Isolation of cereal arabinogalactan-peptides and structural comparison of their carbohydrate and peptide moieties

Arabinogalactan (AG)-peptides were isolated and purified from wheat and durum wheat and for the first time from spelt, triticale, rye and barley using three dedicated isolation procedures. The AG-peptide molecules have molecular weights of approximately 23,500 with the exception of triticale (27,500) and rye (33,000). The fine structure of the carbohydrate parts revealed close resemblances among the purified cereal AG-peptide samples. They consist of a (1→6)-β-d-galactopyranosyl backbone substituted in the C(O)3-position with a single α-l-arabinofuranosyl or a single β-d-galactopyranosyl residue. The latter can also be substituted in its C(O)3-position with a single α-l-arabinofuranosyl residue. The AG-peptide peptide cores typically exist of 15 amino acids including three highly conserved hydroxyprolines (Hyp), each linked to a carbohydrate chain. The peptide amino acid sequence of spelt and durum wheat AG-peptides showed high similarity with the wheat AG-peptide peptide sequence while triticale, rye and barley AG-peptide peptide cores displayed less similarity. Homology with the N-terminal part of cereal grain softness protein (GSP) precursors indicates that the cereal AG-peptide peptides are a processing product of GSP synthesis. An overall structural model is proposed.