MembraneTissue 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) 1 (1-4). Genetic studies have highlighted the 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 the imaginal discs (5). Similarly, mutation of lin-2, a related MAGUK in Caenorhabditits elegans, blocks vulval development (2), and mutation of the postsynaptic density protein PSD-95 impairs synaptic plasticity in mammalian brain (6). MAGUK proteins are composed of a set of protein-protein interaction domains that explain their important roles in organizing protein complexes at sites of cell-cell contact. MAGUKs contain L27 heterodimerization domains, one or three PDZ domains, an SH3 domain and a C-terminal domain homologous to guanylate kinase (GK) (7,8). PDZ domains are small modular motifs that contain a single peptide binding groove that associates with specific sequences often found at the extreme C termini of interacting ion channels (9, 10), cell adhesion molecules (11, 12), and other membrane-associated proteins (13-16). Multiple PDZ domains in PSD-95 organize and accelerate signal transduction at synapses by linking receptors to downstream signaling enzymes (17,18).Whereas the functional roles for PDZ domains in MAGUKs are well established, functions for the SH3 and GK domains are less certain. SH3 domains classically bind to proline-rich motifs (19 -23); however, the structure of the PSD-95 SH3 domain suggests that such interactions are unlikely because a conserved helix in MAGUK SH3 domains occludes the canonical polyproline binding site (24,25). A variety of high affinity protein ligands have been identified for GK domains from several MAGUKs (26 -30), but it is not yet clear how these interactions regulate MAGUK functions. In addition to binding exogenous ligands, protein fragments containing the proposed SH3 and GK regions of MAGUK proteins interact with each other (31-34). The crystal structure of the SH3GK region of PSD-95 reveals that this interaction is the assembly of the SH3 fold from discontinuous structural components (24,25). This SH3/GK interaction, may oligomerize MAGUK scaffolds, but factors that regulate intermolecular SH3 assembly remain uncertain. Despite our limited understanding of the biochemical roles for the SH3 and GK domains, these regions are clearly critical as most genetically identified mutations of MAGUKs occur in the SH3 and GK domains (2, 5).The GK domains of MAGUKs share 40% sequence homology with authentic GK enzymes, which phosphorylate GMP to for...