Plants are the planet's primary
producers of food, fuel, materials and medicines. Therefore, a
self-sustaining civilization must of necessity acquire a profound
understanding of plants. My research program deals with plant cell
walls, particularly their hydroxyproline-rich glycoproteins (HRGPs),
which are major scaffolding components of the extracellular matrix, and
their role in plant form and function. And, as a result of our recent
successes in creating synthetic gene analogs of HRGPs, this research
also emphasizes the utilitarian value of reengineering HRGPs to produce
new plant gums, novel glycoprotein-based biopolymer and long acting
biologically potent human cytokines and hormenes expressed as HRGP
chimeras.
Plant HRGPs are modular structural glycoproteins
-- generally elongated, flexible, rodlike molecules with marked peptide
periodicity. Some become crosslinked to form covalent cell wall
networks that control extension growth, increase the tensile strength
of the cell wall and impart mechanical resistance to attack by plant
pathogens. Other HRGPs such as the arabinogalactan-proteins (AGPs)
remain uncrosslinked and form a protective hydrophilic cushion at the
interface between the plasma membrane and cell wall. Yet others, like
the gum arabic glycoprotein are protective exudate gum components,
acting as plastic sealants at the sites of mechanical injury. Still
other HRGPs contain highly symmetrical motifs with palindromic amino
acid sidechains – the sequence of the amino acid sidechains reads the
same forwards as backwards. Involvement of these peptide palindromes in
more subtle molecular roles such as self-ordering liquid crystals could
lead to biomineralization, epitaxial growth and self-assembly of wall
components at the cell surface, or molecular recognition of pathogens.
Current and Future Work deals with:
HGRP post-translational modifications
and functions at the molecular level.
Synthetic genes for simple repetitive HRGP
constructs can test predicted sites of glycosylation and other
functional properties. Suitably modified they can also be used as
endogenous inhibitors of specific HRGPs to probe function module by
module.
Simple HRGP constructs provide useful substrates
to assay the HRGP-specific prolyl hydroxylases, glycosyltransferases
and crosslinking enzymes yet to be isolated.
HRGP contribution to cell wall networks and
other supramolecular structures.
Synthetic genes are also being used in my lab to
test the role of other motifs in cell wall self-assembly mechanisms:
The sequence Val-Tyr-Lys is a putative
intermolecular crosslink motif responsible for extensin network
formation, to be tested using constructs engineered with Tyr to Phe and
Lys to Leu substitutions. Crosslinking spacing may determine whether
extensin networks are tight or loose, hence their function as
mechanical barriers, ultrafilters or molecular sieves.
The Tyr-Xaa-Tyr intramolecular isodityrosine
crosslink motif rigidifies the extended polypeptide backbone and
results in intermolecular di-isodityrosine crosslinking and the
formation of pathogen-resistant barrier networks.
Association of membrane-anchored AGPs
may form a cell-type specific hydrophilic
cushion between the plasmamembrane and the wall, and assist and direct
cell wall assembly.
The design of new gums and
other hydrocolloids of utilitarian value.
Exudate gums are of widespread industrial use,
mainly due to their HRGP component. For the first time we have the
opportunity of radically reengineering these gums to enhance their
specific properties.
Gum arabic glycoprotein exemplifies
emulsification properties, but the precise mechanism by which this
hydrophilic molecule stabilizes lipophilic suspensions is unknown. We
have designed synthetic constructs to solve that problem.
New precisely engineered hydrogels are envisaged
that can be produced in plants, for example, as monomers for subsequent
crosslinking. Such benign biodegradable ‘green’ polymers produced in
plant ‘factories’ could contribute to our self-sustainability and
reduce our reliance on petrochemicals.
