Change in allosteric network affects binding affinities of PDZ domains: analysis through perturbation response scanning

The allosteric mechanism plays a key role in cellular functions of several PDZ domain proteins (PDZs) and is directly linked to pharmaceutical applications; however, it is a challenge to elaborate the nature and extent of these allosteric interactions. One solution to this problem is to explore the dynamics of PDZs, which may provide insights about how intramolecular communication occurs within a single domain. Here, we develop an advancement of perturbation response scanning (PRS) that couples elastic network models with linear response theory (LRT) to predict key residues in allosteric transitions of the two most studied PDZs (PSD-95 PDZ3 domain and hPTP1E PDZ2 domain). With PRS, we first identify the residues that give the highest mean square fluctuation response upon perturbing the binding sites. Strikingly, we observe that the residues with the highest mean square fluctuation response agree with experimentally determined residues involved in allosteric transitions. Second, we construct the allosteric pathways by linking the residues giving the same directional response upon perturbation of the binding sites. The predicted intramolecular communication pathways reveal that PSD-95 and hPTP1E have different pathways through the dynamic coupling of different residue pairs. Moreover, our analysis provides a molecular understanding of experimentally observed hidden allostery of PSD-95. We show that removing the distal third alpha helix from the binding site alters the allosteric pathway and decreases the binding affinity. Overall, these results indicate that (i) dynamics plays a key role in allosteric regulations of PDZs, (ii) the local changes in the residue interactions can lead to significant changes in the dynamics of allosteric regulations, and (iii) this might be the mechanism that each PDZ uses to tailor their binding specificities regulation.     

Distinct ligand specificity of the Tiam1 and Tiam2 PDZ domains

Guanine nucleotide exchange factor proteins of the Tiam family are activators of the Rho GTPase Rac1 and critical for cell morphology, adhesion, migration, and polarity. These proteins are modular and contain a variety of interaction domains, including a single post-synaptic density-95/discs large/zonula occludens-1 (PDZ) domain. Previous studies suggest that the specificities of the Tiam1 and Tiam2 PDZ domains are distinct. Here, we sought to conclusively define these specificities and determine their molecular origin. Using a combinatorial peptide library, we identified a consensus binding sequence for each PDZ domain. Analysis of these consensus sequences and binding assays with peptides derived from native proteins indicated that these two PDZ domains have overlapping but distinct specificities. We also identified residues in two regions (S(0) and S(-2) pockets) of the Tiam1 PDZ domain that are important determinants of ligand specificity. Site-directed mutagenesis of four nonconserved residues in these two regions along with peptide binding analyses confirmed that these residues are crucial for ligand affinity and specificity. Furthermore, double mutant cycle analysis of each region revealed energetic couplings that were dependent on the ligand being investigated. Remarkably, a Tiam1 PDZ domain quadruple mutant had the same specificity as the Tiam2 PDZ domain. Finally, analysis of Tiam family PDZ domain sequences indicated that the PDZ domains segregate into four distinct families based on the residues studied here. Collectively, our data suggest that Tiam family proteins have highly evolved PDZ domain-ligand interfaces with distinct specificities and that they have disparate PDZ domain-dependent biological functions.     

Genome-wide analysis of PDZ domain binding reveals inherent functional overlap within the PDZ interaction network

Binding selectivity and cross-reactivity within one of the largest and most abundant interaction domain families, the PDZ family, has long been enigmatic. The complete human PDZ domain complement (the PDZome) consists of 267 domains and we applied here a Bayesian selectivity model to predict hundreds of human PDZ domain interactions, using target sequences of 22,997 non-redundant proteins. Subsequent analysis of these binding scores shows that PDZs can be divided into two genome-wide clusters that coincide well with the division between canonical class 1 and 2 PDZs. Within the class 1 PDZs we observed binding overlap at unprecedented levels, mediated by two residues at positions 1 and 5 of the second α-helix of the binding pocket. Eight PDZ domains were subsequently selected for experimental binding studies and to verify the basics of our predictions. Overall, the PDZ domain class 1 cross-reactivity identified here implies that auxiliary mechanisms must be in place to overcome this inherent functional overlap and to minimize cross-selectivity within the living cell. Indeed, when we superimpose PDZ domain binding affinities with gene ontologies, network topology data and the domain position within a PDZ superfamily protein, functional overlap is minimized and PDZ domains position optimally in the binding space. We therefore propose that PDZ domain selectivity is achieved through cellular context rather than inherent binding specificity.     

