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Sci Rep. 2017 Mar 30;7:45341. doi: 10.1038/srep45341.

Identification and evolution of a plant cell wall specific glycoprotein glycosyl transferase, ExAD.

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VKR Research Centre, Department of Plant and Environmental Sciences, University of Copenhagen, DK-1871, Frederiksberg C, Denmark.
Fundación Instituto Leloir and IIBBA-CONICET, Av. Patricia Argentinas 435, Buenos Aires, C1405BWE, Argentina.
Instituto de Fisiología, Biología Molecular y Neurociencias, IFIByNE-CONICET, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Intendente Güiraldes 2160, Ciudad Universitaria, Pabellón II, Buenos Aires, C1428EGA, Argentina.
Institute for Plant Cell Biology and Biotechnology, Heinrich-Heine University, Duesseldorf, Germany.
Carlsberg Research Laboratory, J. C. Jacobsens Gade 4, 1799, Copenhagen V, Denmark.
Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich, NR4 7UH, UK.
Copenhagen Center for Glycomics, Department of Molecular and Cellular Medicine and School of Dentistry, Faculty of Health Sciences, University of Copenhagen, DK-2200, Copenhagen N, Denmark.


Extensins are plant cell wall glycoproteins that act as scaffolds for the deposition of the main wall carbohydrate polymers, which are interlocked into the supramolecular wall structure through intra- and inter-molecular iso-di-tyrosine crosslinks within the extensin backbone. In the conserved canonical extensin repeat, Ser-Hyp4, serine and the consecutive C4-hydroxyprolines (Hyps) are substituted with an α-galactose and 1-5 β- or α-linked arabinofuranoses (Arafs), respectively. These modifications are required for correct extended structure and function of the extensin network. Here, we identified a single Arabidopsis thaliana gene, At3g57630, in clade E of the inverting Glycosyltransferase family GT47 as a candidate for the transfer of Araf to Hyp-arabinofuranotriose (Hyp-β1,4Araf-β1,2Araf-β1,2Araf) side chains in an α-linkage, to yield Hyp-Araf4 which is exclusively found in extensins. T-DNA knock-out mutants of At3g57630 showed a truncated root hair phenotype, as seen for mutants of all hitherto characterized extensin glycosylation enzymes; both root hair and glycan phenotypes were restored upon reintroduction of At3g57630. At3g57630 was named Extensin Arabinose Deficient transferase, ExAD, accordingly. The occurrence of ExAD orthologs within the Viridiplantae along with its' product, Hyp-Araf4, point to ExAD being an evolutionary hallmark of terrestrial plants and charophyte green algae.

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