![]() Here in this review, we will highlight the major breakthroughs of the posttranslational modifications of EXTs and how these modifications affect the function of EXTs within the primary plant cell wall.Īlthough the protein sequences that define the O-glycosylation site on HRGPs are partially known ( Kieliszewski and Lamport, 1994 Shpak et al., 1999, 2001), the glycosyltransferases that transfer the activated sugars to the protein backbone (arabinose and galactose) have remained a mystery for decades. EXTs can also occur in the context of natural, chimeric glycosylated polypeptides, e.g., EXT-arabinogalactan proteins ( Lind et al., 1994), leucine-rich repeat (LRR) EXT proteins ( Rubinstein et al., 1995), or lectin-EXTs ( Kieliszewski and Lamport, 1994). EXTs are plant cell wall O-glycoproteins that belong to the hydroxyproline-rich glycoproteins (HRGPs) superfamily with a modular highly repetitive sequence that includes the O-glycosylated Ser-(Hyp) 3–4 repeats and some of the crosslinking motifs Val- Tyr-Lys, Val- Tyr-Lys- Tyr-Lys, Tyr- Tyr- Tyr-Lys, or Tyr-X- Tyr, all of a hydrophobic nature (Figure 1A Fry, 1982 Brady et al., 1998 Held et al., 2004 Cannon et al., 2008). In addition, a structural role has been clearly assigned to the hydroxyproline-rich O-glycoprotein extensins (EXTs) in building and maintaining the growing cell wall ( Lamport and Northcote, 1960 Hall and Cannon, 2002 Held et al., 2004 Cannon et al., 2008 Lamport et al., 2011 Velasquez et al., 2011a, b). ![]() The major components of primary plant cell walls include a complex composite of networks of cellulose microfibrils, branched xyloglucans, and a diverse pectin matrix ( Somerville et al., 2004 Cosgrove, 2005). We discuss the effects of posttranslational modifications on the structure and function of extensins in plant cell walls. In this review we summarize the enzymes that define the O-glycosylation sites on the O-glycoproteins, i.e., the prolyl 4-hydroxylases (P4Hs), the glycosyltransferases that transfer arabinose units (named arabinosyltransferases, AraTs), and the one responsible for transferring a single galactose (galactosyltransferase, GalT) on the protein EXT backbones. Genes conferring the posttranslational modifications, i.e., proline hydroxylation and subsequent O-glycosylation, of the EXTs have been recently identified. We are now beginning to see the emerging components of the molecular machinery that assembles these complex O-glycoproteins on the plant cell wall. The genetic set up and the enzymes that define the O-glycosylation sites and transfer the activated sugars to cell wall glycoprotein Extensins (EXTs) have remained unknown for a long time. 2 Department of Plant Biology and Biotechnology, Faculty of Life Sciences, University of Copenhagen, Copenhagen, Denmark.1 Facultad de Ciencias Exactas y Naturales, Instituto de Fisiología, Biología Molecular y Neurociencias (IFIByNE-CONICET), Universidad de Buenos Aires, Buenos Aires, Argentina.
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