Proteins containing PDZ (postsynaptic density-95, discs large, zonula occludens) domains play a general role in recruiting receptors and enzymes to specific synaptic sites. In Caenorhabditis elegans, a complex of three PDZ proteins, LIN-2/7/10, mediates basolateral targeting of a receptor tyrosine kinase. Homologs of these LIN proteins have also been identified in higher organisms, and here we analyze the MALS/Veli (mammalian LIN-7/vertebrate homolog of LIN-7) proteins in brain.

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... cal staining and in situ hybridization show that MALS occur differentially in discrete populations of neurons throughout the brain. Most neurons express only one MALS protein, although some cells contain two or even all three MALS isoforms. At the subcellular level, MALS proteins are found in both dendritic and axonal locations, suggesting that they may regulate processes at both pre-and postsynaptic sites. Targeted disruption of MALS-1 and MALS-2 does not yield a detectable phenotype, and hippocampal synaptic function and plasticity are intact in the MALS-1/2 double knockouts. Interestingly, MALS-3 protein is dramatically induced in the MALS-1/2 double knockouts, implying that dynamic changes in protein expression may play an important regulatory role for this family of synaptic PDZ proteins. Neuronal development and function require polarized sorting of protein complexes to appropriate cellular domains within neurons. At synapses neurotransmitter receptors must align at the postsynaptic density (PSD) 1 opposed to the presynaptic active zone. Although mechanisms for protein trafficking to synapses and other specializations in neurons remain poorly understood, recent work identifies a general role for proteins containing PDZ (postsynaptic density-95, discs large, zonula occludens) domains in synaptic targeting (1-3). PDZ motifs are modular 80-amino acid domains that mediate protein-protein interactions in a variety of cellular contexts. PDZ domains contain a conserved peptide-binding grove that associates with the extreme C terminus of interacting protein ligands or with appropriate internal binding motifs (4, 5). In addition to binding to certain receptors, PDZ-containing complexes also recruit cytosolic signaling enzymes to appropriate plasma membrane domains (1-3). Genetic studies of invertebrates have shown that PDZ proteins often play essential roles in regulating cellular signaling pathways. Mutations of Drosophila discs large, a membraneassociated guanylate kinase related to PSD-95, cause overgrowth of imaginal discs and abnormalities of larval neuromuscular junction (6, 7). In Caenorhabditis elegans, mutations of a set of three PDZ proteins, LIN-2, LIN-7, and LIN-10, disrupt differentiation of vulval precursor cells and yield a vulvaless phenotype (8). A series of elegant biochemical and cell biological studies showed that LIN-2/7/10 form a stable complex in which LIN-2 directly binds to both LIN-7 and LIN-10 (8). These interactions do not involve the PDZ domains, which remain free to interact with other cellular proteins (9). Specifically, the PDZ domain from LIN-7 binds the extreme C terminus of LET-23 and mediates basolateral localization of this receptor. The LIN-2/7/10 complex also occurs prominently in neurons. Disruption of lin-10 in C. elegans prevents proper postsynaptic sorting of the GLR-1 glutamate receptor (10). In mammals, close homologs of LIN-2 (CASK), LIN-7 (MALS/Veli), and LIN-10 (Mint/X11) have all been identified, and the LIN-2/7/10 complex occurs at highest levels in brain (9, 11, 12) . However, specific roles for the LIN-2/7/10 complex in brain remain uncertain. Some studies have suggested that this complex regulates presynaptic functions (9), whereas others have implicated postsynaptic roles (12). Immunohistochemical studies show CASK is ubiquitous in brain and occurs at both pre-and postsynaptic sites (13, 14) . Mint has also been found in numerous neuronal populations in brain. The cellular localizations for MALS in brain remain unknown. Here, we have developed antisera to the family of MALS/Veli (mammalian LIN-7/vertebrate homolog of LIN-7) homologs. Immunohistochemical staining of brain shows a heterogeneous distribution for these proteins, such that each MALS shows discrete localization in specific neuronal populations and both dendritic and axonal sites are labeled. In general, cells are endowed with only one MALS protein; however, certain neurons express multiple MALS isoforms. Targeted disruption of MALS-1 and MALS-2 does not yield a detectable phenotype,