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Uneven distribution and local concentration of protein complexes on distinct membrane cortices is a fundamental property in numerous biological processes, including Drosophila neuroblast (NB) asymmetric cell divisions and cell polarity in general. In NBs, the cell fate determinant Numb forms a basal crescent together with Pon and is segregated into the basal daughter cell to initiate its differentiation. Here we discover that Numb PTB domain, using two distinct binding surfaces, recognizes repeating motifs within Pon in a previously unrecognized mode. The multivalent Numb-Pon interaction leads to high binding specificity and liquid-liquid phase separation of the complex. Perturbations of the Numb/Pon complex phase transition impair the basal localization of Numb and its subsequent suppression of Notch signaling during NB asymmetric divisions. Such phase-transition-mediated protein condensations on distinct membrane cortices may be a general mechanism for various cell polarity regulatory complexes.

Uneven distribution and local concentration of protein complexes on distinct membrane cortices is a fundamental property in numerous biological processes, including Drosophila neuroblast (NB) asymmetric cell divisions and cell polarity in general. In NBs, the cell fate determinant Numb forms a basal crescent together with Pon and is segregated into the basal daughter cell to initiate its differentiation. Here we discover that Numb PTB domain, using two distinct binding surfaces, recognizes repeating motifs within Pon in a previously unrecognized mode. The multivalent Numb-Pon interaction leads to high binding specificity and liquid-liquid phase separation of the complex. Perturbations of the Numb/Pon complex phase transition impair the basal localization of Numb and its subsequent suppression of Notch signaling during NB asymmetric divisions. Such phase-transition-mediated protein condensations on distinct membrane cortices may be a general mechanism for various cell polarity regulatory complexes.

Uneven distribution and local concentration of protein complexes on distinct membrane cortices is a fundamental property in numerous biological processes, including Drosophila neuroblast (NB) asymmetric cell divisions and cell polarity in general. In NBs, the cell fate determinant Numb forms a basal crescent together with Pon and is segregated into the basal daughter cell to initiate its differentiation. Here we discover that Numb PTB domain, using two distinct binding surfaces, recognizes repeating motifs within Pon in a previously unrecognized mode. The multivalent Numb-Pon interaction leads to high binding specificity and liquid-liquid phase separation of the complex. Perturbations of the Numb/Pon complex phase transition impair the basal localization of Numb and its subsequent suppression of Notch signaling during NB asymmetric divisions. Such phase-transition-mediated protein condensations on distinct membrane cortices may be a general mechanism for various cell polarity regulatory complexes.

The tumor suppressor Merlin/NF2 functions upstream of the core Hippo pathway kinases Lats1/2 and Mst1/2, as well as the nuclear E3 ubiquitin ligase CRL4(DCAF1). Numerous mutations of Merlin have been identified in Neurofibromatosis type 2 and other cancer patients. Despite more than two decades of research, the upstream regulator of Merlin in the Hippo pathway remains unknown. Here we show by high-resolution crystal structures that the Lats1/2-binding site on the Merlin FERM domain is physically blocked by Merlin's auto-inhibitory tail. Angiomotin binding releases the auto-inhibition and promotes Merlin's binding to Lats1/2. Phosphorylation of Ser518 outside the Merlin's auto-inhibitory tail does not obviously alter Merlin's conformation, but instead prevents angiomotin from binding and thus inhibits Hippo pathway kinase activation. Cancer-causing mutations clustered in the angiomotin-binding domain impair angiomotin-mediated Merlin activation. Our findings reveal that angiomotin and Merlin respectively interface cortical actin filaments and core kinases in Hippo signaling, and allow construction of a complete Hippo signaling pathway.

The tumor suppressor Merlin/NF2 functions upstream of the core Hippo pathway kinases Lats1/2 and Mst1/2, as well as the nuclear E3 ubiquitin ligase CRL4(DCAF1). Numerous mutations of Merlin have been identified in Neurofibromatosis type 2 and other cancer patients. Despite more than two decades of research, the upstream regulator of Merlin in the Hippo pathway remains unknown. Here we show by high-resolution crystal structures that the Lats1/2-binding site on the Merlin FERM domain is physically blocked by Merlin's auto-inhibitory tail. Angiomotin binding releases the auto-inhibition and promotes Merlin's binding to Lats1/2. Phosphorylation of Ser518 outside the Merlin's auto-inhibitory tail does not obviously alter Merlin's conformation, but instead prevents angiomotin from binding and thus inhibits Hippo pathway kinase activation. Cancer-causing mutations clustered in the angiomotin-binding domain impair angiomotin-mediated Merlin activation. Our findings reveal that angiomotin and Merlin respectively interface cortical actin filaments and core kinases in Hippo signaling, and allow construction of a complete Hippo signaling pathway.

