An internal PDZ-binding motif in Densin-180 promotes activity-dependent SHANK scaffold remodelling

SHANK proteins form core postsynaptic density (PSD) scaffolds that organise signalling complexes through multiple interaction domains, including PDZ domains that typically recognise C-terminal peptide motifs. Here, we identify an internal PDZ recognition mechanism that links Densin-180 to SHANK and promotes SHANK scaffold assembly. We map SHANK binding to an internal PDZ-binding motif in Densin-180 (residues 843-863) and show by NMR spectroscopy and fluorescence polarisation that this motif binds SHANK1-3 PDZ domains with high affinity and specificity, competing with canonical C-terminal ligands. The crystal structure of the Densin-180-SHANK complex reveals that Phe858 inserts into the hydrophobic pocket of the PDZ domain despite the absence of a terminal carboxylate. In neurons, this interaction mediates Densin-180 recruitment to dendritic spines and drives activity-dependent reorganisation of postsynaptic SHANK3 assemblies, whereas mutation of Phe858 disrupts both processes. Disrupting the Densin-180-SHANK interaction using a Densin-180-derived peptide impairs activity-dependent spine structural plasticity, accompanied by defects in PSD organisation and actin cytoskeleton remodelling. These findings define an internal mode of PDZ recognition and reveal how Densin-180 couples neuronal activity to SHANK scaffold remodelling.