ANCHORING of ion channels at specific subcellular sites is critical for neuronal signalling, but the mechanisms underlying channel localization and clustering are largely unknown (reviewed in ref. 1). Voltage-gated K+ channels are concentrated in various neuronal domains, including presynaptic terminals, nodes of Ranvier and dendrites, where they regulate local membrane excitability. Here we present functional and biochemical evidence that cell-surface clustering of Shaker-subfamily K+ channels is mediated by the PSD-95 family of membrane-associated putative guanylate kinases, as a result of direct binding of the carboxy-terminal cytoplasmic tails to the K+ channel subunits to two PDZ (also known as GLGF or DHR) domains in the PSD-95 protein. The ability of PDZ domains to function as independent modules for protein-protein interaction, and their presence in other junction-associated molecules (such as ZO-1 (ref. 3) and syntrophin), suggest that PDZ-domain-containing polypeptides may be widely involved in the organization of proteins at sites of membrane specialization.

Selective concentration and anchoring of ionotropic receptors at the synapse is essential for neuronal signaling. Little is known about the molecules that mediate receptor clustering in the CNS. With use of the yeast two-hybrid system to screen a rat brain cDNA library and by in vitro binding assays, we have identified an interaction between NMDA receptor subunits 2A and 2B (NR2A and NR2B) and three distinct members of the PSD-95/SAP90 family of membrane-associated putative guanylate kinases. The interaction is mediated by binding of the C terminus of the NMDA receptor subunits to the first two PDZ (also known as GLGF or DHR) domains of PSD-95/SAP90, an abundant synaptic protein associated with the membrane cytoskeleton. PSD-95 is also known to bind and cluster Shaker-type voltage-gated K+ channels. Similarities between the C-termini of NR2 subunits and K+ channels suggest a common C-terminal binding motif for PDZ domains. These data suggest that PDZ domains can function as modules for protein-protein interactions. Members of the PSD-95 family might serve to anchor NMDA receptors to the submembrane cytoskeleton and aid in the assembly of signal transduction complexes at postsynaptic sites.

Disruption of one allele of the LIS1 gene causes a severe developmental brain abnormality, type I lissencephaly. In Aspergillus nidulans, the LIS1 homolog, NUDF, and cytoplasmic dynein are genetically linked and regulate nuclear movements during hyphal growth. Recently, we demonstrated that mammalian LIS1 regulates dynein functions. Here we characterize NUDEL, a novel LIS1-interacting protein with sequence homology to gene products also implicated in nuclear distribution in fungi. Like LIS1, NUDEL is robustly expressed in brain, enriched at centrosomes and neuronal growth cones, and interacts with cytoplasmic dynein. Furthermore, NUDEL is a substrate of Cdk5, a kinase known to be critical during neuronal migration. Inhibition of Cdk5 modifies NUDEL distribution in neurons and affects neuritic morphology. Our findings point to cross-talk between two prominent pathways that regulate neuronal migration.