The scaffold protein GRIP1 (glutamate receptor interacting protein 1) binds to and regulates both the trafficking and membrane organization of a large number of transmembrane proteins. Mutation of GRIP1 in mice displays essentially the same phenotype of the mutations of Fras1 or Frem2, which are the animal models of the human genetic disorder Fraser syndrome. However, the molecular basis governing the interaction between GRIP1 and Fras1/Frem2 is unknown. Here, we show that interaction between Fras1 and GRIP1 requires the first two PDZ domains (PDZ1 and PDZ2) to be connected in tandem, as the folding of PDZ1 strictly depends on the covalent attachment of PDZ2. The crystal structure of GRIP1 PDZ12 in complex with the Fras1 C-terminal peptide reveals that the PDZ12 tandem forms a supramodule in which only the peptide-binding groove of PDZ1 is bound with the Fras1 peptide. The GRIP1 PDZ12/Fras1 peptide complex not only provides a mechanistic explanation of the link between GRIP1 and the Fraser syndrome but may also serve as a foundation for searching for potential mutations in GRIP1 that could lead to the Fraser syndrome. (C) 2007 Elsevier Ltd. All rights reserved.

The scaffold protein GRIP1 (glutamate receptor interacting protein 1) binds to and regulates both the trafficking and membrane organization of a large number of transmembrane proteins. Mutation of GRIP1 in mice displays essentially the same phenotype of the mutations of Fras1 or Frem2, which are the animal models of the human genetic disorder Fraser syndrome. However, the molecular basis governing the interaction between GRIP1 and Fras1/Frem2 is unknown. Here, we show that interaction between Fras1 and GRIP1 requires the first two PDZ domains (PDZ1 and PDZ2) to be connected in tandem, as the folding of PDZ1 strictly depends on the covalent attachment of PDZ2. The crystal structure of GRIP1 PDZ12 in complex with the Fras1 C-terminal peptide reveals that the PDZ12 tandem forms a supramodule in which only the peptide-binding groove of PDZ1 is bound with the Fras1 peptide. The GRIP1 PDZ12/Fras1 peptide complex not only provides a mechanistic explanation of the link between GRIP1 and the Fraser syndrome but may also serve as a foundation for searching for potential mutations in GRIP1 that could lead to the Fraser syndrome. (C) 2007 Elsevier Ltd. All rights reserved.

The scaffold protein GRIP1 (glutamate receptor interacting protein 1) binds to and regulates both the trafficking and membrane organization of a large number of transmembrane proteins. Mutation of GRIP1 in mice displays essentially the same phenotype of the mutations of Fras1 or Frem2, which are the animal models of the human genetic disorder Fraser syndrome. However, the molecular basis governing the interaction between GRIP1 and Fras1/Frem2 is unknown. Here, we show that interaction between Fras1 and GRIP1 requires the first two PDZ domains (PDZ1 and PDZ2) to be connected in tandem, as the folding of PDZ1 strictly depends on the covalent attachment of PDZ2. The crystal structure of GRIP1 PDZ12 in complex with the Fras1 C-terminal peptide reveals that the PDZ12 tandem forms a supramodule in which only the peptide-binding groove of PDZ1 is bound with the Fras1 peptide. The GRIP1 PDZ12/Fras1 peptide complex not only provides a mechanistic explanation of the link between GRIP1 and the Fraser syndrome but may also serve as a foundation for searching for potential mutations in GRIP1 that could lead to the Fraser syndrome. (C) 2007 Elsevier Ltd. All rights reserved.

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.

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.

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.

Members of the X11/Mint family of multidomain adaptor proteins are composed of a divergent N terminus, a conserved PTB domain and a pair of C-terminal PDZ domains. Many proteins can interact with the PDZ tandem of X11 proteins, although the mechanism of such interactions is unclear. Here we show that the highly conserved C-terminal tail of X11 alpha folds back and inserts into the target-binding groove of the first PDZ domain. The binding of this tail occludes the binding of other target peptides. This autoinhibited conformation of X11 requires that the two PDZ domains and the entire C-terminal tail be covalently connected to form an integral structural unit. The autoinhibited conformation of the X11 PDZ tandem provides a mechanistic explanation for the unique target-binding properties of the protein and hints at potential regulatory mechanisms for the X11-target interactions.

