PLANT PHYSIOLOGY , Vol 114, Issue 1 245-254, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Xyloglucan Galactosyl- and Fucosyltransferase Activities from Pea Epicotyl Microsomes
A. Faik, C. Chileshe, J. Sterling and G. Maclachlan
Department of Biology, McGill University, 1205 Avenue Docteur Penfield, Montreal, Quebec, Canada H3A 1B1
Microsomal membranes from growing tissue of pea (Pisum sativum L.)
epicotyls were incubated with the substrate UDP-[14C]galactose (Gal) with
or without tamarind seed xyloglucan (XG) as a potential galactosyl
acceptor. Added tamarind seed XG enhanced incorporation of [14C]Gal into
high-molecular-weight products (eluted from columns of Sepharose CL-6B in
the void volume) that were trichloroacetic acid-soluble but insoluble in
67% ethanol. These products were hydrolyzed by cellulase to fragments
comparable in size to XG subunit oligosaccharides. XG-dependent
galactosyltransferase activity could be solubilized, along with XG
fucosyltransferase, by the detergent
3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate. When this
enzyme was incubated with tamarind (Tamarindus indica L.) seed XG or
nasturtium (Tropaeolum majus L.) seed XG that had been partially
degalactosylated with an XG-specific [beta]-galactosidase, the rates of Gal
transfer increased and fucose transfer decreased compared with controls
with native XG. The reaction products were hydrolyzed by cellulase to 14C
fragments that were analyzed by gel-filtration and high-performance liquid
chromatography fractionation with pulsed amperometric detection. The major
components were XG subunits, namely one of the two possible monogalactosyl
octasaccharides (-XXLG-) and digalactosyl nonasaccharide (-XLLG-), whether
the predominant octasaccharide in the acceptor was XXLG (as in tamarind
seed XG) or XLXG (as in nasturtium seed XG). It is concluded that the first
xylosylglucose from the reducing end of the subunits was the Gal acceptor
locus preferred by the solubilized pea transferase. These observations are
incorporated into a model for the biosynthesis of cell wall XGs.