[著者版] ; The carbohydrate moieties of arabinogalactan-proteins (AGPs), which are mainly composed of Gal, L-Ara, GlcA, and 4Me-GlcA residues, are essential for the physiological functions of these proteoglycans in higher plants. For this study, we have identified two genes encoding family 79 beta-glueuronidases, designated AnGlcAase and NcGlcAase, in Aspergillus niger and Neurospora crassa, respectively, based on the amino acid sequence of a native beta-glucuronidase purified from a commercial pectolytic enzyme preparation from A. niger. Although the deduced protein sequences of AnGlcAase and NcGlcAase were highly similar, the recombinant enzymes expressed in Pichia pastoris exhibited distinct substrate specificity toward 4-Me-GlcA residues of AGPs: recombinant ADGlcAase (rAnGlcAase) substantially liberated both GlcA and 4-Me-GlcA residues from radish AGPs, whereas recombinant NcGlcAase (rNcGlcAase) activity on the 4-Me-GlcA residues of AGPs was very low. Maximum activity of rAnGlcAase hydrolyzing PNP beta-GlcA occurred at pH 3.0-4.0, whereas the maximum rNcGlcAase activity was at pH 6.0. The apparent K. values of rAnGlcAase were 30.4 mu M for PNP beta-GlcA and 422 mu M for beta-GIcA-(1 -> 6)-Gal, and those of rNcGlcAase were 38.3 mu M and 378 mu M, respectively. Similar to the native enzyme, rAnGlcAase was able to catalyze the transglycosylation of GlcA residues from PNP beta-GlcA to various monosaccharide acceptors such as Glc, Gal, and Xyl. We propose that both AnGlcAase and NcGlcAase are instances of a novel type of beta-glucuronidase with the capacity to hydrolyze beta-GlcA and 4-Me-beta-GlcA residues of AGPs, although they differ significantly in their preferences. (c) 2008 Elsevier Ltd. All rights reserved.