Pleckstrin homology (PH) domains play diverse roles in cytoskeletal dynamics and signal transduction. Split PH domains represent a unique subclass of PH domains that have been implicated in interactions with complementary partial PH domains `hidden' in many proteins. Whether partial PH domains exist as independent structural units alone and whether two halves of a split PH domain can fold together to form an intact PH domain are not known. Here, we solved the structure of the PHN-PDZ-PHC tandem of alpha(-)syntrophin. The split PH domain of alpha-syntrophin adopts a canonical PH domain fold. The isolated partial PH domains of alpha-syntrophin, although completely unfolded, remain soluble in solution. Mixing of the two isolated domains induces de novo folding and yields a stable PH domain. Our results demonstrate that two complementary partial PH domains are capable of binding to each other to form an intact PH domain. We further showed that the PHN-PDZ-PHC tandem forms a functionally distinct supramodule, in which the split PH domain and the PDZ domain function synergistically in binding to inositol phospholipids.

Initially identified in Caenorhabditis elegans Lin-2 and Lin-7, L27 domain is a protein-protein interaction domain capable of organizing scaffold proteins into supramolecular assemblies by formation of heteromeric L27 domain complexes. L27 domain-mediated protein assemblies have been shown to play essential roles in cellular processes including asymmetric cell division, establishment and maintenance of cell polarity, and clustering of receptors and ion channels. The structural basis of L27 domain heteromeric complex assembly is controversial. We determined the high-resolution solution structure of the prototype L27 domain complex formed by mLin-2 and mLin-7 as well as the solution structure of the L27 domain complex formed by Patj and Pals1. The structures suggest that a tetrameric structure composed of two units of heterodimer is a general assembly mode for cognate pairs of L27 domains. Structural analysis of the L27 domain complex structures further showed that the central four-helix bundles mediating tetramer assembly are highly distinct between different pairs of L27 domain complexes. Biochemical studies revealed that the C-terminal a-helix responsible for the formation of the central helix bundle is a critical specificity determinant for each L27 domain in choosing its binding partner. Our results provide a unified picture for L27 domain-mediated protein-protein interactions.

Members of the X11/Mint family of multidomain adaptor proteins are composed of a divergent N terminus, a conserved PTB domain and a pair of C-terminal PDZ domains. Many proteins can interact with the PDZ tandem of X11 proteins, although the mechanism of such interactions is unclear. Here we show that the highly conserved C-terminal tail of X11 alpha folds back and inserts into the target-binding groove of the first PDZ domain. The binding of this tail occludes the binding of other target peptides. This autoinhibited conformation of X11 requires that the two PDZ domains and the entire C-terminal tail be covalently connected to form an integral structural unit. The autoinhibited conformation of the X11 PDZ tandem provides a mechanistic explanation for the unique target-binding properties of the protein and hints at potential regulatory mechanisms for the X11-target interactions.

Initially identified in Caenorhabditis elegans Lin-2 and Lin-7, L27 domain is a protein-protein interaction domain capable of organizing scaffold proteins into supramolecular assemblies by formation of heteromeric L27 domain complexes. L27 domain-mediated protein assemblies have been shown to play essential roles in cellular processes including asymmetric cell division, establishment and maintenance of cell polarity, and clustering of receptors and ion channels. The structural basis of L27 domain heteromeric complex assembly is controversial. We determined the high-resolution solution structure of the prototype L27 domain complex formed by mLin-2 and mLin-7 as well as the solution structure of the L27 domain complex formed by Patj and Pals1. The structures suggest that a tetrameric structure composed of two units of heterodimer is a general assembly mode for cognate pairs of L27 domains. Structural analysis of the L27 domain complex structures further showed that the central four-helix bundles mediating tetramer assembly are highly distinct between different pairs of L27 domain complexes. Biochemical studies revealed that the C-terminal a-helix responsible for the formation of the central helix bundle is a critical specificity determinant for each L27 domain in choosing its binding partner. Our results provide a unified picture for L27 domain-mediated protein-protein interactions.

