RHAMNOGALACTURONAN-I AND RHAMNOGALACTURONAN-II ARE PECTIC SUBSTRATES FOR FLAX-CELL METHYLTRANSFERASES

The methyltransferase activity (EC 2.1.1) of proteins solubilized with Triton X-100 from an endomembrane extract of flax (Linum usitatissimu m L.) has been assayed, using exogenous rhamnogalacturonan (RG) substr ates: (a) a RG-I solubilized with EDTA-Na-2 from mechanically-isolated flax fibres and (b) RG-II samples isolated from red wine. The transfe r of labelled methyl groups to each isolated RG was evidenced from co- elution of the radioactivity and exogenous rhamnogalacturonans after s ize exclusion chromatography of the incubation medium. The methyl tran sfer on RG-I was higher in Golgi apparatus enriched-membranes (densiti es 1.12-1.14) at pH 7-7.5. The RG-II methyltransferase activity was ma ximum at pH 6.5-8 and independent of the degree of methylesterificatio n of the uronic acids in the molecule (about 20%).