Characterization of two membrane-associated β-glucosidases from maize (Zea mays L.) coleoptiles

Abstract

We isolated membrane vesicles from maize (Zea mays L.) coleoptiles and identified in these vesicles a 58 kDa (pm58) and a 60 kDa (pm60) protein by photoaffinity labelling with 5-azido-[7-3H]indole-3-acetic acid ([3H]N3IAA). Photoaffinity labelling was effectively competed for by auxins as well as by flavonoids. The labelled proteins were solubilized by Triton X-114 from the vesicles and partially purified. Microsequence analysis revealed that pm60 is a β-glucosidase. This was confirmed by biochemical and immunological analysis. We show that pm60 has a β-D-glucoside glucohydrolase (EC 3.2.1.21) activity. It uses p-nitrophenyl β-D-glucopyranoside (PNPG) as a substrate, with a pH optimum of 5.0. The K(m) for PNPG is 0.652 mM and the V(max.) 6.24 μmol·min-1·mg-1. The β-glucosidase activity of pm60 was competitively inhibited by IAA and 1-naphthylacetic acid as well as by gluconolactam and glucose. N-terminal amino-acid-sequence analysis of pm58 revealed similarity to pm60, suggesting that both proteins are encoded by different members of a gene family.