Isolation of the protein backbone of an arabinogalactan-protein from the styles of Nicotiana alata and characterization of a corresponding cDNA

Arabinogalactan-proteins (AGPs) from the styles of Nicotiana alata were isolated by ion exchange and gel filtration chromatography. After deglycosylation by anhydrous hydrogen fluoride, the protein backbones were fractionated by reversed-phase HPLC. One of the protein backbones, containing mainly hydroxyproline, alanine, and serine residues (53% of total residues), was digested with proteases, and the peptides were isolated and sequenced. This sequence information allowed the cloning of a 712-bp cDNA, AGPNa1. AGPNa1 encodes a 132-amino acid protein with three domains: an N-terminal secretion signal sequence, which is cleaved from the mature protein; a central sequence, which contains most of the hydroxyproline/proline residues; and a C-terminal hydrophobic region. AGPNa1 is expressed in many tissues of N. alata and related species. The arrangement of domains and amino acid composition of the AGP encoded by AGPNa1 are similar to that of an AGP from pear cell suspension culture filtrate, although the only sequence identity is at the N termini of the mature proteins.