Proteome scanning to predict PDZ domain interactions using support vector machines

Background  

PDZ domains mediate protein-protein interactions involved in important biological processes through the recognition of short linear motifs in their target proteins. Two recent independent studies have used protein microarray or phage display technology to detect PDZ domain interactions with peptide ligands on a large scale. Several computational predictors of PDZ domain interactions have been developed, however they are trained using only protein microarray data and focus on limited subsets of PDZ domains. An accurate predictor of genomic PDZ domain interactions would allow the proteomes of organisms to be scanned for potential binders. Such an application would require an accurate and precise predictor to avoid generating too many false positive hits given the large amount of possible interactors in a given proteome. Once validated these predictions will help to increase the coverage of current PDZ domain interaction networks and further our understanding of the roles that PDZ domains play in a variety of biological processes.     

On the accuracy of inferring energetic coupling between distant sites in protein families from evolutionary imprints: Illustrations using lattice model

It is suspected that correlated motions among a subset of spatially separated residues drive conformational dynamics not only in multidomain but also in single domain proteins. Sequence and structure‐based methods have been proposed to determine covariation between two sites on a protein. The statistical coupling analysis (SCA) that compares the changes in probability at two sites in a multiple sequence alignment (MSA) and a subset of the MSA has been used to infer the network of residues that encodes allosteric signals in protein families. The structural perturbation method (SPM), that probes the response of a local perturbation at all other sites, has been used to probe the allostery wiring diagram in biological machines and enzymes. To assess the efficacy of the SCA, we used an exactly soluble two dimensional lattice model and performed double‐mutant cycle (DMC) calculations to predict the extent of physical coupling between two sites. The predictions of the SCA and the DMC results show that only residues that are in contact in the native state are accurately identified. In addition, covariations among strongly interacting residues are most easily identified by the SCA. These conclusions are consistent with the DMC experiments on the PDZ family. Good correlation between the SCA and the DMC is only obtained by performing multiple experiments that vary the nature of amino acids at a given site. In contrast, the energetic coupling found in experiments for the PDZ domain are recovered using the SPM. We also predict, using the SPM, several residues that are coupled energetically. Proteins 2009. © 2009 Wiley‐Liss, Inc.     

Role of the PDZ domains in Escherichia coli DegP protein

PDZ domains are modular protein interaction domains that are present in metazoans and bacteria. These domains possess unique structural features that allow them to interact with the C-terminal residues of their ligands. The Escherichia coli essential periplasmic protein DegP contains two PDZ domains attached to the C-terminal end of the protease domain. In this study we examined the role of each PDZ domain in the protease and chaperone activities of this protein. Specifically, DegP mutants with either one or both PDZ domains deleted were generated and tested to determine their protease and chaperone activities, as well as their abilities to sequester unfolded substrates. We found that the PDZ domains in DegP have different roles; the PDZ1 domain is essential for protease activity and is responsible for recognizing and sequestering unfolded substrates through C-terminal tags, whereas the PDZ2 domain is mostly involved in maintaining the hexameric cage of DegP. Interestingly, neither of the PDZ domains was required for the chaperone activity of DegP. In addition, we found that the loops connecting the protease domain to PDZ1 and connecting PDZ1 to PDZ2 are also essential for the protease activity of the hexameric DegP protein. New insights into the roles of the PDZ domains in the structure and function of DegP are provided. These results imply that DegP recognizes substrate molecules targeted for degradation and substrate molecules targeted for refolding in different manners and suggest that the substrate recognition mechanisms may play a role in the protease-chaperone switch, dictating whether the substrate is degraded or refolded.     

