Interaction of synaptic scaffolding molecule and β-catenin

Synaptic scaffolding molecule (S-SCAM) is a synaptic membrane-associated guanylate kinase with inverted domain organization (MAGI) that interacts with NMDA receptor subunits and neuroligin. In epithelial cells, the non-neuronal isoform of S-SCAM (MAGI-1) is localized at tight or adherens junctions. Recent studies have revealed that the polarized targeting of MAGI-1 to the lateral membrane is mediated by its C-terminal region and that MAGI-1 interacts with beta-catenin in epithelial cells. In this article, we report that S-SCAM interacts with beta-catenin in neurons. beta-Catenin is coimmunoprecipitated with S-SCAM from rat brain. Both S-SCAM and beta-catenin are localized at synapses and are partially colocalized. The C-terminal region of S-SCAM binds to the C-terminal region of beta-catenin. We have tested how the interaction between S-SCAM and beta-catenin plays a role in the synaptic targeting of S-SCAM and beta-catenin. S-SCAM is targeted to synapses via the C-terminal postsynaptic density-95/Dlg-A/ZO-1 (PDZ) domain. beta-Catenin is targeted to synapses with armadillo repeats. The over-expressed C-terminal region of beta-catenin blocks the synaptic targeting of S-SCAM. The overexpressed C-terminal region of S-SCAM is partially targeted to synapses and forms a small number of clusters. In the presence of overexpressed beta-catenin, the C-terminal region of S-SCAM forms more clusters at synapses. These data suggest that the synaptic targeting of S-SCAM is mediated by the interaction with beta-catenin.