NE-dlg/SAP102, a neuronal and endocrine tissue-specific membrane- associated guanylate kinase family protein, is known to bind to C-terminal ends of N-methyl-D-aspartate receptor 2B (NR2B) through its PDZ (PSD- 95/Dlg/ZO-1) domains. NE-dlg/SAP102 and NR2B colocalize at synaptic sites in cultured rat hippocampal neurons, and their expressions increase in parallel with the onset of synaptogenesis. We have identified that NE-dlg/SAP102 interacts with calmodulin in a Ca2+-dependent manner. The binding site for calmodulin has been determined to lie at the putative basic α-helix region located around the src homology 3 (SH3) domain of NE-dlg/SAP102. Using a surface plasmon resonance measurement system, we detected specific binding of recombinant NE-dlg/SAP102 to the immobilized calmodulin with a K(d) value of 44 nM. However, the binding of Ca2+/calmodulin to NE-dlg/SAP102 did not modulate the interaction between PDZ domains of NE-dlg/SAP102 and the C- terminal end of rat NR2B. We have also identified that the region near the calmodulin binding site of NE-dlg/SAP102 interact with the GUK-like domain of PSD-95/SAP90 by two-hybrid screening. Pull down assay revealed that NE- dlg/SAP102 can interact with PSD-95/SAP90 in the presence of both Ca2+ and calmodulin. These findings suggest that the Ca2+/calmodulin modulates interaction of neuronal membrane-associated guanylate kinase proteins and regulates clustering of neurotransmitter receptors at central synapses.
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