Cadherins function in the adhesion of presynaptic and postsynaptic membranes at excitatory synapses. Here we show that the cadherinassociated protein neural plakophilin-related arm protein (NPRAP; also called ␦-catenin) binds via a postsynaptic density-95 (PSD-95)/ discs large/zona occludens-1 (PDZ) interaction to AMPA receptor (AMPAR)-binding protein (ABP) and the related glutamate receptor (GluR)-interacting protein (GRIP), two multi-PDZ proteins that bind the GluR2 and GluR3 AMPAR subunits. The resulting cadherin-NPRAP-ABP/GRIP complexes serve as anchorages for AMPARs. Exogenous NPRAP that was bound to cadherins at adherens junctions of Madin-Darby canine kidney cells recruited ABP from the cytosol to form cadherin-NPRAP-ABP complexes, dependent on NPRAP interaction with the ABP PDZ domain 2. The cadherin-NPRAP-ABP complexes also bound GluR2. In cultured hippocampal neurons, dominant-negative mutants of NPRAP designed to disrupt tethering of ABP to NPRAP-cadherin complexes reduced surface levels of endogenous GluR2, indicating that interaction with cadherin-NPRAP-ABP complexes stabilized GluR2 at the neuronal plasma membrane. Cadherins, NPRAP, GRIP, and GluR2 copurified in the fractionation of synaptosomes and the postsynaptic density, two fractions enriched in synaptic proteins. Furthermore, synaptosomes contain NPRAP-GRIP complexes, and NPRAP localizes with the postsynaptic marker PSD-95 and with AMPARs and GRIP at spines of hippocampal neurons. Thus, tethering is likely to take place at synaptic or perisynaptic sites. NPRAP also binds PSD-95, which is a scaffold for NMDA receptors, for AMPARs in complexes with auxiliary subunits, the TARPs (transmembrane AMPA receptor regulator proteins), and for adhesion molecules. Thus, the interaction of scaffolding proteins with cadherin-NPRAP complexes may anchor diverse signaling and adhesion molecules at cadherins.
Ca2ϩ channel inactivation is a key element in controlling the level of Ca 2ϩ entry through voltage-gated Ca 2ϩ channels. Interaction between the pore-forming ␣ 1 subunit and the auxiliary  subunit is known to be a strong modulator of voltage-dependent inactivation. Here, we demonstrate that an N-terminal membrane anchoring site (MAS) of the  2a subunit strongly reduces ␣ 1A (Ca V 2.1) Ca 2ϩ channel inactivation. This effect can be mimicked by the addition of a transmembrane segment to the N terminus of the  2a subunit. Inhibition of inactivation by  2a also requires a link between MAS and another important molecular determinant, the  interaction domain (BID). Our data suggest that mobility of the Ca 2ϩ channel I-II loop is necessary for channel inactivation. Interaction of this loop with other identified intracellular channel domains may constitute the basis of voltage-dependent inactivation. We thus propose a conceptually novel mechanism for slowing of inactivation by the  2a subunit, in which the immobilization of the channel inactivation gate occurs by means of MAS and BID.
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