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 simp le, highly biased amino acid composition indicating a repetitive polypeptid e backbone. Previous work (Qi, W., Pong, C., Lamport, D.T.A., 1991. Plant P hysiology 96, 848), suggested small (similar to 11 residue) repetitive pept ide motifs each with three Hyp-arabinoside attachment sites and a single Hy p-arabinogalactan polysaccharide attachment site. We tested that hypothesis by sequence analysis of the GAGP polypeptide after HF-deglycosylation. A f amily of closely related peptides confirmed the presence of a repetitive 19 -residue consensus motif However, the motif: Ser-Hyp-Hyp-Hyp-Thr-Leu-Ser-Hy p-Ser-Hyp-Thr-Hyp-Thr-Hyp-Hyp was about twice the size anticipated. Thus, j udging 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 hypot hesis which predicts arabinosides on contiguous Hyp residues and arabinogal actan polysaccharides on clustered non-contiguous Hyp residues, i.e. the GA GP 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 so me of the remarkable properties of gum arabic. (C) 2000 Elsevier Science Lt d. All rights reserved.