Tissue development, differentiation, and physiology require specialized cellular adhesion and signal transduction at sites of cell-cell contact. Scaffolding proteins that tether adhesion molecules, receptors, and intracellular signaling enzymes organize macromolecular protein complexes at cellular junctions to integrate these functions. One family of such scaffolding proteins is the large group of membrane-associated guanylate kinases (MAGUKs). Genetic studies have highlighted critical roles for MAGUK proteins in the development and physiology of numerous tissues from a variety of metazoan organisms. Mutation of Drosophila discs large (dlg) disrupts epithelial septate junctions and causes overgrowth of imaginal discs. Similarly, mutation of lin-2, a related MAGUK in Caenorhabditis elegans, blocks vulval development, and mutation of the postsynaptic density protein PSD-95 impairs synaptic plasticity in mammalian brain. These diverse roles are explained by recent biochemical and structural analyses of MAGUKs, which demonstrate their capacity to assemble well--efined--yet adaptable--protein complexes at cellular junctions.

Trafficking of AMPA receptors (AMPA-Rs) to and from synapses controls the strength of excitatory synaptic transmission. However, proteins that cluster AMPA-Rs at synapses remain poorly understood. Here we show that PSD-95-like membrane-associated guanylate kinases (PSD-MAGUKs) mediate this synaptic targeting, and we uncover a remarkable functional redundancy within this protein family. By manipulating endogenous neuronal PSD-MAGUK levels, we find that both PSD-95 and PSD-93 independently mediate AMPA-R targeting at mature synapses. We also reveal unanticipated synapse heterogeneity as loss of either PSD-95 or PSD-93 silences largely nonoverlapping populations of excitatory synapses. In adult PSD-95 and PSD-93 double knockout animals, SAP-102 is upregulated and compensates for the loss of synaptic AMPA-Rs. At immature synapses, PSD-95 and PSD-93 play little role in synaptic AMPA-R clustering; instead, SAP-102 dominates. These studies establish a PSD-MAGUK-specific regulation of AMPA-R synaptic expression that establishes and maintains glutamatergic synaptic transmission in the mammalian central nervous system.

RNA editing by A-to-I modification has been recognized as an important molecular mechanism for generating RNA and protein diversity. In mammals, it is mediated by a family of adenosine deaminases that act on RNAs (ADARs). The large version of the editing enzyme ADAR1 (ADAR1-L), expressed from an interferon-responsible promoter, has a Z-DNA/Z-RNA binding domain at its N-terminus. We have tested the in vitro ability of the enzyme to act on a 50 bp segment of dsRNA with or without a Z-RNA forming nucleotide sequence. A-to-I editing efficiency is markedly enhanced in presence of the sequence favoring Z-RNA. In addition, an alteration in the pattern of modification along the RNA duplex becomes evident as reaction times decrease. These results suggest that the local conformation of dsRNA molecules might be an important feature for target selectivity by ADAR1 and other proteins with Z-RNA binding domains.

Kidney development and physiology require polarization of epithelia that line renal tubules. Genetic studies show that polarization of invertebrate epithelia requires the crumbs, partition-defective-3, and discs large complexes. These evolutionarily conserved protein complexes occur in mammalian kidney; however, their role in renal development remains poorly defined. Here, we find that mice lacking the small PDZ protein mammalian LIN-7c (MALS-3) have hypomorphic, cystic, and fibrotic kidneys. Proteomic analysis defines MALS-3 as the only known core component of both the crumbs and discs large cell polarity complexes. MALS-3 mediates stable assembly of the crumbs tight junction complex and the discs large basolateral complex, and these complexes are disrupted in renal epithelia from MALS-3 knockout mice. Interestingly, MALS-3 controls apico-basal polarity preferentially in epithelia derived from metanephric mesenchyme, and defects in kidney architecture owe solely to MALS expression in these epithelia. These studies demonstrate that defects in epithelial cell polarization can cause cystic and fibrotic renal disease.