Membrane-associated guanylate kinases (MAGUKs) contain multiple protein-binding domains that allow them to assemble specific multiprotein complexes in particular regions of the cell. CASK/LIN-2, a MAGUK required for EGF receptor localization and signalling in Caenorhabditis elegans, contains a calmodulin-dependent protein kinase-like domain followed by PDZ, SH3 and guanylate kinase-like domains. In adult rat brain, CASK is concentrated at neuronal synapses and binds to the cell-surface proteins neurexin and syndecan and the cytoplasmic proteins Mint/LIN-10 and Veli/LIN-7. Here we report that, through its guanylate kinase domain, CASK interacts with Tbr-1, a T-box transcription factor that is involved in forebrain development. CASK enters the nucleus and binds to a specific DNA sequence (the T-element) in a complex with Tbr-1. CASK acts as a coactivator of Tbr-1 to induce transcription of T-element containing genes, including reelin, a gene that is essential for cerebrocortical development. Our findings show that a MAGUK which is usually associated with cell junctions has a transcription regulation function.

The circadian clock in the suprachiasmatic nucleus (SCN) is thought to drive daily rhythms of behavior by secreting factors that act locally within the hypothalamus. In a systematic screen, we identified transforming growth factor-alpha (TGF-alpha) as a likely SCN inhibitor of locomotion. TGF-alpha is expressed rhythmically in the SCN, and when infused into the third ventricle it reversibly inhibited locomotor activity and disrupted circadian sleep-wake cycles. These actions are mediated by epidermal growth factor (EGF) receptors on neurons in the hypothalamic subparaventricular zone. Mice with a hypomorphic EGF receptor mutation exhibited excessive daytime locomotor activity and failed to suppress activity when exposed to light. These results implicate EGF receptor signaling in the daily control of locomotor activity, and identify a neural circuit in the hypothalamus that likely mediates the regulation of behavior both by the SCN and the retina.

Members of the postsynaptic density-95 (PSD-95)/SAP90 family of membrane-associated guanylate kinase (MAGUK) proteins function as multimodular scaffolds that organize proteinsignaling complexes at neuronal synapses. MAGUK proteins contain PDZ, Src homology 3 (SH3), and guanylate kinase (GK)-like domains, all of which can function as sites for specific protein-protein interactions. We report here a direct proteinprotein interaction between the SH3 domain and the GK region in the PSD-95 family of MAGUKs. The SH3 domain of the PSD-95 family appears to have an atypical binding specificity, because the classical SH3 binding (-P-X-X-P-) motif is absent from the GK domain. Although SH3-GK binding can occur in either an intramolecular or intermolecular manner, the intramolecular mode is preferred, possibly because of additional tertiary interactions available when the SH3 and GK domains are adjacent in the same polypeptide. Mutations disrupting the intramolecular SH3-GK interaction do not interfere with PSD-95 association with the K ϩ channel Kv1.4 or with the GK domain-binding protein GKAP. The same mutations, however, inhibit the clustering of Kv1.4 by PSD-95, suggesting that the intramolecular SH3-GK interaction may modulate the clustering activity of PSD-95. Key words: PDZ domain; ion channel clustering; postsynaptic density; Src tyrosine kinase; polyproline helix; disks largeSynaptic transmission in neuronal synapses requires the proper organization of ion channels, neurotransmitter receptors, and signaling molecules in the synaptic junction. An emerging theme for the micro-organization of synapses is that functionally coupled proteins are placed in close proximity to each other via their interactions with scaffold proteins. The PSD-95 family of membrane-associated guanylate kinases (MAGUKs) plays such a role at postsynaptic sites of glutamatergic synapses by interacting with a variety of ion channels, receptors, cytoskeletal components, and intracellular signaling proteins (Sheng and Wyszynski, 1997;Ziff, 1997;Craven and Bredt, 1998;Kennedy, 1998;O'Brien et al., 1998).The PSD-95 family of proteins (PSD-95/SAP90, SAP97/hdlg, chapsyn-110/PSD-93, and SAP102) share a common domain organization, consisting of three PDZ domains in their N-terminal half, a central Src homology 3 (SH3) domain, and a guanylate kinase (GK) domain at their C terminus. The PDZ domains of PSD-95 family proteins interact with the C terminal sequence motif (consensus -X-T/S-X-V) found in a variety of membrane and intracellular proteins, including Shaker K ϩ channels, NMDA receptors, and SynGAP (Kim et al., 1995Kornau et al., 1995;Doyle et al., 1996;Chen et al., 1998). In addition, PSD-95 binds to neuronal nitric oxide synthase (nNOS) through a PDZ-PDZ/␤-finger interaction (Brenman et al., 1996a;Hillier et al., 1999). In vitro and in vivo evidence suggests that these PDZmediated interactions are involved in the clustering of specific membrane proteins at synaptic sites (Kim et al., 1995Thomas et al., 1997).The GK domain of the PSD-95 family of MA...