Analysis of metabotropic glutamate receptor 7 as a potential substrate for SUMOylation

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. Research highlights ► mGluR7 is a presynaptic GPCR that modulates transmission. ► We confirm that SUMO can be conjugated to c-terminal fragments of mGluR7 in vitro. ► We show that mutation of lysine 889 abolishes SUMOylation in in vitro assays. ► We cannot detect any biochemical or functional evidence for SUMOylation of full-length mGluR7. ► Our data question if native mGluR7 is a physiologically relevant SUMO substrate. Keywords: SUMO; SUMOylation; mGluR7; Ubc9; Ubiquitin; Post-translational modification; GPCR; mGluR7; Metabotropic glutamate receptor