Group III metabotropic glutamate receptors (mGluRs) undergo post-translational 
modification by SUMO in in vitro assays but the SUMOylation of full-length 
mGluRs in mammalian cells has not been reported. Here we investigated 
SUMOylation of mGluR7 in HEK293 cells and primary cortical neurons in an attempt 
to confirm SUMOylation and define physiological effects on mGluR7 function. 
Using a recombinant bacterial expression assay we validated in vitro SUMOylation 
of the C-terminal domain of mGluR7 by both SUMO-1 and SUMO-2 and show that a 
single lysine residue (K889) in mGluR7 is required for SUMOylation. However, 
using a range of approaches, we were unable to detect SUMOylation of full-length 
mGluR7 in either heterologous cells or neurons. Further, we observed no 
differences in receptor stability or surface expression between wild-type and a 
non-SUMOylatable point mutant mGluR7. Thus, our results question whether mGluR7, 
and by implication other group III mGluRs, are physiologically relevant neuronal 
SUMO substrates.