vertebrate enzymes. A similar apparent Mr of about 40,000 was also found for prolyl 4-hydroxylase from the green alga Enteromorpha intestinalis, whereas the enzyme from various vascular plants gave an apparent Mr greater than 300,000. SDS/polyacrylamide-gel electrophoresis demonstrated in the highly purified Volvox enzyme the presence of a major protein band doublet with a Mr of about 65,000 and a minor doublet of Mr about 55,000-57,000. A polyclonal antiserum, prepared against the Mr-65,000 doublet, stained in immunoblotting the Mr-65,000 doublet as well as the alpha subunit, but not the beta subunit, of the vertebrate prolyl 4-hydroxylase. An antiserum against the beta subunit of the vertebrate enzyme stained in immunoblotting a Mr-50,000 polypeptide in a partially purified Volvox enzyme preparation, but did not stain either the Mr-65,000 or the Mr-55,000-57,000 doublet of the highly purified enzyme. The data thus suggest that the active Volvox carteri prolyl 4-hydroxylase is an enzyme monomer antigenically related to the alpha subunit of the vertebrate enzyme.

Prolyl 4-hydroxylase in the foot of the marine mussel Mytilus edulis L.: purification and characterization

The mussel foot secretes a variety of unusual hydroxyproline-containing collagenous and noncollagenous proteins. Prolyl 4-hydroxylase acting on one or more of the secreted proteins was isolated from the foot by using conventional gel filtration and ion exchange chromatography. Mr of the intact enzyme was 230,000 (alpha 2 beta 2) composed of two subunits with Mr of 60,000 (alpha) and 57,000 (beta) as estimated by HPLC gel filtration and SDS-PAGE. The enzyme utilized (Pro-Pro-Gly)10 as a substrate with an apparent Km value of 0.17 mM. Cofactors and inhibitors were very similar to animal, plant, and microbial prolyl hydroxylases previously described. The enzyme had a relatively sharp pH optimum in the range of 7.8-8.3 and the hydroxyproline formed increased in proportion to the rise in the temperature between 5 and 20 degrees C. No detectable hydroxylation occurred with poly-L-proline or the unhydroxylated decapeptide analog (Ala-Lys-Pro-Ser-Tyr-Pro-Pro-Thr-Tyr-Lys) of the polyphenolic protein. Kinetic studies, however, revealed that the mussel prolyl 4-hydroxylase was competitively inhibited by poly-L-proline and uncompetitively inhibited by the decapeptide. These results suggest that the decapeptide binds the enzyme-substrate i.e. (Pro-Pro-Gly)10 complex. It is not yet clear whether this enzyme acts exclusively on collagenous substrates or whether its catalytic purview extends as well to the polyphenolic protein.

Characterization of a low-relative-molecular-mass prolyl 4-hydroxylase from the green alga Chlamydomonas reinhardii

Prolyl 4-hydroxylase was partially purified and characterized from the unicellular green alga, Chlamydomonas reinhardii. This enzyme differed from all the animal and plant prolyl 4-hydroxylases studied so far in that its Mr was only about 40,000 by gel filtration, being thus less than one-sixth of those determined for the vertebrate and higher-plant enzymes. The algal enzyme did not hydroxylate to any significant extent chick-embryo protocollagen or triple-helical (Pro-Pro-Gly)10, whereas a low hydroxylation rate was found with denatured (Pro-Pro-Gly)10. Poly(L-proline), which is an effective inhibitor of the vertebrate enzymes but acts as a substrate for some higher-plant enzymes, was a good substrate. In the absence of poly(L-proline) the enzyme catalysed an uncoupled decarboxylation of 2-oxoglutarate. Studies of the Km values for the co-substrates and cofactors and the specificity of the 2-oxoglutarate requirement, as well as inhibition studies with selected 2-oxoglutarate analogues, suggested that the catalytic site of the algal enzyme is similar to, but not identical with, those of the vertebrate enzymes. The existence of distinct similarities was further demonstrated by an inhibition of the algal enzyme activity with a monoclonal antibody to the beta-subunit of human prolyl 4-hydroxylase. The amount of prolyl 4-hydroxylase activity in the algal cells was not altered by signals which recognize the presence or absence of the cell wall, as determined in studies on experimental cell-wall regeneration and wall-less mutants.

