Arabinogalactan protein

Arabinogalactan-proteins (AGP) are members of the hydroxyproline (Hyp)-rich cell wall glycoprotein superfamily and are extensively glycosylated. AGPs contains a protein backbone of varied length (5-30 kDa) with N-terminal secretory peptide followed by AGP, fasciclin (FAS) domains, and a C-terminal glycosylphosphatidylinositol (GPI) lipid anchor site.[1] There are 85 predicted AGPs in Arabidopsis,[2] with most of them containing a GPI plasma membrane anchor sequence that ties the extracellular AGPs to the plasma membrane and positions them to function as potential signaling molecules.[3] To date, 18 different genes have been functionally characterized and many more involved with AGP glycosylation are expected to be identified as research progresses.

AGPs are widely distributed in plants and typically comprise only 2 to 10% protein by weight.[4] The protein family has been earlier reported to contain O-linked glycans, whereas recent efforts employing mass spectrometry have revealed the presence of N-linked glycans as well within this protein family isolated from elongating cotton fiber cells.[5] AGPs are implicated in various aspects of plant growth and development, including root elongation, somatic embryogenesis, hormone responses, xylem differentiation, pollen tube growth and guidance, programmed cell death, cell expansion, salt tolerance, host-pathogen interactions, and cellular signaling.[6][7]

Structure

AGPs contains protein backbone of varied lengths. In some plant cells, the length of the mature protein backbone is only 10-13 residues long and they are therefore called as arabinogalactan (AG) peptides.[8] The protein backbone contains domain rich in hydroxyproline/proline (Hyp/Pro), serine (Ser), alanine (Ala) and threonine (Thr) amino acids. The repeated occurrence of Ala/Ser/Thr-Pro stretch (glycomodules) and the presence of Hyp suggests the sites for O-linked glycosylation and arabinogalactan modification.[4] The AGs are O-glycosidically linked to clustered non-contiguous Hyp residues on the protein backbone (Hyp contiguity hypothesis).[9] On some AGPs, single galactose (Gal) residues may also be found to be O-glycosidically attached to Ser/Thr, for example, in green algae.[10]

The carbohydrate moieties of AGPs are rich in arabinose and galactan, but other sugars may also be found such as L-rhamnopyranose (L-Rhap), D-mannopyranose (Manp), D-xylopyranose (Xylp), L-fucose (Fuc), D-glucopyranose (Glcp), D-glucuronic acid (GlcA) and its 4-O-methyl derivative, and D-galacturonic acid (GalA) and its 4-O-methyl derivative.[11][12] The AG found in AGPs is of type II (type II AGs) – that is, a galactan backbone of (1-3)-linked β-D-galactopyranose (Galp) residues, with branches (between one and three residues long) of (1,6)-linked β-D-Galp. In most cases, the Gal residues terminate with α-L-arabinofuranose (Araf) residues. Some AGPs are rich in uronic acids (GlcA), resulting in a charged polysaccharide moiety, and others have short oligosaccharides of Araf.[13] Specific sets of hydroxyproline O-β-galactosyltransferases, β-1,3-galactosyltransferases, β-1,6-galactosyltransferases, α-arabinosyltransferases, β-glucuronosyltransferases, α-rhamnosyltransferases, and α- fucosyltransferases are responsible for the synthesis of these complex structures.[14]

One of the features of type II AGs, particularly the (1,3)-linked β-D-Galp residues, is their ability to bind to the Yariv phenylglycosides. Yariv phenylglycosides are widely used as cytochemical reagents to perturb the molecular functions of AGPs as well as for the detection, quantification, purification, and staining of AGPs.[11] Recently, it was reported that interaction with Yariv was not detected for β-1,6-galacto-oligosaccharides of any length.[15] Yariv phenylglycosides were concluded to be specific binding reagents for β-1,3-galactan chains longer than five residues. Seven residues and longer are sufficient for cross-linking, leading to precipitation of the glycans with the Yariv phenylglycosides, which are observed with classical AGPs binding to β-Yariv dyes. The same results were observed where in AGPs appear to need at least 5–7 β-1,3-linked Gal units to make aggregates with the Yariv reagent.[16]

Functionally characterized genes involved in AGP glycosylation

Bioinformatics analysis using mammalian β-1,3-galactosyltransferase (GalT) sequences as templates suggested involvement of the Carbohydrate-Active enZYmes (CAZy) glycosyltransferase (GT) 31 family in the synthesis of the galactan chains of the AG backbone.[17] Members of the GT31 family have been grouped into 11 clades, with four clades being plant-specific: Clades 1, 7, 10, and 11. Clades 1 and 11 domains and motifs are not well-defined; while Clades 7 and 10 have domain similarities with proteins of known GalT function in mammalian systems.[17] Clade 7 proteins contain both GalT and galectin domains, while Clade 10 proteins contain a GalT-specific domain.[18] The galectin domain is proposed to allow the GalT to bind to the first Gal residue on the polypeptide backbone of AGPs; thus, determining the position of subsequent Gal residues on the protein backbone, similar to the activity of human galectin domain-containing proteins.[17]

