SUMOylation is a post-translation modification whereby a member of the SUMO family is covalently attached to other proteins to modify their function, similar to ubiquitination. SUMOylation, has been found to have many functions, the most studied of which are nuclear-cytosolic transport and transcriptional regulation. However, more recently SUMO was found to be involved in the activity dependent removal of GluK2-containing kainate receptors (KAR) from the plasma membrane (1). PKC has been shown to have a role in both removal (2;3) and insertion of KAR (4;5). In addition, the GluK2 subunit has a predicted PKC phosphorylation site close to the consensus sequence for SUMOylation. We therefore investigated the role of PKC in SUMO-dependent internalisation of GluK2-containing KAR using whole cell patch clamp electrophysiology in CA3 pyramidal neurons in transverse hippocampal slices from P14 Wistar rats. All data are given as a percentage of the initial response and error values stated refer to the standard error of the mean. In agreement with Martin et al. (2007), inclusion of SUMO-1 in the patch pipette solution decreases the amplitude of KAR-EPSCs to 50.0 ± 4.4% (n=13). In addition, inclusion of SENP-1, an enzyme responsible for de-SUMOylation, increases amplitude to 134.9 ± 7.2% (n=7), thought to be due to reinsertion of receptors that are no longer SUMOylated. However, application of phorbol 12-myristate 13-acetate (PMA; 1 µM), a PKC activator, increased KAR-EPSC amplitude to 140.0 ± 11.0% (n=7; P<0.05). Whilst application of a PKC inhibitor, chelerythrine (5 µM) decreased KAR-EPSC amplitude to 73.4 ± 8.3% (n=7; P<0.05). These data imply that phosphorylation of PKC does not lead to SUMOylation. Conversely, preincubation with PMA (1 µM) for 15 minutes leads to an increase in the SUMO-1 dependent rundown of KAR-mediated EPSCs (55.6 ± 7.7% vs 26.1 ± 5.2%; n=8; P<0.05). Also, incubation with chelerythrine occludes the SUMO-1 dependent rundown (93.56 ± 8.3%, n=8). We propose that phosphorylation of GluK2 leads to insertion or stabilisation of GluK2-containing receptors in the membrane, increasing the number of receptors in the membrane that may subsequently be SUMOylated. When receptor recycling is blocked using chelerythrine there is a slow rundown in the number of synaptic receptors and therefore less receptors to be SUMOylated. In support of this, preincubation with PMA has no effect on the run-up in KAR-EPSC amplitude caused by SENP-1 (138.15 ± 6.7% vs. 139.34 ± 9.5%), but blocking PKC also occludes the SENP-1 run-up (94.1 ± 7.4%). Taken together, these data suggest that PKC is involved in the trafficking of KAR at mossy fibre synapses in CA3. Rather than leading directly to SUMOylation, phosphorylation of GluK2 by PKC may increase or stabilise receptors at the synapse, and these receptors are likely to be from the same pool of KAR that can be SUMOylated.