Pleckstrin homology (PH) domains play diverse roles in cytoskeletal dynamics and signal transduction. Split PH domains represent a unique subclass of PH domains that have been implicated in interactions with complementary partial PH domains `hidden' in many proteins. Whether partial PH domains exist as independent structural units alone and whether two halves of a split PH domain can fold together to form an intact PH domain are not known. Here, we solved the structure of the PHN-PDZ-PHC tandem of alpha(-)syntrophin. The split PH domain of alpha-syntrophin adopts a canonical PH domain fold. The isolated partial PH domains of alpha-syntrophin, although completely unfolded, remain soluble in solution. Mixing of the two isolated domains induces de novo folding and yields a stable PH domain. Our results demonstrate that two complementary partial PH domains are capable of binding to each other to form an intact PH domain. We further showed that the PHN-PDZ-PHC tandem forms a functionally distinct supramodule, in which the split PH domain and the PDZ domain function synergistically in binding to inositol phospholipids.

Initially identified in Caenorhabditis elegans Lin-2 and Lin-7, L27 domain is a protein-protein interaction domain capable of organizing scaffold proteins into supramolecular assemblies by formation of heteromeric L27 domain complexes. L27 domain-mediated protein assemblies have been shown to play essential roles in cellular processes including asymmetric cell division, establishment and maintenance of cell polarity, and clustering of receptors and ion channels. The structural basis of L27 domain heteromeric complex assembly is controversial. We determined the high-resolution solution structure of the prototype L27 domain complex formed by mLin-2 and mLin-7 as well as the solution structure of the L27 domain complex formed by Patj and Pals1. The structures suggest that a tetrameric structure composed of two units of heterodimer is a general assembly mode for cognate pairs of L27 domains. Structural analysis of the L27 domain complex structures further showed that the central four-helix bundles mediating tetramer assembly are highly distinct between different pairs of L27 domain complexes. Biochemical studies revealed that the C-terminal a-helix responsible for the formation of the central helix bundle is a critical specificity determinant for each L27 domain in choosing its binding partner. Our results provide a unified picture for L27 domain-mediated protein-protein interactions.

Pleckstrin homology (PH) domains play diverse roles in cytoskeletal dynamics and signal transduction. Split PH domains represent a unique subclass of PH domains that have been implicated in interactions with complementary partial PH domains `hidden' in many proteins. Whether partial PH domains exist as independent structural units alone and whether two halves of a split PH domain can fold together to form an intact PH domain are not known. Here, we solved the structure of the PHN-PDZ-PHC tandem of alpha(-)syntrophin. The split PH domain of alpha-syntrophin adopts a canonical PH domain fold. The isolated partial PH domains of alpha-syntrophin, although completely unfolded, remain soluble in solution. Mixing of the two isolated domains induces de novo folding and yields a stable PH domain. Our results demonstrate that two complementary partial PH domains are capable of binding to each other to form an intact PH domain. We further showed that the PHN-PDZ-PHC tandem forms a functionally distinct supramodule, in which the split PH domain and the PDZ domain function synergistically in binding to inositol phospholipids.

Members of the X11/Mint family of multidomain adaptor proteins are composed of a divergent N terminus, a conserved PTB domain and a pair of C-terminal PDZ domains. Many proteins can interact with the PDZ tandem of X11 proteins, although the mechanism of such interactions is unclear. Here we show that the highly conserved C-terminal tail of X11 alpha folds back and inserts into the target-binding groove of the first PDZ domain. The binding of this tail occludes the binding of other target peptides. This autoinhibited conformation of X11 requires that the two PDZ domains and the entire C-terminal tail be covalently connected to form an integral structural unit. The autoinhibited conformation of the X11 PDZ tandem provides a mechanistic explanation for the unique target-binding properties of the protein and hints at potential regulatory mechanisms for the X11-target interactions.

