Galacturonosyltransferase (GAUT)1 and GAUT7 are the core of a plant cell wall pectin biosynthetic homogalacturonan:galacturonosyltransferase complex

Abstract

Plant cell wall pectic polysaccharides are arguably the most complex carbohydrates in nature. Progress in understanding pectin synthesis has been slow due to its complex structure and difficulties in purifying and expressing the low-abundance, Golgi membranebound pectin biosynthetic enzymes. Arabidopsis galacturonosyltransferase (GAUT) 1 is an α-1,4-galacturonosyltransferase (GalAT) that synthesizes homogalacturonan (HG), the most abundant pectic polysaccharide. We now show that GAUT1 functions in a protein complex with the homologous GAUT7. Surprisingly, although both GAUT1 and GAUT7 are type II membrane proteins with single Nterminal transmembrane-spanning domains, the N-terminal region of GAUT1, including the transmembrane domain, is cleaved in vivo. This raises the question of how the processed GAUT1 is retained in the Golgi, the site of HG biosynthesis. We show that the anchoring of GAUT1 in the Golgi requires association with GAUT7 to form the GAUT1:GAUT7 complex. Proteomics analyses also identified 12 additional proteins that immunoprecipitate with the GAUT1:GAUT7 complex. This study provides conclusive evidence that the GAUT1: GAUT7 complex is the catalytic core of an HG:GalAT complex and that cell wall matrix polysaccharide biosynthesis occurs via protein complexes. The processing of GAUT1 to remove its N-terminal transmembrane domain and its anchoring in the Golgi by association with GAUT7 provides an example of how specific catalytic domains of plant cell wall biosynthetic glycosyltransferases could be assembled into protein complexes to enable the synthesis of the complex and developmentally and environmentally plastic plant cell wall. disulfide bond | biomass | primary cell wall | secondary cell wall

Biosynthesis of the Fungal Cell Wall Polysaccharide Galactomannan Requires Intraluminal GDP-mannose

“…This study demonstrates, however, that both the backbone and side chain assembly of galactomannan require nucleotide sugar transporters and thus provide indirect evidence of the involvement of the Golgi apparatus in the biosynthesis of a fungal cell wall polysaccharide. In plants, the contribution of the Golgi apparatus to the elaboration of hemicellulosic and pectic polysaccharides has long been recognized, and it has recently been demonstrated that type II glycosyltransferases can elaborate the polysaccharide backbone (55,56). Similarly, biosynthesis of the unique polysaccharide capsule of the basiodiomycete Cryptococcus neoformans requires intraluminal GDP-mannose, and capsule material has even been observed within intracellular vesicles (25,57,58).…”

GALACTURONOSYLTRANSFERASE-LIKE5 Is Involved in the Production of Arabidopsis Seed Coat Mucilage

“…For example, GAUT7, another protein in the GT8 family, is thought to play a noncatalytic role in HG biosynthesis in association with GAUT1 (Sterling et al, 2006;Atmodjo et al, 2011). Recent studies (Atmodjo et al, 2011) demonstrate that the transmembrane domain of GAUT1 is cleaved off posttranslationally and show that the mature GAUT1 protein is retained in the Golgi by association with GAUT7, which retains its transmembrane domain, as part of a large biosynthetic protein complex. A further example where a GT has been suggested to play a noncatalytic role in plant cell wall polysaccharide synthesis is in xyloglucan biosynthesis .…”

“…AtGATL5 belongs to Carbohydrate-Active Enzymes family GT8 (Yin et al, 2010), several of whose members have been implicated in the synthesis of HG (Bouton et al, 2002;Leboeuf et al, 2005;Orfila et al, 2005;Sterling et al, 2006;Atmodjo et al, 2011), including one functionally characterized GalAT, GAUT1 (Sterling et al, 2006). Thus, we tested whether AtGATL5 functions as a HG:GalAT.…”

“…Recently, several putative GTs involved in the biosynthesis of different pectins have been identified using mutational and/or biochemical approaches (for review, see Mohnen, 2008;Harholt et al, 2010). Among these genes, GALACTURONOSYLTRANSFERASE1 (GAUT1) has been shown to encode a functional galacturonosyltransferase (GalAT; Sterling et al, 2006) and acts in a complex with GAUT7 to synthesize the HG backbone (Atmodjo et al, 2011). QUASIMODO1 (QUA1)/GAUT8 was also implicated in HG backbone synthesis due to the observation that the qua1 mutant showed decreased activity of HG GalA transferase (Bouton et al, 2002;Orfila et al, 2005).…”

AspWood: High-Spatial-Resolution Transcriptome Profiles Reveal Uncharacterized Modularity of Wood Formation in Populus tremula

“…We identified the Populus genes encoding glycosyl transferases involved in the biosynthesis of homogalacturonan (Atmodjo et al, 2011), RG-I (Harholt et al, 2006;Liwanag et al, 2012), RG-II (Egelund et al, 2006), and xyloglucan (Cavalier and Keegstra, 2006;Zabotina et al, 2008;Chou et al, 2012) (Supplemental Data Set 9). As expected, the majority of these genes were highly expressed in primary walled cambial and radially expanding tissues (sample cluster ii; Figure 5A).…”