Plant Molecular Biology 47: 161–176, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
The complex structures of arabinogalactan-proteins and the journey
towards understanding function
, Kim L. Johnson
, James A. McKenna
, Antony Bacic
and Carolyn J.
Plant Cell Biology Research Centre, University of Melbourne, 3010, Australia (
author for correspondence; e-
Cooperative Research Centre for Bioproducts, University of Melbourne, 3010,
Current address: Department of Plant Science, Adelaide University, RMB1 Glen Osmond, SA 5064,
Key words: Arabidopsis, arabinogalactan-protein, cell wall, fasciclin, glycosylphosphatidylinositol-anchor,
Arabinogalactan-proteins (AGPs) are a family of complex proteoglycans found in all higher plants. Although
the precise function(s) of any single AGP is unknown, they are implicated in diverse developmental roles such
as differentiation, cell-cell recognition, embryogenesis and programmed cell death. DNA sequencing projects
have made possible the identiﬁcation of the genes encoding a large number of putative AGP protein backbones.
In contrast, our understanding of how AGPs undergo extensive post-translational modiﬁcation is poor and it is
important to understand these processes since they are likely to be critical for AGP function. Genes believed to be
responsible for post-translational modiﬁcation of an AGP protein backbone, include prolyl hydroxylases, glycosyl
transferases, proteases and glycosylphosphatidylinositol-anchor synthesising enzymes. Here we examine models
for proteoglycan function in animals and yeast to highlight possible strategies for determining the function(s) of
individual AGPs in plants.
Abbreviations: AGP, arabinogalactan-protein; AG, arabinogalactan; GlcY, β-glucosyl Yariv reagent; ER, en-
doplasmic reticulum; GlcNH
, glucosamine; GPI, glycosylphosphatidylinositol; HRGP, hydroxyproline-rich
glycoprotein; PLC, phospholipase C; PLD, phospholipase D; RP-HPLC, reversed-phase high-performance liquid
Plant cell walls play a pivotal role in determining
the ﬁnal shape and function of plant cells and tis-
sues (Bacic et al., 1988; Carpita and Gibeaut, 1993).
They consist of complex macromolecules and are prin-
cipally comprised of carbohydrate building blocks.
Small proportions of the macromolecules contain both
carbohydrate and protein and are known collectively
as proteoglycans or glycoproteins, depending on the
relative proportions of carbohydrate and protein. The
protein component of these molecules often contain
abundant amounts of hydroxyproline (Hyp), an imino
acid that is also found in proteins of the extracellular
matrix in animals, although in animals Hyp residues
are not glycosylated. This family of plant molecules is
known as the Hyp-rich glycoprotein (HRGP) family,
and includes the extensins, arabinogalactan-proteins
(AGPs), Pro/Hyp-rich glycoproteins and the solana-
ceous lectins (Showalter, 1993; Nothnagel, 1997;
Sommer-Knudsen et al., 1997; Bacic et al., 2000). In
terms of structure, these sub-classes of HRGPs should
not be viewed as discrete families but rather as a con-
tinuum of molecules ranging from basic, minimally
glycosylated proteins, for example Pro-rich proteins,
to acidic highly glycosylated proteoglycans, such as
AGPs (Sommer-Knudsen et al., 1998).