Gum arabic glycoprotein is a twisted hairy rope. A new model based on O-galactosylhydroxyproline as the polysaccharide attachment site

Separation of the wound exudate from Acacia senegal (L.) Willd., "gum arabic," on a preparative Superose-6 column gave two major fractions: a high molecular weight gum arabic glycoprotein (GAGP) containing about 90% carbohydrate and a lower molecular weight heterogenous gum arabic polysacc...

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Bibliographic Details
Published in:Plant physiology (Bethesda) 1991-07, Vol.96 (3), p.848-855
Main Authors: Qi, W. (Beijing Agricultural Univeristy, Beijing, P.R. China), Fong, C, Lamport, T.A
Format: Article
Language:English
Subjects:
550200 - Biochemistry
ACACIA SENEGAL
ACIDE AMINE
AGENTES NOCIVOS
AMINES
AMINO ACIDS
AMINOACIDOS
AZOLES
BASIC BIOLOGICAL SCIENCES
Biological and medical sciences
CARBOHYDRATES
CARBOXYLIC ACIDS
Cell walls
Chemical constitution
COMPOSICION QUIMICA
COMPOSITION CHIMIQUE
ELECTRON MICROSCOPY
EXSUDAT
EXUDADOS
FACTEUR NUISIBLE
Fundamental and applied biological sciences. Psychology
Gels
GLICOPROTEINAS
GLYCOPROTEINE
GLYCOPROTEINS
Glycosides
GOMA ARABIGA
GOMME ARABIQUE
Gum arabic
GUMS
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROXY ACIDS
HYDROXYPROLINE
MICROSCOPY
MOLECULAR STRUCTURE
Molecules
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PEPTIDES
Plant physiology and development
Plants
POLYPEPTIDES
POLYSACCHARIDES
PROTEINS
PYRROLES
PYRROLIDINES
SACCHARIDES
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Summary:Separation of the wound exudate from Acacia senegal (L.) Willd., "gum arabic," on a preparative Superose-6 column gave two major fractions: a high molecular weight gum arabic glycoprotein (GAGP) containing about 90% carbohydrate and a lower molecular weight heterogenous gum arabic polysaccharide fraction. Hydrogen fluoride-deglycosylation of GAGP gave a large (approximately 400 residue) hydroxyproline-rich polypeptide backbone (dGAGP). Alkaline hydrolysis of GAGP showed that most of the carbohydrate was attached to the polypeptide backbone as small (approximately 30 residue) hydroxyproline (Hyp)-polysaccharide substituents. After partial acid hydrolysis of the Hyp-polysaccharide fraction we identified O-galactosylhydroxyproline as the glycopeptide linkage, identical with that of hydroxyproline-rich arabinogalactan-proteins (AGPs). However, unlike the acidic alanine-rich AGPs, GAGP is basic and notably deficient in alanine. Thus, while the GAGP polypeptide backbone more closely resembles that of the Hyp-rich cell wall protein extensin, the GAGP polysaccharide sidechains resemble AGPs. Possibly all three proteins comprise a phylogenetically related extensin superfamily of extended rod-like macromolecules. The "wattle-blossom" model for AGP and gum arabic predicts a few large polysaccharide substituents along the polypeptide backbone of a spheroidal macromolecule. On the contrary, our data imply a rod-like molecule with numerous small polysaccharide substituents (attached to 24% of the Hyp residues), regularly arranged along a highly periodic polypeptide backbone based, hypothetically, on a 10 to 12 residue repetitive peptide motif. Thus, a simple statistical model of the gum arabic glycoprotein predicts a repeating polysaccharide-peptide subunit of about 7 kilodaltons. The small polysaccharide substituents will maximize intramolecular hydrogen bonding if aligned along the long axis of the molecule, forming in effect a twisted hairy rope. Electron micrographs of rotary shadowed GAGP molecules support that prediction and may also explain how such apparently large molecules can exit the cell by endwise reptation through the small pores of the primary cell wall
ISSN:0032-0889
1532-2548
DOI:10.1104/pp.96.3.848