An expanded view of the protein folding landscape of PDZ domains

Most protein domains fold in an apparently co-operative and two-state manner with only the native and denatured states significantly populated at any experimental condition. However, the protein folding energy landscape is often rugged and different transition states may be rate limiting for the folding reaction under different conditions, as seen for the PDZ protein domain family. We have here analyzed the folding kinetics of two PDZ domains and found that a previously undetected third transition state is rate limiting under conditions that stabilize the native state relative to the denatured state. In light of these results, we have re-analyzed previous folding data on PDZ domains and present a unified folding mechanism with three distinct transition states separated by two high-energy intermediates. Our data show that sequence composition tunes the relative stabilities of folding transition states within the PDZ family, while the overall mechanism is determined by topology. This model captures the kinetic folding mechanism of all PDZ domains studied to date.     

Alteration of the C-Terminal Ligand Specificity of the Erbin PDZ Domain by Allosteric Mutational Effects

Modulation of protein binding specificity is important for basic biology and for applied science. Here we explore how binding specificity is conveyed in PDZ (postsynaptic density protein-95/discs large/zonula occludens-1) domains, small interaction modules that recognize various proteins by binding to an extended C terminus. Our goal was to engineer variants of the Erbin PDZ domain with altered specificity for the most C-terminal position (position 0) where a Val is strongly preferred by the wild-type domain. We constructed a library of PDZ domains by randomizing residues in direct contact with position 0 and in a loop that is close to but does not contact position 0. We used phage display to select for PDZ variants that bind to 19 peptide ligands differing only at position 0. To verify that each obtained PDZ domain exhibited the correct binding specificity, we selected peptide ligands for each domain. Despite intensive efforts, we were only able to evolve Erbin PDZ domain variants with selectivity for the aliphatic C-terminal side chains Val, Ile and Leu. Interestingly, many PDZ domains with these three distinct specificities contained identical amino acids at positions that directly contact position 0 but differed in the loop that does not contact position 0. Computational modeling of the selected PDZ domains shows how slight conformational changes in the loop region propagate to the binding site and result in different binding specificities. Our results demonstrate that second-sphere residues could be crucial in determining protein binding specificity.     

Redox-regulated affinity of the third PDZ domain in the phosphotyrosine phosphatase PTP-BL for cysteine-containing target peptides

PDZ domains are protein-protein interaction modules that are crucial for the assembly of structural and signalling complexes. They specifically bind to short C-terminal peptides and occasionally to internal sequences that structurally resemble such peptide termini. The binding of PDZ domains is dominated by the residues at the P(0) and P(-2) position within these C-terminal targets, but other residues are also important in determining specificity. In this study, we analysed the binding specificity of the third PDZ domain of protein tyrosine phosphatase BAS-like (PTP-BL) using a C-terminal combinatorial peptide phage library. Binding of PDZ3 to C-termini is preferentially governed by two cysteine residues at the P(-1) and P(-4) position and a valine residue at the P(0) position. Interestingly, we found that this binding is lost upon addition of the reducing agent dithiothrietol, indicating that the interaction is disulfide-bridge-dependent. Site-directed mutagenesis of the single cysteine residue in PDZ3 revealed that this bridge formation does not occur intermolecularly, between peptide and PDZ3 domain, but rather is intramolecular. These data point to a preference of PTP-BL PDZ3 for cyclic C-terminal targets, which may suggest a redox state-sensing role at the cell cortex.     

Energetic determinants of internal motif recognition by PDZ domains

PDZ domains are protein-protein interaction modules that organize intracellular signaling complexes. Most PDZ domains recognize specific peptide motifs followed by a required COOH-terminus. However, several PDZ domains have been found which recognize specific internal peptide motifs. The best characterized example is the syntrophin PDZ domain which, in addition to binding peptide ligands with the consensus sequence -E-S/T-X-V-COOH, also binds the neuronal nitric oxide synthase (nNOS) PDZ domain in a manner that does not depend on its precise COOH-terminal sequence. In the structure of the syntrophin-nNOS PDZ heterodimer complex, the two PDZ domains interact in a head-to-tail fashion, with an internal sequence from the nNOS PDZ domain binding precisely at the peptide binding groove of the syntrophin PDZ domain. To understand the energetic basis of this alternative mode of PDZ recognition, we have undertaken an extensive mutagenic and biophysical analysis of the nNOS PDZ domain and its interaction with the syntrophin PDZ domain. Our data indicate that the presentation of the nNOS internal motif within the context of a rigid beta-hairpin conformation is absolutely essential to binding; amino acids crucial to the structural integrity of the hairpin are as important or more important than residues that make direct contacts. The results reveal the general rules of PDZ recognition of diverse ligand types.     

