Selectivity and promiscuity of the first and second PDZ domains of PSD-95 and synapse-associated protein 102

PDZ domains typically interact with the very carboxyl terminus of their binding partners. Type I PDZ domains usually require valine, leucine, or isoleucine at the very COOH-terminal (PO) position, and serine or threonine 2 residues upstream at P-2. We quantitatively defined the contributions of carboxyl-terminal residues to binding selectivity of the prototypic interactions of the PDZ domains of postsynaptic density protein 95 (PSD-95) and its homolog synapse-associated protein 90 (SAP102) with the NR2b subunit of the N-methyl-D-aspartate-type glutamate receptor. Our studies indicate that all of the last five residues of NR2b contribute to the binding selectivity. Prominent were a requirement for glutamate or glutamine at P-3 and for valine at P-o for high affinity binding and a preference for threonine over serine at P-2, in the context of the last 11 residues of the NR2b COOH terminus. This analysis predicts a COOH-terminal (E/Q)(S/T)XV consensus sequence for the strongest binding to the first two PDZ domains of PSD-95 and SAP102. A search of the human genome sequences for proteins with a COOH-terminal (E/Q)(S/T)XV motif yielded 50 proteins, many of which have not been previously identified as PSD-95 or SAP102 binding partners. Two of these proteins, brain-specific angiogenesis inhibitor 1 and protein kinase Calpha, co-immunoprecipitated with PSD-95 and SA-P102 from rat brain extracts.