A protein from maize labeled with azido‐IAA has novel β‐glucosidase activity

A biologically active and photolabile auxin analog, 5‐azido‐[7‐ 3 H]indole‐3‐acetic acid ([ 3 H]N 3 IAA), was used to search for auxin‐binding proteins in cytosolic extracts from maize coleoptiles ( Zea mays L.) and identified a protein with a molecular mass of 60 kDa (p60). Binding of [ 3 H]N 3 IAA is highly specific as demonstrated by competition analysis with functionally relevant auxin analogs. p60 is found in coleoptiles and roots of etiolated maize seedlings and was detected in cytosolic as well as in microsomal fractions. The protein binds to 1‐naphthylacetic acid (1‐NAA) sepharose and is eluted with auxins. A purification scheme resulting in homogenous p60 protein was devised and it was shown that p60 has β‐ d ‐glucoside glucohydrolase activity (E.C.3.2.1.21). The hydrolytic activity of p60 for the synthetic substrate p ‐nitro‐phenyl‐β‐ d ‐glucopyranoside is diminished by 1‐NAA. p60 shows high substrate specificity since it hydrolyzes indoxyl‐O‐glucoside, but not β‐(1,4)‐cellobiose, IAA‐inositol or IAA‐amino acid conjugates. The present data suggest that p60 might be involved in the hydrolysis of auxin conjugates.