Recent experimental evidence indicates that many scaffolding proteins localize at and make up synapses. These scaffolding proteins play important roles in both the maintenance of the architecture of synapses and the organization of neurotransmitters signaling complexes within synapses. Many scaffolding proteins are multidomain in nature, and these multimodular scaffold proteins often contain several repeat domains, which mainly mediate protein-protein interactions, arranged in tandem. The functional significances of such tandem arrangements of protein-protein interaction domains in scaffold proteins are largely unknown. In this thesis, I have presented the supramodular structure of the first two PDZ domains of PSD-95 (PDZ12). The structure of PDZ12 reveals that the two tandem PDZ domains are connected by a short and conserved inter-domain linker sequence ('KPPAE'). The PDZ 12 tandem structure further showes that the C-terminal peptide-binding groove on each of the two PDZ domains are aligned with a near parallel orientation. The supramodular arrangement of PSD-95 PDZ12 thereby affords the synergistic binding of the scaffold protein to multimerized carboxyl peptides. Therefore, my data provide a mechanistic explanation for PSD-95 mediated clustering of receptors and ion-channels. Indeed, abolition of the inter-domain orientation by insertion of nine amino acids ('GSSGSSGSS') into the linker region of PDZ1 and PDZ2 lowered the binding affinity of PDZ12 to its multimeric targets. I further went to demonstrate this insertion could decrease the clustering of potassium channel Kv4.1 in heterologous cells. The second part of this thesis addresses the supramodular nature of the tandem PDZ domains found in another neuronal scaffold protein, X11α. The three dimensional structures of the isolated PDZ domains as well as the tandem PDZ domains of X11α were determined. The structure shows that the first PDZ domain of X11α (PDZ1) binds to a carboxy1 peptide by recognizing only the

To look for regulators of the mitogen-activated protein kinase ( MAPK) kinase 6 (MKK6), a yeast two-hybrid screen was initiated using MKK6 as bait. p150(Glued) dynactin, a key component of the cytoplasmic dynein-dynactin motor complex, was found to specifically interact with MKK6 and its close homologue MKK3. Silencing of p150(Glued) expression by small interference RNA reduced the stimulus-induced phosphorylation of MKK3/6 and p38 MAPKs. The similar adverse effect was also seen when the cytoplasmic dynein motor was disrupted by other means. Like p150(Glued), MKK3/6 directly associate with microtubules. Disruption of microtubules prior to cell stimulation specifically inhibits the stimulus-induced phosphorylation of both MKK3/6 and p38 MAPKs. Our unexpected findings reveal a specific requirement for p150(Glued)/dynein/functional microtubules in activation of MKK3/6 and p38 MAPKs in vivo.

To look for regulators of the mitogen-activated protein kinase ( MAPK) kinase 6 (MKK6), a yeast two-hybrid screen was initiated using MKK6 as bait. p150(Glued) dynactin, a key component of the cytoplasmic dynein-dynactin motor complex, was found to specifically interact with MKK6 and its close homologue MKK3. Silencing of p150(Glued) expression by small interference RNA reduced the stimulus-induced phosphorylation of MKK3/6 and p38 MAPKs. The similar adverse effect was also seen when the cytoplasmic dynein motor was disrupted by other means. Like p150(Glued), MKK3/6 directly associate with microtubules. Disruption of microtubules prior to cell stimulation specifically inhibits the stimulus-induced phosphorylation of both MKK3/6 and p38 MAPKs. Our unexpected findings reveal a specific requirement for p150(Glued)/dynein/functional microtubules in activation of MKK3/6 and p38 MAPKs in vivo.

L27 domain, initially identified in the Caenorhabditis elegans Lin-2 and Lin-7 proteins, is a protein interaction module that exists in a large family of scaffold proteins. The domain can function as an organization center of large protein assemblies required for establishment and maintenance of cell polarity. We have solved the high-resolution NMR structure of a tetrameric complex of L27 domains containing two SAP97-mLin-2 L27 domain heterodimers. Each L27 domain contains three alpha-helices. The first two helices of each domain are packed together to form a four-helical bundle in the heterodimer. The third helix of each L27 domain forms another four-helical bundle that assembles the two heterodimers into a tetramer. The structure of the complex provides a mechanistic explanation for L27 domain mediated polymerization of scaffold proteins, a process that is crucial for the assembly of supramolecular complexes in asymmetric cells.

To look for regulators of the mitogen-activated protein kinase ( MAPK) kinase 6 (MKK6), a yeast two-hybrid screen was initiated using MKK6 as bait. p150(Glued) dynactin, a key component of the cytoplasmic dynein-dynactin motor complex, was found to specifically interact with MKK6 and its close homologue MKK3. Silencing of p150(Glued) expression by small interference RNA reduced the stimulus-induced phosphorylation of MKK3/6 and p38 MAPKs. The similar adverse effect was also seen when the cytoplasmic dynein motor was disrupted by other means. Like p150(Glued), MKK3/6 directly associate with microtubules. Disruption of microtubules prior to cell stimulation specifically inhibits the stimulus-induced phosphorylation of both MKK3/6 and p38 MAPKs. Our unexpected findings reveal a specific requirement for p150(Glued)/dynein/functional microtubules in activation of MKK3/6 and p38 MAPKs in vivo.

