A [beta]-Glucosidase from Lodgepole Pine Xylem Specific for the Lignin Precursor Coniferin

Abstract

Coniferin, the glucoside of the monolignol coniferyl alcohol, accumulates to high levels in gymnosperms during spring-cambial reactivation. A cinnamyl alcohol glucoside/[beta]-glucosidase system is thought to play a key role in lignification by releasing the monolignol aglycones. Investigation of such an enzyme system in the xylem of Pinus contorta var latifolia Engelm. revealed two major [beta]-glucosidases. One efficiently hydrolyzed the native substrate, coniferin, and the other was more active against synthetic glucosides. The coniferin [beta]-glucosidase was purified to apparent homogeneity using anion exchange, hydrophobic interaction, and size-exclusion chromatography. The apparent native molecular weight was estimated to be 60,000. A dominant 28-kD protein and a minor 24-kD protein were detected in the purified preparation following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunological evidence from polyclonal antibodies directed against the synthetic N-terminal peptide of the 24-kD protein suggested that the native protein is a dimer of 28-kD subunit size. The N-terminal sequence showed that coniferin [beta]-glucosidase has high homology to known plant [beta]-glucosidases. Coniferin, syringin, and a synthetic coniferin analog were preferred substrates for the coniferin [beta]-glucosidase. In situ localization using the chromogenic coniferin analog showed the exclusive presence of [beta]-glucosidase activity in the differentiating xylem, similar to peroxidase activity.