Synaptic signal complexes of metabotropic glutamate receptors: mGluR8 interacts with SUMO ligases and Band 4.1 proteins

Neurotransmission requires a highly ordered and tightly controlled interplay between synaptic proteins, both in space and time. This is accomplished by synaptic signal complexes that integrate functionally related proteins, e.g. neurotransmitter receptors, enzymes and scaffolds. Here, we identified and characterized proteins interacting with intracellular C-termini of the metabotropic glutamate receptors mGluR8a and mGluR8b.

The Sumo E3-ligases Pias1 and Pias3L co-localized with Ubc9, Sumo1 and mGluR8b in the ganglion cell layer of the retina. Pias1 interacted strongly with mGluR8b and enhanced its sumoylation at K882 and K903. Although K903 is part of the non-canonical motif VKSG, modelling studies suggested that this sequence is able to contact the E2-conjugating Ubc9.

Expression of the Sumo E3-ligase RanBPM was restricted to the inner plexiform layer of the retina where it co-localized with the mGluR8b isoform and processes of cholinergic amacrine cells. These processes express mGluR2 and indeed, RanBPM also bound mGluR2.

Binding of Band 4.1B to mGluR8a/b facilitated their cell surface expression and increased cAMP concentrations. Protein interactions were mediated by exons 19/20 of 4.1B and four basic residues in the mGluR8 C-termini. MGluR8 isoforms bound 4.1B, 4.1G, 4.1N and 4.1R and all interactors were distributed similarly in retinal layers.

In conclusion, we characterized new players and molecular mechanisms guiding synaptic neurotransmission.

Supported by the DFG