Biosynthesis and assembly of pectin and glucuronoarabinoxylan in plants
Nascent pectin and glucuronarabinoxylan (GAX), synthesized in vitro by membrane-bound enzymes from etiolated pea (Pisum sativum L.) epicotyls, were found to bind to pea xyloglucan in a pH-dependent manner. The binding was maximum at low pH (3-4), and decreased to almost zero at pH 6. The binding seemed to occur instantaneously, to be non-covalent, and to require both terminal fucose residues of xyloglucan, in addition to the non-reduced acid residues of GAX and pectin. Removal by protease of the proteins attached to nascent pectin and GAX, greatly reduced the maximum binding and abolished the pH-dependence. The proteins involved seem to have approximate molecular weights of 14 and 94 kDa. The pH-dependent binding of nascent pectin and GAX is not completely specific to xyloglucan, since some binding occurred to a range of other matrix polysaccharides, though at a lower level than to pea xyloglucan. Newly-deposited pectin was extracted from peas that were incubated with radioactively labelled sucrose. It was shown to behave in a similar manner as nascent pectin, exhibiting the same pH-dependent binding pattern to xyloglucan. Protease treatment of pectin decreased the binding, indicating the possible presence of proteins attached to pectin in the cell wall, and revealing the role of those proteins in the interaction of pectin with other matrix polysaccharides, particularly xyloglucan. The pH-dependence of the binding suggests a functional interaction with the mechanisms that control growth, since the wall pH decreases when elongation growth is initiated. The proteins involved would play a significant role in cell-wall assembly and cell-wall elongation.