Figure 2.

Mn²⁺ and Ca²⁺ dependence of glycosylation in the Golgi. Enzymes are either marked in magenta for those experimentally confirmed to require Mn²⁺ or in gray for those predicted to be Mn²⁺-dependent. Enzymes that are also activated by Ca²⁺ are framed in blue. An enzyme that does not absolutely depend on Mn²⁺ is marked in brown. A, Schematic representation of enzymes involved in synthesis of O-glycans attached to plant AGPs and EXTs, and specific complex-type N-glycans attached to plant glycoproteins. The glycan models presented are modified from Nguema-Ona et al. (2014) and Showalter and Basu (2016). Among those core enzymes of deglycosylation (insert) and glycosylation, MNS1, MNS2, MNS3 (Liebminger et al., 2009), and FUT11 (Both et al., 2011) are activated by Ca²⁺ or Mn²⁺. XYLT (Pagny et al., 2003), GALT1 (Strasser et al., 2007), and GALT2 to 6 (Basu et al., 2015a, 2015b) are reported to coordinate Mn²⁺ for their catalytic activity. Activity of XYLT is stimulated or inhibited by Mn²⁺ (Bencúr et al., 2005). Based on the Uniprot database (www.uniprot.org), some further Arabidopsis GTs involved in N- or O-glycosylation are predicted to contain a DxD motif or to bind Mn²⁺ by sequence similarity, that is, β-1,2-N-acetylglucosaminyltransferase (GNT1 and 2), β-1,2-arabinosyltransferase (RRA1,2,3 and XEG113), hydroxyproline-O-galactosyltransferase (HPGT1,2,3), β-1,3-galactosyltransferase (B3GALT7), β-1,6-galactosyltransferase (GALT31A), and β-arabinofuranosyltransferase (RAY1). B, Schematic representation of enzymes involved in the synthesis of matrix sugars of the plant cell wall. The structures of matrix sugars are modified from Burton et al. (2010). Enzymes shown to require Mn²⁺ are galactan synthase 1 (AtGALS1) that catalyzes the addition of galactose from UDP-α-D-Gal to β-1,4-galactan chains of rhamnogalacturonan I and the transfer of an arabinopyranose from UDP-β-L-Ara_p to galactan chains (Ebert et al., 2018; Laursen et al., 2018); a polygalacturonate (1,4)-α-D-galacturonosyltransferase complex (GAUT1:GAUT7) that catalyzes the transfer of galacturonic acid onto homogalacturonan (Amos et al., 2018); (1,3)-α-D-xylosyltransferases (AtRGXT1 and 2) that synthesize rhamnogalacturonan-II (RG II; Egelund et al., 2006; Petersen et al., 2009); xyloglucan xylosyltransferases (XXT1 and 2) that are involved in xyloglucan biosynthesis (Culbertson et al., 2016; 2018); and GUX1 that adds GlcA to xylan (Rennie et al., 2012). In addition, based on the Uniprot database (www.uniprot.org), further Arabidopsis GTs involved in matrix sugar biosynthesis are predicted to contain a DxD motif or to bind Mn²⁺ by sequence similarity, that is, rhamnogalacturonan α-1,3-D-xylosyltransferase (MGP4 and RGXT3) for RGII biosynthesis, and UDP-GlcA:xylan glucuronyltransferase (GUX2,3,4,5) for heteroxylan biosynthesis.