A comparison of proteins from the developing xylem of compression and non‐compression wood of branches of Sitka spruce (Picea sitchensis) reveals a differentially expressed laccase

Soluble and cell wall‐associated proteins were extracted from the developing xylem of the compression and non‐compression sides of branches of Sitka spruce (Picea sitchensis (Bong) Carr.) by an identical procedure. Equal amounts of proteins were separated by SDS‐PAGE, and polypeptides were identified that were more abundant in soluble and cell wall‐associated extracts from the developing xylem of either compression or non‐compression wood. Two polypeptides (at apparent Mrs of 48 kDa and 120 kDa) that were more adundant in cell wall‐associated extracts of the developing xylem of the compression tissues were selected for amino‐terminal protein sequencing. The 48 kDa polypeptide yielded an amino‐terminal sequence that had no homology with known protein, gene or EST database sequences. The amino‐terminal sequence of the 120 kDa polypeptide was homologous to a number of laccase‐type polyphenol oxidases (EC 1.10.3.2) thought to be involved in lignin biosynthesis in trees. Using non‐denaturing SDS‐PAGE, the 120 kDa laccase was confirmed as a major oxidase activity in extracts of lignifying compression xylem but it was barely detectable in the non‐compression extracts where an 85 kDa oxidase was the predominant activity. The differential expression of oxidases in compression and non‐compression xylem is discussed.