New Conformational State of NHERF1-CXCR2 Signaling Complex Captured by Crystal Lattice Trapping

NHERF1 is a PDZ adaptor protein that scaffolds the assembly of diverse signaling complexes and has been implicated in many cancers. However, little is known about the mechanism responsible for its scaffolding promiscuity or its ability to bind to multiple targets. Computational studies have indicated that PDZ promiscuity may be attributed to its conformational dynamics, but experimental evidence for this relationship remains very limited. Here we examine the conformational flexibility of the NHERF1 PDZ1 domain using crystal lattice trapping via solving PDZ1 structure of a new crystal form. The structure, together with prior PDZ1 structures of a different space group, reveals that 4 of 11 ligand-interacting residues undergo significant crystal packing-induced structural changes. Most of these residues correspond to the residues involved in allosteric transition when a peptide ligand binds. In addition, a subtle difference in ligand conformations causes the same peptide to bind in slightly different modes in different crystal forms. These findings indicate that substantial structural flexibility is present in the PDZ1 peptide-binding pocket, and the structural substate trapped in the present crystal form can be utilized to represent the conformational space accessible to the protein. Such knowledge will be critical for drug design against the NHERF1 PDZ1 domain, highlighting the continued need for experimentally determined PDZ1-ligand complexes.     

The structural flexibility of the shank1 PDZ domain is important for its binding to different ligands

The PDZ domain of the shank protein interacts with numerous cell membrane receptors and cytosolic proteins via the loosely defined binding motif X-(Ser/Thr)-X-Φ-COOH (Φ represents hydrophobic residues) at the carboxyl terminus of its target protein. This enables shank to serve as a membrane-associated scaffold for the assembly of signaling complexes. As the list of proteins that bind to the shank PDZ domain grows, it is not immediately clear what structural element(s) mediate this domain’s target specificity or the plasticity required to bind its different targets. Here, we have determined the crystal structure of the shank1 PDZ in complex with the βPIX C-terminal pentapeptide (642–646, DETNL) at 2.3 Å resolution and modeled shank1 PDZ binding to selected pentapeptide ligands. The resulting structures revealed a large hydrophobic pocket within the PDZ domain that can accommodate a variety of ligand residues at the P(0) position. A H-bond between His735 and Ser/Thr at the P(−2) position is invariant throughout the model structures. In addition, we identified multiple PDZ domain residues that are able to form H-bonds and salt bridges with an incoming target protein. Overall, our study provides a new level of understanding of the specificity and structural plasticity of the shank PDZ domain.     

Detection of a hidden folding intermediate of the third domain of PDZ

The folding pathway of the third domain of PDZ from the synaptic protein PSD-95 was characterized using kinetic and equilibrium methods by monitoring the fluorescence signal from a Trp residue that is incorporated at a near-surface position. Kinetic folding of this domain showed multiple exponential phases, whereas unfolding showed a single exponential phase. The slow kinetic phases were attributed to isomerization of proline residues, since there are five proline residues in this domain. We found that the logarithms of the rate constants for the fast phase of folding and unfolding are linearly dependent on the concentrations of denaturant. The unfolding free energy derived from these rate constants at zero denaturant was close to the value measured using the equilibrium method, suggesting the absence of detectable sub-millisecond folding intermediates. However, native-state hydrogen exchange experiments detected a partially unfolded intermediate under native conditions. It was further confirmed by a protein engineering study. These data suggest that a hidden intermediate exists after the rate-limiting step in the folding of the third domain of PDZ.     

