XYLOGLUCAN GALACTOSYLTRANSFERASE AND FUCOSYLTRANSFERASE ACTIVITIES FROM PEA EPICOTYL MICROSOMES

Microsomal membranes from growing tissue of pea (Pisum sativum L.) epi cotyls were incubated with the substrate UDP-[C-14]galactose (Gal) wit h or without tamarind seed xyloglucan (XG) as a potential galactosyl a cceptor. Added tamarind seed XG enhanced incorporation of [C-14]Gal in to high-molecular-weight products (eluted from columns of Sepharose CL -6B in the void volume) that were trichloroacetic acid-soluble but ins oluble in 67% ethanol. These products were hydrolyzed by cellulase to fragments comparable in size to XG subunit oligosaccharides. XG-depend ent galactosyltransferase activity could be solubilized, along with XG fucosyltransferase, by the detergent lamidopropyl)-dimethylammonio]-1 -propanesulfonate. When this enzyme was incubated with tamarind (Tamar indus indica L.) seed XG or nasturtium (Tropaeolum majus L.) seed XG t hat had been partially degalactosylated with an XG-specific beta-galac tosidase, the rates of Cal transfer increased and fucose transfer decr eased compared with controls with native XG. The reaction products wer e hydrolyzed by cellulase to C-14 fragments that were analyzed by gel- filtration and high-performance liquid chromatography fractionation wi th pulsed amperometric detection. The major components were XG subunit s, namely one of the two possible monogalactosyl octasaccharides (-XXL G-) and digalactosyl nonasaccharide (-XLLG-), whether the predominant octasaccharide in the acceptor was XXLG (as in tamarind seed XG) or XL XG (as in nasturtium seed XG). It is concluded that the first xylosylg lucose from the reducing end of the subunits was the Cal acceptor locu s preferred by the solubilized pea transferase. These observations are incorporated into a model for the biosynthesis of cell wall XGs.