Molecular Analysis of Genes Encoding Arabinogalactan-Proteins

Known arabinogalactan-proteins (AGPs) are acidic and hydrophilic glycoproteins associated with the cell wall and with the plasma membrane and contain 65–98% carbohydrate covalently linked to a protein backbone. The AGPs belong to the Hyp-rich glycoprotein (HRGP) family of proteins, which includes extensins, Pro-rich proteins, some nodulins and solanaceous lectins. The AGPs are present in all tissues and are likely components of cell-cell signaling pathways. The protein moiety is typically rich in Hyp, Pro, Ser, Thr and Ala residues (reviewed in Du et al 1996a, Nothnagel 1997). Most of the known AGP backbones are O-glycosylated. The polysaccharide is composed of mostly galactose and arabinose and consists of a backbone of (l–3)-linked (β-D-Gal residues substituted at C(O)6 by short chains of (l–6)-linked (β-D-Gal (Nothnagel 1997). It has been proposed that the HRGP family is a continuum of macromolecules substituted by mono-, oligo-, or polysaccharides (Du et al 1996a). The composition and the structure of recently characterized proteins (e.g., so-called non-classical AGPs and chimeric molecules) rather suggest that AGPs are distinct from the other HRGPs. The AGPs are defined by a high percentage of galactose and arabinose in the carbohydrate, a central Pro-rich domain in the protein backbone, and the ability to bind the β-glucosyl Yariv reagent (Yariv et al 1962). Classical AGPs have a three domain structure: a signal sequence, a central domain and a hydrophobic C-terminal domain. Most of the known AGPs are secreted into the extracellular matrix. The AGPs are also known to be associated with the plasma membrane and are expected to be heterogeneous, since during Triton X-114 fractionation (Bordier 1981) they partition either to the detergent phase (Smallwood et al 1996) or to the water phase (Norman et al 1990, Pennell et al 1991, Stohr et al 1996, Kjellbom et al 1997). Serpe and Nothnagel (1996) also obtained heterogeneous AGP fractions from plasma membranes of cultured rose cells. On the basis of the cDNA sequences of secreted AGPs of pear and tobacco cell cultures, Du et al (1996a) distinguished between “classical AGPs,” with a predicted transmembrane helix in the C-terminal domain, and “non-classical” AGPs, with a hydrophilic Asn-rich domain in the C-terminal part of the molecule. This part of the protein is predicted to be cleaved and not to be present in the mature protein (Mau et al 1995).