MOLECULAR-CLONING OF A GENE ENCODING AN ARABINOGALACTAN PROTEIN FROM PEAR (PYRUS-COMMUNIS) CELL-SUSPENSION CULTURE

Arabinogalactan proteins (AGPs) are proteoglycans containing a high pr oportion of carbohydrate (typically >90%) linked to a protein backbone rich in hydroxyproline (Hyp), Ala, Ser, and Thr. They are widely dist ributed in plants and may play a role in development. The structure of the carbohydrate of some AGPs is known in detail but information rega rding the protein backbone is restricted to a few peptide sequences. H ere we report isolation and partial amino acid sequencing of the prote in backbone of an AGP, This AGP is a member of one of four major group s of AGPs isolated from the filtrate of pear cell suspension culture. A cDNA encoding this protein backbone (145 amino acids) was cloned; th e deduced protein is rich in Hyp, Ala, Ser, and Thr, which together ac count for >75% of total residues. It has three domains, an N-terminal secretion signal, a central hydrophilic domain containing all of the P ro residues, and a hydrophobic C-terminal domain that is predicted to be a transmembrane helix. Approximately 93% of the Pro residues are hy droxylated and hence are potential sites for glycosylation.