Gum arabic glycoprotein contains glycomodules of both extensin and arabinogalactan-glycoproteins.

Gum arabic glycoprotein (GAGP) is a large molecular weight, hydroxyproline-rich arabinogalactan-protein (AGP) component of gum arabic. GAGP has a simple, highly biased amino acid composition indicating a repetitive polypeptide backbone. Previous work (Qi, W., Fong, C., Lamport, D.T.A., 1991. Plant Physiology 96, 848), suggested small (approximately 11 residue) repetitive peptide motifs each with three Hyp-arabinoside attachment sites and a single Hyp-arabinogalactan polysaccharide attachment site. We tested that hypothesis by sequence analysis of the GAGP polypeptide after HF-deglycosylation. A family of closely related peptides confirmed the presence of a repetitive 19-residue consensus motif. However, the motif: Ser-Hyp-Hyp-Hyp-Thr-Leu-Ser-Hyp-Ser- Hyp-Thr-Hyp-Thr-Hyp-Hyp-Leu-Gly-Pro-His, was about twice the size anticipated. Thus, judging by Hyp-glycoside profiles of GAGP, the consensus motif contained six Hyp-arabinosides rather than three and two Hyp-polysaccharides rather than one. We inferred the glycosylation sites based on the Hyp contiguity hypothesis which predicts arabinosides on contiguous Hyp residues and arabinogalactan polysaccharides on clustered non-contiguous Hyp residues, i.e. the GAGP motif would consist of arabinosylated contiguous Hyp blocks flanking two central Hyp-polysaccharides. We predict this rigidifies the glycoprotein, enhances the overall symmetry of the glycopeptide motif, and may explain some of the remarkable properties of gum arabic.