Purification and cloning of an arabinogalactan-protein from xylem of loblolly pine

An arabinogalactan-protein (AGP) was purified from differentiating xylem of loblolly pine (Pinus taeda L.) and the N-terminal sequence used to identif y a cDNA clone. The protein, PtaAGP3, was not coded for by any previously i dentified AGP-like genes. Moreover, PtaAGP3 was abundantly and preferential ly expressed in differentiating xylem. The encoded protein contains four do mains, a signal peptide, a cleaved hydrophilic region. a region rich in ser ine, alanine, and proline/hydroxyproline, and a hydrophobic C-terminus. It is postulated to contain a GPI (glycosylphosphatidylinositol) anchor site. If the protein is cleaved at the putative GPI anchor site, as has been obse rved in other classical AGPs, all but the Ser-Ala-Pro/Hyp-rich domain may b e missing from the mature protein. Xylem-specific AGPs are hypothesized to be involved in xylem development.