Localisation of methyltransferases involved in glucuronoxylan and pectin methylation in the Golgi apparatus in etiolated pea epicotyls

Membranes from etiolated pea epicotyls were fractionated by discontinuous sucrose-density-gradient centrifugation into endoplasmic reticulum, Golgi apparatus and plasma membrane. The Golgi apparatus was further fractionated on a shallow, continuous sucrose density gradient into Golgi subfractions of low, medium and high density. Methyltransferases forming both EDTA-soluble and EDTA-insoluble methylated polysaccharides were found to be concentrated in the Golgi apparatus, chiefly in the Low and medium density Golgi membranes. The products were characterised by acid and alkaline hydrolysis and enzymic digestion. These tests indicated that the EDTA-soluble methyltransferase product consisted mainly of methyl-esterified pectin, probably rhamnogalacturonan 1, while the EDTA-insoluble methyltransferase product was mostly 4-O-methylglucuronoxylan. It was concluded chat both pectin methyltransferase and glucuronoxylan methyltransferase are located in the low and medium density Golgi membranes, which may correspond to the cis- and medial-Golgi cisternae identified by electron microscopy.