AbstractAt neuronal synapses, multiprotein complexes of trans-synaptic adhesion molecules, scaffold proteins and neurotransmitter receptors assemble to essential building blocks required for synapse formation and maintenance. Here we describe a novel role for the membrane-associated guanylate kinase (MAGUK) protein MPP2 (MAGUK p55 subfamily member 2) at synapses of rat central neurons. Through interactions mediated by its C-terminal SH3-GK domain module, MPP2 binds to the abundant postsynaptic scaffold proteins PSD-95 and GKAP and localises to postsynaptic sites in hippocampal neurons. MPP2 also colocalises with the synaptic adhesion molecule SynCAM1. We demonstrate that the SynCAM1 C-terminus interacts directly with the MPP2 PDZ domain and that MPP2 does not interact in this manner with other highly abundant postsynaptic transmembrane proteins. Our results highlight a previously unexplored role for MPP2 at postsynaptic sites as a scaffold that links SynCAM1 cell adhesion molecules to core proteins of the postsynaptic density.