L27 domain, initially identified in the Caenorhabditis elegans Lin-2 and Lin-7 proteins, is a protein interaction module that exists in a large family of scaffold proteins. The domain can function as an organization center of large protein assemblies required for establishment and maintenance of cell polarity. We have solved the high-resolution NMR structure of a tetrameric complex of L27 domains containing two SAP97-mLin-2 L27 domain heterodimers. Each L27 domain contains three alpha-helices. The first two helices of each domain are packed together to form a four-helical bundle in the heterodimer. The third helix of each L27 domain forms another four-helical bundle that assembles the two heterodimers into a tetramer. The structure of the complex provides a mechanistic explanation for L27 domain mediated polymerization of scaffold proteins, a process that is crucial for the assembly of supramolecular complexes in asymmetric cells.

PDZ domain proteins play critical roles in binding, clustering and subcellular targeting of membrane receptors and ion channels. PDZ domains in multi-PDZ proteins often are arranged in groups with highly conserved spacing and intervening sequences; however, the functional significance of such tandem arrangements of PDZs is unclear. We have solved the three-dimensional structure of the first two PDZ domains of postsynaptic density protein-95 (PSD-95 PDZ1 and PDZ2), which are closely linked to each other in the PSD-95 family of scaffold proteins. The two PDZs have limited freedom of rotation and their C-terminal peptide-binding grooves are aligned with each other with an orientation preference for binding to pairs of C termini extending in the same direction. Increasing the spacing between PDZ1 and PDZ2 resulted in decreased binding between PDZ12 and its dimeric targets. The same mutation impaired the functional ability of PSD-95 to cluster Kv1.4 potassium channels in heterologous cells. The data presented provide a molecular basis for preferential binding of PSD-95 to multimeric membrane proteins with appropriate C-terminal sequences. (C) 2003 Elsevier Science Ltd. All rights reserved.

PDZ domain proteins play critical roles in binding, clustering and subcellular targeting of membrane receptors and ion channels. PDZ domains in multi-PDZ proteins often are arranged in groups with highly conserved spacing and intervening sequences; however, the functional significance of such tandem arrangements of PDZs is unclear. We have solved the three-dimensional structure of the first two PDZ domains of postsynaptic density protein-95 (PSD-95 PDZ1 and PDZ2), which are closely linked to each other in the PSD-95 family of scaffold proteins. The two PDZs have limited freedom of rotation and their C-terminal peptide-binding grooves are aligned with each other with an orientation preference for binding to pairs of C termini extending in the same direction. Increasing the spacing between PDZ1 and PDZ2 resulted in decreased binding between PDZ12 and its dimeric targets. The same mutation impaired the functional ability of PSD-95 to cluster Kv1.4 potassium channels in heterologous cells. The data presented provide a molecular basis for preferential binding of PSD-95 to multimeric membrane proteins with appropriate C-terminal sequences. (C) 2003 Elsevier Science Ltd. All rights reserved.

PDZ domain proteins play critical roles in binding, clustering and subcellular targeting of membrane receptors and ion channels. PDZ domains in multi-PDZ proteins often are arranged in groups with highly conserved spacing and intervening sequences; however, the functional significance of such tandem arrangements of PDZs is unclear. We have solved the three-dimensional structure of the first two PDZ domains of postsynaptic density protein-95 (PSD-95 PDZ1 and PDZ2), which are closely linked to each other in the PSD-95 family of scaffold proteins. The two PDZs have limited freedom of rotation and their C-terminal peptide-binding grooves are aligned with each other with an orientation preference for binding to pairs of C termini extending in the same direction. Increasing the spacing between PDZ1 and PDZ2 resulted in decreased binding between PDZ12 and its dimeric targets. The same mutation impaired the functional ability of PSD-95 to cluster Kv1.4 potassium channels in heterologous cells. The data presented provide a molecular basis for preferential binding of PSD-95 to multimeric membrane proteins with appropriate C-terminal sequences. (C) 2003 Elsevier Science Ltd. All rights reserved.