The Arabidopsis thaliana Glutamate-like Receptor Family (AtGLR) - Communication in Plants: Neuronal Aspects of Plant Life - page 194

13.3.3
Are At GLRs Ion Channels?
At GLRs are thought to encode subunits of plant NSCCs (Davenport 2002;
Demidchik et al. 2002; Demidchik, this volume). However, despite profound
aminoacidsequencesimilaritywithiGluRstheresidueswithintheputative
pore region of At GLRs are unlike any other ion channel known (Davenport
2002). This makes predictions about their ion selectivity, if they are indeed
channels, impossible. Furthermore, although At GLRs are divided into three
clear clades upon the sequence similarity of their whole sequence if only the
M1, P, and M2 domains are compared then the similarity in and between
clades breaks down. This may indicate either a loss of channel function
or similar selectivity and/or gating properties between At GLR clades, in
contrast to iGluR subfamilies (Chiu et al. 2002).
Attempts to functionally characterise At GLRs in heterologous expres-
sion systems have so far proved inconclusive. The expression of several
group 2 members in Xenopus oocytes did not result in any detectable cur-
rent, even following fusion with the signal peptide from the rat GluR6 which
facilitated the functional characterisation of the previously non-functional
Synechocystis GluR0 (Lacombe et al. 2001b; B.R. Davies and R. Davenport,
unpublished). The injection of AtGLR3.4 and AtGLR3.7 complementary
RNA (cRNA) results in the activation of cation-permeable conductances at
theoocyteplasmamembraneandtheconsecutiveactivationoftheintrinsic
Ca 2+ -activated chloride current (ICl Ca )(D.Becker,unpublished;Cheffings
2001; Gilliham 2005, unpublished; Lacombe et al. 2001b). Activation of the
ICl Ca does indicate that [Ca 2+ ] cyt has been elevated but it is not proof that
the injected cRNA forms a channel that catalyses the influx of Ca 2+ across
the plasma membrane. ICl Ca activation may be the sole result of upregula-
tion of additional endogenous channels in intracellular compartments or
theplasmamembranesuchasCa 2+ -permeable hyperpolarisation-activated
cation channels (Tzounopoulos 1995; Kuruma et al. 2000) or the disruption
of cellular homeostasis (Weber 1999). Cheffings (2001) has suggested that
At GLR3.7 is constitutively active and catalyses the voltage-independent
movement of Na + ,K + ,andCa 2+ across the plasma membrane when ex-
pressed in Xenopus oocytes. Injection of the At GLR3. 7 coding sequence
containing stop codons reduced the instantaneous current component at all
voltages to wild-type levels, indicating that At GLR3. 7 may encode a chan-
nel. The possibility of constitutive At GLRchannelactivityissupportedby
the sequence of At GLR M2 domains, which resemble sequences known to
induce constitutive activity in some iGluRs (Chiu et al. 1999; Yuzaki 2003).
Disappointingly, no currents have been activated by L-glutamate, glycine,
or a combination of both, at any voltage, at various pH ext ,witharangeof
external cations, in Xenopus oocytes at levels over control following the