O-Acetylation of plant cell wall polysaccharides:: identification and partial characterization of a rhamnogalacturonan O-acetyl-transferase from potato suspension-cultured cells

A microsomal preparation from suspension-cultured potato stem cells (Solanum tuberosum L. cv. AZY) was incubated with [C-14]acetyl-CoA resulting in a precipitable radiolabeled product. Analysis of the product revealed that it consisted mostly of acetylated proteins and cell wall polysaccharides, including xyloglucan, homogalacturonan and rhamnogalacturonan I. Thus, acetyl-CoA is a donor-substrate for the O-acetylation of wall polysaccharides. A rhamnogalacturonan acetylesterase was used to develop an assay to measure and characterize rhamnogalacturonan O-acetyl transferase activity in the microsomal preparation. Using this assay, it was shown that the transferase activity was highest during the linear growth phase of the cells, had a pH-optimum at pH 7.0, a temperature optimum at 30 degrees C, an apparent K-m of 35 mu M and an apparent V-max of 0.9 pkat per mg protein. Further analysis of the radiolabeled acetylated product revealed that it had a molecular mass > 500 kDa.