Isolation and characterization of prolyl hydroxylase from Chlamydomonas reinhardii

Prolyl hydroxylase, which is responsible for the hydroxylation of peptidyl proline residues, has been isolated and purified from the green alga Chlamydomonas reinhardii. The enzyme, which appears to be loosely associated with microsomal membranes, was released into solution by sonication in the presence of detergent. Purification was achieved by ion-exchange chromatography followed by affinity chromatography using the immobilized substrate poly-L-proline. Apart from its differing substrate specificity the enzyme appears to possess similar molecular characteristics to prolyl hydroxylase isolated from animal tissues: the active enzyme is a tetramer of about 240-250 kDa and nonidentical monomers of 65 and 60 kDa. The monomers are capsule shaped having a dimension of 12×7 nm.

Elicitor-induced prolyl hydroxylase from French bean (Phaseolus vulgaris). Localization, purification and properties

The enzyme prolyl hydroxylase (proline: 2-oxoglutarate dioxygenase, EC 1.14.11.12), induced in suspension-cultured cells of Phaseolus vulgaris L. (French bean) by treatment with an elicitor preparation from the phytopathogenic fungus Colletotrichum lindemuthianum, has been investigated. The enzyme, which catalyses the hydroxylation of poly-L-proline with the stoichiometric decarboxylation of 2-oxoglutarate, has been shown to be localized mainly in smooth endoplasmic reticulum. After solubilization from microsomal membranes, the hydroxylase was purified by ion-exchange chromatography and affinity chromatography on poly-L-proline-Sepharose 4B. The subunit Mr, as assessed by sodium dodecyl sulphate/poly-acrylamide-gel electrophoresis, was 65 000, the subunit apparently being recovered as a doublet: the subunits associate under non-denaturing conditions to give at least a tetramer. The bean hydroxylase has kinetic properties and cofactor requirements similar to those previously reported for the enzyme from other plants. Elicitor treatment of suspension-cultured bean cells leads to a rapid induction of prolyl hydroxylase activity concomitant with induction of a protein: arabinosyl-transferase and increased levels of an arabinosylated hydroxyproline-rich protein.

Intracellular localization of posttranslational modifications in the synthesis of hydroxyproline-rich glycoproteins. Peptidyl proline hydroxylation in maize roots

The enzyme prolyl hydroxylase which is responsible for the hydroxylation of peptidyl proline has been investigated in extracts of maize roots. The optimum conditions under which this enzyme can be assayed have been determined using both a colorometric and a radiochemical assay. The enzyme has certain features in common with vertebrate prolyl hydroxylase (pH optimum, requirement for ferrous ion, inhibition by tricine and phosphate buffers, stimulation by bovine serum albumin) but prefers poly-L-proline to collagenous substrates. Centrifugation studies shows that the enzyme is mainly membrane-bound and is primarily localized in the endoplasmic reticulum, although the presence of small amounts in the Golgi apparatus cannot be ruled out.

Organelles involved in the synthesis and transport of hydroxyproline-containing glycoproteins in carrot root discs

Isopycnic sucrose density gradients of homogenates from carrot root tissue were analyzed radiochromatographically, radiochemically, and photometrically for the presence of hydroxyproline residues. Significant amounts were found in endoplasmic reticulum (ER), Golgi apparatus (GA) and plasma membrane (PM) fractions as designated by the presence of marker enzymes for these membranes. Some hydroxyproline-containing macromolecules could be detected in the soluble cytoplasm (cytosol) but this was interpreted as an artefact due to homogenization. Hydroxyproline-rich polymers can be released from a mixed ER/PM fraction by freezing and thawing in water. The PM-associated hydroxyproline polymer is suggested to be an arabinogalactan protein rather than cell wall extensin. Nevertheless, the polypeptides of both glycoproteins are considered as being synthesized at the ER and transported via the GA to the PM.

