Effects of N-glycosylation on the folding and structure of plant proteins

Abstract

wide range of experimental approaches. A large amount of information has been collected by following the folding The synthesis of many of the proteins that are transloc- of purified proteins in a well-defined in vitro environment. ated into the endoplasmic reticulum is accompanied In addition, protein folding has been studied in the cell, by the co-translational attachment of preformed oligo- where the acquisition of the 3-D structure can also saccharide chains to certain Asn residues. These gly- be linked to other events, including co- and postcans can play a variety of roles in the mature proteins, translational processing of the polypeptide chain. One of including the one of stabilizing the protein and pro- these events is the co-translational transfer of an oligosactecting the polypeptide backbone from the action of charide chain to certain Asn residues present on the proteases. In addition, they can have a crucial function nascent polypeptide. This covalent modification, termed during the process of polypeptide folding, when N-glycosylation, has been shown to play an important aggregation with other proteins would hamper the role in the folding of many proteins. However, the mechacquisition of the native conformation. Their influence anism(s) by which these chains manage to improve the on protein folding can be direct, or mediated by inter- folding of polypeptides has been questioned for a long actions with endoplasmic reticulum-located molecular time, and only recently has started to become clear. chaperones. The elucidation of the mechanisms that The role of glycan chains in protein folding, stability govern glycoprotein folding in the plant endoplasmic and function has beeen discussed in a number of reviews reticulum should contribute to the understanding of ( Elbein, 1991; Lis and Sharon, 1993; Varki, 1993; how much plant cells rely on glycan chains to achieve Bergeron et al., 1994; Helenius, 1994; O’Connor and the efficient folding of many proteins under diverse Imperiali, 1996; Wyss and Wagner, 1996; Helenius et al., environmental conditions. In addition, a better know- 1997) and aspects specific to plant glycoproteins have ledge of the level of conservation of the in vivo folding been addressed in Sturm (1995). In this work, what is mechanisms will be important for the exploitation currently known about the role of N-glycosylation in the of plant cells in the production of heterologous folding of proteins in plant cells is summarized and an glycoproteins. attempt made to place this information within the larger framework provided by the studies on animal and yeast