Purification of several pectin methyltransferases from cell suspension cultures of flax (Linum usitatissimum L.)

Three pectin methyltransferases (PMT5, PMT7, PMT18; EC 2.1.1.6.x) were solubilized from the endo-membrane complex of flax cells, with 0.05 % Triton X-100. After a 3 step-chromatography procedure, PMT7 and PMT5 were purified to apparent homogeneity. PMT5 and PMT7 differed regarding their optimum pH (5 or 7), the methyl acceptor (low or highly methylesterified pectin), their focusing pH range (6–7 or 8–9) and relative molecular mass (40 ± 5 or 110 ± 10 kDa). SDS-PAGE of PMT5 and PMT7 did not reveal bands at 40 or 110 kDa but only a silver stained band of about 18 kDa. Two independent methods (photo labelling and enzymatic activity) showed that this silver-stained band corresponded to a methyltransferase with affinity for pectins. This polypeptide was of the same size as the enzyme designed PMT18 (18 ± 3 kDa; pI 4–4.5) recovered during size exclusion chromatography of either PMT7 or PMT5, suggesting that PMT18 bears the catalytic site of PMT5 and PMT7.