The ubiquitin-proteasome pathway (UPP) regulates synaptic function, but little is known about specific UPP targets and mechanisms in mammalian synapses. We report here that the SCF(beta-TRCP) complex, a multisubunit E3 ubiquitin ligase, targets the postsynaptic spine-associated Rap GTPase activating protein (SPAR) for degradation in neurons. SPAR degradation by SCF(beta-TRCP) depended on the activity-inducible protein kinase Polo-like kinase 2 (Plk2). In the presence of Plk2, SPAR physically associated with the SCF(beta-TRCP) complex through a canonical phosphodegron. In hippocampal neurons, disruption of the SCF(beta-TRCP) complex by overexpression of dominant interfering beta-TRCP or Cul1 constructs prevented Plk2-dependent degradation of SPAR. Our results identify a specific E3 ubiquitin ligase that mediates degradation of a key postsynaptic regulator of synaptic morphology and function.