NMDA receptor (NMDAR)-dependent long-term depression (LTD) in the hippocampus is mediated primarily by the calcium-dependent removal of AMPA receptors (AMPARs) from the postsynaptic density. The AMPAR-binding, PDZ (PSD-95/Dlg/ZO1) and BAR (Bin/amphiphysin/Rvs) domain-containing protein PICK1 has been implicated in the regulation of AMPAR trafficking underlying several forms of synaptic plasticity. Using a strategy involving small hairpin RNA-mediated knockdown of PICK1 and its replacement with recombinant PICK1, we performed a detailed structure–function analysis of the role of PICK1 in hippocampal synaptic plasticity and the underlying NMDAR-induced AMPAR trafficking. We found that PICK1 is not necessary for maintenance of the basal synaptic complement of AMPARs or expression of either metabotropic glutamate receptor-dependent LTD or NMDAR-dependent LTP. Rather, PICK1 function is specific to NMDAR-dependent LTD and the underlying AMPAR trafficking. Furthermore, although PICK1 does not regulate the initial phase of NMDAR-induced AMPAR endocytosis, it is required for intracellular retention of internalized AMPARs. Detailed biophysical analysis of an N-terminal acidic motif indicated that it is involved in intramolecular electrostatic interactions that are disrupted by calcium. Mutations that interfered with the calcium-induced structural changes in PICK1 precluded LTD and the underlying NMDAR-induced intracellular retention of AMPARs. These findings support a model whereby calcium-induced modification of PICK1 structure is critical for its function in the retention of internalized AMPARs that underlies the expression of hippocampal NMDAR-dependent LTD.