MCB 741
_____________________________________________________Yan Liang 2010 Fall
Investigations on the galactosyltransferease activities involved in arabinogalactanprotein glycosylation
Arabinogalactan-proteins (AGPs) are a family of plant cell wall proteins implicated in
many physiological processes, including plant somatic embryogenesis, programmed cell
death, wound responses, and hormone signaling pathways (Seifert and Roberts, 2007;
Ellis et al., 2010). AGPs are of commercial importance as well. For example, larch AG and
gum arabic are widely used as emulsifier and stabilizer in the food and pharmaceutical
industries (Showalter, 2001; Ellis et al., 2010). Structurally, AGPs belong to
hydroxyproline- rich glycoproteins (HRGPs) that are frequently characterized by the
presence of [Ala-Hyp] ([AO]) repetitive units (Nothnagel, 1997; Showalter, 2001).
Carbohydrate components of AGPs generally count for > 90% of the molecular mass.
Recent studies provided detailed information about the AG side chain structure (Tan et
al., 2004), however, little is known about its biosynthetic mechanism. In the current
study, an in vitro GalT reaction system was developed using UDP-[14C]Gal as the donor
substrate and the acceptor substrates composed of [AO] repetitive units, specifically, a
chemically synthesized [AO]7 acceptor and a transgenically produced and deglycosylated
d[AO]51 acceptor. Using the GalT reaction system, GalT activities were detected for the
addition of the first and second Gal residues to the [AO] peptides in both tobacco and
Arabidopsis microsomal membranes. This result indicated that AGP O-glycosylation
follows a sequential sugar addition mechanism, at least in the initial steps. The
[AO]7:GalT activity was shown to be specific for peptidyl Hyp in AGP sequences, which
provided further support for the Hyp-contiguity hypothesis (Kieliszewski and Lamport,
1994). In addition, subcellular localization of the two GalT activities indicated that AGP is
synthesized through a secretary pathway involving the endomembrane system.
References
Ellis M, Egelund J, Schultz CJ, Bacic A (2010) Arabinogalactan-proteins: key regulators at the cell
surface? Plant Physiol 153: 409-419
Kieliszewski MJ, Lamport DTA (1994) Extensin - repetitive motifs, functional sites,
posttranslational codes, and phylogeny. Plant J 5: 157-172
Nothnagel EA (1997) Proteoglycans and related components in plant cells. In International
Review of Cytology - a Survey of Cell Biology, Vol 174, Vol 174, pp 195-291
Seifert GJ, Roberts K (2007) The biology of arabinogalactan proteins. Annu Rev Plant Biol 58:
137-161
Showalter AM (2001) Arabinogalactan-proteins: structure, expression and function. Cell Mol
Life Sci 58: 1399-1417
Tan L, Qiu F, Lamport DT, Kieliszewski MJ (2004) Structure of a hydroxyproline (Hyp)arabinogalactan polysaccharide from repetitive Ala-Hyp expressed in transgenic
Nicotiana tabacum. J Biol Chem 279: 13156-13165