title: Plant O-hydroxyproline arabinogalactans are composed of repeating trigalactosyl subunits with short bifurcated side chains.
abstract: Classical arabinogalactan proteins partially defined by type II O-Hyp-linked arabinogalactans (Hyp-AGs) are structural components of the plant extracellular matrix. Recently we described the structure of a small Hyp-AG putatively based on repetitive trigalactosyl subunits and suggested that AGs are less complex and varied than generally supposed. Here we describe three additional AGs with similar subunits. The Hyp-AGs were isolated from two different arabinogalactan protein fusion glycoproteins expressed in tobacco cells; that is, a 22-residue Hyp-AG and a 20-residue Hyp-AG, both isolated from interferon alpha2b-(Ser-Hyp)(20), and a 14-residue Hyp-AG isolated from (Ala-Hyp)(51)-green fluorescent protein. We used NMR spectroscopy to establish the molecular structure of these Hyp-AGs, which share common features: (i) a galactan main chain composed of two 1-->3 beta-linked trigalactosyl blocks linked by a beta-1-->6 bond; (ii) bifurcated side chains with Ara, Rha, GlcUA, and a Gal 6-linked to Gal-1 and Gal-2 of the main-chain trigalactosyl repeats; (iii) a common side chain structure composed of up to six residues, the largest consisting of an alpha-L-Araf-(1-->5)-alpha-L-Araf-(1-->3)-alpha-L-Araf-(1-->3- unit and an alpha-L-Rhap-(1-->4)-beta-D-GlcUAp-(1-->6)-unit, both linked to Gal. The conformational ensemble obtained by using nuclear Overhauser effect data in structure calculations revealed a galactan main chain with a reverse turn involving the beta-1-->6 link between the trigalactosyl blocks, yielding a moderately compact structure stabilized by H-bonds.
authors.affiliation: Department of Chemistry and Biochemistry, Biochemistry Research Facility, Ohio University, Athens, Ohio 45701, USA.
authors.forename: Li
authors.initials: L
authors.lastname: Tan
authors.forename: Peter
authors.initials: P
authors.lastname: Varnai
authors.forename: Derek T A
authors.initials: DT
authors.lastname: Lamport
authors.forename: Chunhua
authors.initials: C
authors.lastname: Yuan
authors.forename: Jianfeng
authors.initials: J
authors.lastname: Xu
authors.forename: Feng
authors.initials: F
authors.lastname: Qiu
authors.forename: Marcia J
authors.initials: MJ
authors.lastname: Kieliszewski
mesh_terms: D002240:Carbohydrate Sequence; D002241:Carbohydrates / Q000737:chemistry; D002473:Cell Wall / Q000378:metabolism; D005685:Galactans / Q000737:chemistry*; D005690:Galactose / Q000737:chemistry*; D006023:Glycoproteins / Q000737:chemistry; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006860:Hydrogen Bonding; D006909:Hydroxyproline / Q000737:chemistry*; D007371:Interferon-gamma / Q000378:metabolism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008969:Molecular Sequence Data; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014026:Nicotiana / Q000235:genetics / Q000378:metabolism
chemical_list: D002241:Carbohydrates; D005685:Galactans; D006023:Glycoproteins; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D007371:Interferon-gamma; D006909:Hydroxyproline; D005690:Galactose