Eight enzymes belonging to the GT31 family demonstrated the ability to place the first Gal residue onto Hyp residues in AGP core proteins. These enzymes are named GALT2, GALT3, GALT4, GALT5, GALT6,[19] which are Clade 7 members, and HPGT1, HPGT2, and HPGT3,[20] which are Clade 10 members. Preliminary enzyme substrate specificity studies demonstrated that another GT31 Clade 10 enzyme, At1g77810, had β-1,3-GalT activity.[17] A GT31 Clade 10 gene, KNS4/UPEX1, encodes a β-1,3-GalT capable of synthesizing β-1,3-Gal linkages found in type II AGs present in AGPs and/or pectic rhamnogalacturonan I (RG-I).[21] Another GT31 Clade 10 member, named GALT31A, encodes a β-1,6-GalT when heterologously expressed in E. coli and Nicotiana benthamiana and elongated β-1,6-galactan side chains of AGP glycans.[22] GALT29A, a member of GT29 family was identified as being co-expressed with GALT31A and act co-operatively and form complexes.[23]

Three members of GT14 named GlcAT14A, GlcAT14B, and GlcAT14C were reported to add GlcA to both β-1,6- and β-1,3-Gal chains in an in vitro enzyme assay following heterologous expression in Pichia pastoris.[24] Two α-fucosyltransferase genes, FUT4 and FUT6, both belonging to GT37 family, encode enzymes which add α-1,2-fucose residues to AGPs.[25][26] They appear to be partially redundant as they display somewhat different AGP substrate specificities.[25] A GT77 family member, REDUCED ARABINOSE YARIV (RAY1), was found to be a β-arabinosyltransferase that adds a β-Araf to methyl β-Gal of a Yariv-precipitable wall polymer.[27] More research is expected to functionally identify other genes involved in AGP glycosylation and their interactions with other plant cell wall components.

Biological roles of AGP

The functions of AGPs in plant growth and development processes rely heavily on the incredible diversity of their glycan and protein backbone moieties.[7] In particular, it is the AG polysaccharides that are most likely to be involved in development.[28] Most of the biological roles of AGPs have been identified through T-DNA insertional mutants characterization of genes or enzymes involved in AGP glycosylation, primarily in Arabidopsis thaliana. The galt2-6 single mutants revealed some physiological phenotypes under normal growth conditions, including reduced root hair length and density, reduced seed set, reduced adherent seed coat mucilage, and premature senescence.[1] However, galt2galt5 double mutants showed more severe and pleiotropic physiological phenotypes than the single mutants with respect to root hair length and density and seed coat mucilage.[1] Similarly, hpgt1hpgt2hpgt3 triple mutants showed several pleiotropic phenotypes including longer lateral roots, increased root hair length and density, thicker roots, smaller rosette leaves, shorter petioles, shorter inflorescence stems, reduced fertility, and shorter siliques.[20] In the case of GALT31A, it has been found to be essential for embryo development in Arabidopsis. A T-DNA insertion in the 9th exon of GALT31A resulted in embryo lethality of this mutant line.[22] Meanwhile, knockout mutants for KNS4/UPEX1 have collapsed pollen grains and abnormal pollen exine structure and morphology.[29] In addition, kns4 single mutants exhibited reduced fertility, confirming that KNS4/UPEX1 is critical for pollen viability and development.[21] Knockout mutants for FUT4 and FUT6 showed severe inhibition in root growth under salt conditions[26] while knockout mutants for GlcAT14A, GlcAT14B, and GlcAT14C showed enhanced cell elongation rates in dark grown hypocotyls and light grown roots during seedling growth.[30] In the case of ray1 mutant seedlings grown on vertical plates, the length of the primary root was affected by RAY1 mutation. In addition, the primary root of ray1 mutants grew with a slower rate compared to wild-type Arabidopsis.[27] Taken together, these studies provide evidence that proper glycosylation of AGPs is important to AGP function in plant growth and development.

gollark: (I do agree that you should be typechecking, but it's a bit noisy)
gollark: Also, all those asserttypes - wow, lua could really use a proper type system.
gollark: "self-documenting" is a horrible myth.
gollark: Oh, also, the lack of comments in the libraries.
gollark: It's mostly just code examples and a bit of annotation.

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Arabinogalactan Proteoglycan

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