THE PLANT CELL, Vol 9, Issue 4 597-609, Copyright © 1997 by American Society of Plant Biologists
The Rate of Phaseolin Assembly Is Controlled by the Glucosylation State of Its N-Linked Oligosaccharide Chains
F. Lupattelli, E. Pedrazzini, R. Bollini, A. Vitale and A. Ceriotti
Istituto Biosintesi Vegetali, Consiglio Nazionale delle Ricerche, via Bassini 15, 20133 Milan, Italy
Many of the proteins that are translocated into the endoplasmic reticulum
are glycosylated with the addition of a 14-saccharide core unit
(Glc3Man9GlcNAc2) to specific asparagine residues of the nascent
polypeptide. Glucose residues are then removed by endoplasmic
reticulum-located glucosidases, with diglucosylated and monoglucosylated
intermediates being formed. In this study, we used a cell-free system
constituted of wheat germ extract and bean microsomes to examine the role
of glucose trimming in the structural maturation of phaseolin, a trimeric
glycoprotein that accumulates in the protein storage vacuoles of bean
seeds. Removal of glucose residues from the N-linked chains of phaseolin
was blocked by the glucosidase inhibitors castanospermine and
N-methyldeoxynojirimycin. If glucose trimming was not allowed to occur, the
assembly of phaseolin was accelerated. Conversely, polypeptides bearing
partially trimmed glycans were unable to form trimers. The effect of
castanospermine on the rate of assembly was much more pronounced for
phaseolin polypeptides that have two glycans but was also evident when a
single glycan chain was present, indicating that glycan clustering can
modulate the effect of glucose trimming on the rate of trimer formation.
Therefore, the position of glycan chains and their accessibility to the
action of glucosidases can be fundamental elements in the control of the
structural maturation of plant glycoproteins.
