FIGURE 4.

SUH1 encodes a transmembrane protein containing a Domain of Unknown Function 266 (DUF266) that is highly similar to the GT14 protein. (a) Multiple alignment analysis of SUH1 and predicted DUF266‐containing proteins from plants. The predicted amino acid sequences from Arabidopsis (SUH1, B6IDH4), rice (BC10, Q65XS5), sorghum (So, C5Z134), maize (Ma, B4FL81), poplar (Pp, B9GHR6), and grape (Gr, A5AN07) were aligned using the ClustalW program (mobyle.pasteur.fr/cgi‐bin/portal.py). White letters on a black background indicate invariant residues; other conserved amino acids are shaded in gray. The dotted line indicates the predicted transmembrane region (amino acids 20–39), and the positions of suh1 point mutations are indicated by asterisks. (b) Partial amino acid sequence alignment of DUF266 protein members and the human Core2 1,6 β‐N‐acetylglucosaminyltransferase (Q02742) in the GT14 family. Bioinformatic studies revealed structural similarities and invariant amino acid residues conserved between DUF266 proteins and leukocyte type core‐2 β‐1,6‐N‐acetylglucosaminyltransferase (C2GnT‐L), a member of the GT14 family, suggesting a distant relationship between DUF266 proteins and GT14. The first region presumably corresponds to the Rossmann‐fold motif for nucleotide binding. The second region is a structural domain that interacts with the donor and acceptor substrates. The conservation of the putative catalytic residue Glu‐320 in C2GnT in DUF266 members is particularly interesting. The putative catalytic residue (Glu‐320 in C2GnT) is marked with an inverted triangle. The third region at the C‐terminus of the catalytic domain shows three invariant residues. One of them, Lys‐401 in C2GnT (marked with a circle), which is expected to interact with the diphosphate group of the nucleotide sugar, is also conserved in the DUF266 family.