The Arabidopsis thaliana Glutamate-like Receptor Family (AtGLR) - Communication in Plants: Neuronal Aspects of Plant Life - page 193

C-terminus indicated strongest expression in developing floral stems and
vasculature (Kim et al. 2001; Turano et al. 2002). A number of At GLRs
show high expression in senescent leaves, including all At GLR1, At GLR 2.5 ,
At GLR2.7-9, and At GLR3.4 (Zimmermann et al. 2004; Meyerhoff et al.
2005). Promoter:GUS fusions for AtGLR2.8 , AtGLR2.9 ,and AtGLR3.4 have
indicated their expression throughout the leaf mesophyll but are greatest
surrounding vascular bundles as well as hydathodes, suggestive of a role in
metabolite re-mobilisation during senescence (O. Meyerhoff and D. Becker,
unpublished).
Most At GLRs have a hydrophobic N-terminus predicted to be cleaved
after targeting the nascent protein to the endoplasmic reticulum for pro-
cessing (Davenport 2002). No clear picture has yet emerged concerning the
subcellular distribution of individual plant glutamate receptors. However,
using green fluorescent protein (GFP) or GUS-GFP tags several At GLR
( At GLR3.4, 3.7, and a radish GLR) have been localised to the plasma
membrane of onion epidermis cells, Arabidopsis and tobacco protoplasts
(D. Becker, unpublished; R. Davenport, unpublished; Kang and An 2003).
However, these studies should be interpreted with caution since incorpo-
ration of tags and/or overexpression can cause mistargeting ( At GLR3.4 has
also been reported to be expressed in plastids) (Szabo et al. 2004). An
antibody to the C-terminus localised At GLR3.2tothemembranefraction
but was not used to distinguish the membrane involved (Turano et al.
2002).

13.3.2
Amino Acid Binding and At GLR Regulation

AtGLR s have been predicted to function in an analogous manner to iGluRs.
Phylogenetic analysis predicts that At GLRs share greatest sequence simi-
larity with NMDA receptors despite sharing more invariant residues with
GluR δ receptors (Chiu et al. 1999, 2002). Subunit-ligand interactions for

At GLRweremodelledbysuperimposingalignedsequencesontotheligand-
binding domains of a rat iGLR α crystal structure (Dubos et al. 2003). The

docking algorithm OXDOCK (Glick et al. 2002) found that only At GLR1.1
would be predicted to bind glutamate, whereas the remaining 19 predicted
At GLR subunits would not owing to steric hindrance in the ligand-binding
site (Dubos et al. 2003). Instead, these 19 subunits share a high degree of
similarity with glycine-binding domains, and are predicted to bind glycine.
It is intriguing that Arabidopsis possesses a greater number of At GLR sub-
unitsthatarepredictedtobindglycine,asopposedtoglutamate,andmay
suggestthatglycineplaysamoreimportantroleinsignallingthanhad
previously been suspected.