Proline-rich synapse-associated protein-1/cortactin binding protein 1(ProSAP1/CortBP1) is a PDZ-domain protein highly enriched in the postsynaptic density

The postsynaptic density (PSD) is crucially involved in the structural and functional organization of the postsynaptic neurotransmitter reception appa ratus. Using antisera against rat brain synaptic junctional protein prepara tions, we isolated cDNAs coding for proline-rich synapse-associated protein -1 (ProSAP1), a PDZ-domain protein. This protein was found to be identical to the recently described cortactin-binding protein-1 (CortBP1). Homology s creening identified a related protein, ProSAP2. Specific antisera raised ag ainst a C-terminal fusion construct and a central part of ProSAP1 detect a cluster of immunoreactive bands of 180 kDa in the particulate fraction of r at brain homogenates that copurify with the PSD fraction. Transcripts and i mmunoreactivity are widely distributed in the brain and are upregulated dur ing the period of synapse formation in the brain. In addition, two short N- terminal insertions are detected; they are differentially regulated during brain development. Confocal microscopy of hippocampal neurons showed that P roSAP1 is predominantly localized in synapses, and immunoelectron microscop y in situ revealed a strong association with PSDs of hippocampal excitatory synapses. The accumulation of ProSAP1 at synaptic structures was analyzed in the developing cerebral cortex. During early postnatal development, stro ng immunoreactivity is detectable in neurites and somata, whereas from post natal day 10 (P10) onward a punctate staining is observed. At the ultrastru ctural level, the immunoreactivity accumulates at developing PSDs starling from P8. Both interaction with the actin-binding protein cortactin and earl y appearance at postsynaptic sites suggest that ProSAP1/ CortBP1 may be inv olved in the assembly of the PSD during neuronal differentiation.