Molecular characterisation of a xyloglucan oligosaccharide-acting α-D-xylosidase from nasturtium (Tropaeolum majus L.) cotyledons that resembles plant 'apoplastic' α-D-glucosidases

We report the isolation, sequencing and analysis of the cDNA corresponding to an α-D-xylosidase involved in the mobilisation of xyloglucan from the cotyledons of germinated nasturtium (Tropaeolum majus L.) seeds. The translated open reading frame (2,808 bp including the stop codon), gave a polypeptide of 935 amino acids. It included the sequences of eleven peptides obtained by endo-proteinase digestion of the protein, and a putative hydrophobic signal sequence characteristic of a protein that is directed through the plasma membrane. The deduced molecular weight of the translated protein was appreciably higher than the molecular weight determined by sodium dodecyl sulphate–polyacrylamide gel electrophoresis, suggesting post-translational modification. The protein sequence showed high homology (76.0% identity over 896 amino acids) with a putative α-xylosidase sequence from Arabidopsis thaliana and there was homology with several α-glucosidases, notably those associated with the plant cell apoplast. The enzyme is a member of Family 31 of the glycosyl hydrolases and it fits into Clade 1 of the phylogenic analysis of α-glucosidases. Although in vivo the nasturtium enzyme catalyses mobilisation of cell wall xyloglucan, the homology of its primary sequence with α-glucosidases prompted study of its action on a range of α-glucosides. It was active against several α-(1→4)- and α-(1→6)-linked substrates, the former being hydrolysed faster. The functional and evolutionary relationships between this α-D-xylosidase and plant "apoplastic" α-D-glucosidases are discussed.