A new prolyl hydroxylase acting on poly-L-proline, from suspension cultured cells of Vinca rosea

A new prolyl hydroxylase having a novel substrate specificity was isolated from the suspension-cultured cells of Vinca rosea. This enzyme was solubilized with 0.05 M Tris-HCl buffer (pH 7.4) containing 0.1% Triton X-100, 0.3 M NaCl and 0.5 mM beta-mercaptoethanol from the membrane fractions of the cells, and was partially purified by (NH4)2SO4 fractionation and DEAE-Sephadex A-50 column chromatography. The enzyme preparation was found to require O2, Fe2+, ascorbate, alpha-ketoglutarate and poly-L-proline to attain maximum activity. The plant enzyme does not hydroxylate free proline and di-, tri- and tetra-L-proline, but hydroxylates octa-L-proline and poly-L-proline (Mr greater than 2000). Model peptides of unhydroxylated collagen, (Pro-Pro-Gly)5 and (Pro-Pro-Gly)10 are poor substrates for the plant enzyme. This means that the plant enzyme has a novel substrate specificity in regard to peptidyl substrate, and this differs from vertebrate prolyl hydroxylase, proline,2-oxoglutarate dioxygenase (prolyl-glycyl-peptide, 2-oxoglutarate: oxygen oxidoreductase, EC 1.14.11.2).

Studies on the biochemistry and fine structure of silica shell formation in diatoms : Formation of hydroxyproline and dihydroxyproline in Nitzschia angularis

In diatoms, the siliceous cell walls are enveloped by an organic component which includes 4-hydroxyproline and 3,4-dihydroxy-L-proline. The formation of these two amino acids were studied in Nitzschia angularis in Si-starvation synchrony. Both appear to arise from peptidyl proline. Its conversion to peptidyl hydroxyproline was shown in cell-free extracts and in kinetic studies using [(14)C]proline. Two lines of evidence indicate that dihydroxyproline does not arise from the further hydroxylation of peptidyl hydroxyproline: First, there was a lag of several minutes between the incorporation of [(14)C]proline into protein and the appearance therein of [(14)C]hydroxyproline but no such lag for the appearance of dihydroxyproline. Second, α,α'-dipyridyl blocked the formation of hydroxyproline, but not of dihydroxypyroline, from peptidyl proline. Cell walls made in the presence of dipyridyl differed little in overall chemical composition from walls made in its absence and were morphologically identical. [(14)C]dehydroproline was rapidly metabolized in the cells, with [(14)C]dihydroxyproline a prominent product. Studies of the conversion of [(14)C]proline to [(14)C]hydroxyproline at various stages of wall formation showed an increased synthesis of [(14)C]dihydroxyproline at the end of cell separation.

Glycoprotein of the wall of sycamore tissue-culture cells

1. A glycoprotein containing a large amount of hydroxyproline is present in the cell walls of sycamore callus cells. This protein is insoluble and remained in the alpha-cellulose when a mild separation procedure was used to obtain the polysaccharide fractions of the wall. The glycoprotein contained a high proportion of arabinose and galactose. 2. Soluble glycopeptides were prepared from the alpha-cellulose fraction when peptide bonds were broken by hydrazinolysis. The soluble material was fractionated by gel filtration and one glycopeptide was further purified by electrophoresis; it had a composition of 10% hydroxyproline, 35% arabinose and 55% galactose, and each hydroxyproline residue carried a glycosyl radical so that the oligosaccharides on the glycopeptide had an average degree of polymerization of 9. 3. The extraction of the glycopeptides was achieved without cleavage of glycosyl bonds, so that the glycoprotein cannot act as a covalent cross-link between the major polysaccharides of the wall. 4. The wall protein approximates in conformation to polyhydroxyproline and therefore it probably has similar physicochemical properties to polyhydroxyproline. This is discussed in relation to the function of the glycoprotein and its effect on the physical and chemical nature of the wall.