Abstract
The major plant sugar l-arabinose (l-Ara) has two different ring forms, l-arabinofuranose (l-Araf) and l-arabinopyranose (l-Arap). Although l-Ara mainly appears in the form of α-l-Araf residues in cell wall components, such as pectic α-1,3:1,5-arabinan, arabinoxylan, and arabinogalactan-proteins (AGPs), lesser amounts of it can also be found as β-l-Arap residues of AGPs. Even though AGPs are known to be rapidly metabolized, the enzymes acting on the β-l-Arap residues remain to be identified. In the present study, four enzymes, which we call β-l-ARAPASE (APSE) and α-GALACTOSIDASE 1 (AGAL1), AGAL2, and AGAL3, are identified as those enzymes that are likely to be responsible for the hydrolysis of the β-l-Arap residues in Arabidopsis thaliana. An Arabidopsis apse-1 mutant showed significant reduction in β-l-arabinopyranosidase activity, and an apse-1 agal3-1 double-mutant exhibited even less activity. The apse-1 and the double-mutants both had more β-l-Arap residues in the cell walls than wild-type plants. Recombinant APSE expressed in the yeast Pichia pastoris specifically hydrolyzed β-l-Arap residues and released l-Ara from gum arabic and larch arabinogalactan. The recombinant AGAL3 also showed weak β-l-arabinopyranosidase activity beside its strong α-galactosidase activity. It appears that the β-l-Arap residues of AGPs are hydrolysed mainly by APSE and partially by AGALs in Arabidopsis.