Solution structure of the RIM1alpha PDZ domain in complex with an ELKS1b C-terminal peptide

PDZ domains are widespread protein modules that commonly recognize C-terminal sequences of target proteins and help to organize macromolecular signaling complexes. These sequences usually bind in an extended conformation to relatively shallow grooves formed between a beta-strand and an alpha-helix in the corresponding PDZ domains. Because of this binding mode, many PDZ domains recognize primarily the C-terminal and the antepenultimate side-chains of the target protein, which commonly conform to motifs that have been categorized into different classes. However, an increasing number of PDZ domains have been found to exhibit unusual specificities. These include the PDZ domain of RIMs, which are large multidomain proteins that regulate neurotransmitter release and help to organize presynaptic active zones. The RIM PDZ domain binds to the C-terminal sequence of ELKS with a unique specificity that involves each of the four ELKS C-terminal residues. To elucidate the structural basis for this specificity, we have determined the 3D structure in solution of an RIM/ELKS C-terminal peptide complex using NMR spectroscopy. The structure shows that the RIM PDZ domain contains an unusually deep and narrow peptide-binding groove with an exquisite shape complementarity to the four ELKS C-terminal residues in their bound conformation. This groove is formed, in part, by a set of side-chains that is conserved selectively in RIM PDZ domains and that hence determines, at least in part, their unique specificity.     

β-Propeller repeats and a PDZ domain in the tricorn protease: predicted self-compartmentalisation and C-terminal polypeptide-binding strategies of substrate selection

Prokaryotic proteases demonstrate a variety of substrate-selection strategies that prevent uncontrolled protein degradation. Proteasomes and ClpXP-like proteases form oligomeric structures that exclude large substrates from central solvated chambers containing their active sites. Monomeric prolyl oligopeptidases have been shown to contain beta-propeller structures that similarly reduce access to their catalytic residues. By contrast, Tsp-like enzymes contain PDZ domains that are thought to specifically target C-terminal polypeptides. We have investigated the sequence of Thermoplasma acidophilum tricorn protease using recently-developed database search methods. The tricorn protease is known to associate into a 20 hexamer capsid enclosing an extremely large cavity that is 37 nm in diameter. It is unknown, however, how this enzyme selects its small oligopeptide substrates. Our results demonstrate the presence in tricorn protease of a PDZ domain and two predicted six-bladed beta-propeller domains. We suggest that the PDZ domain is involved in targeting non-polar C-terminal peptides, similar to those generated by the T. acidophilum proteasome, whereas the beta-propeller domains serve to exclude large substrates from the tricorn protease active site in a similar manner to that previously indicated for prolyl oligopeptidase.     

Distinct binding specificity of the multiple PDZ domains of INADL, a human protein with homology to INAD from Drosophila melanogaster

PDZ domains are protein-protein interaction modules that typically bind to short peptide sequences at the carboxyl terminus of target proteins. Proteins containing multiple PDZ domains often bind to different trans-membrane and intracellular proteins, playing a central role as organizers of multimeric complexes. To characterize the rules underlying the binding specificity of different PDZ domains, we have assembled a novel repertoire of random peptides that are displayed at high density at the carboxyl terminus of the capsid D protein of bacteriophage lambda. We have exploited this combinatorial library to determine the peptide binding preference of the seven PDZ domains of human INADL, a multi-PDZ protein that is homologous to the INAD protein of Drosophila melanogaster. This approach has permitted the determination of the consensus ligand for each PDZ domain and the assignment to class I, class II, and to a new specificity class, class IV, characterized by the presence of an acidic residue at the carboxyl-terminal position. Homology modeling and site-directed mutagenesis experiments confirmed the involvement of specific residues at contact positions in determining the domain binding preference. However, these experiments failed to reveal simple rules that would permit the association of the chemical characteristics of any given residue in the peptide binding pocket to the preference for specific amino acid sequences in the ligand peptide. Rather, they suggested that to infer the binding preference of any PDZ domain, it is necessary to simultaneously take into account all contact positions by using computational procedures. For this purpose we extended the SPOT algorithm, originally developed for SH3 domains, to evaluate the probability that any peptide would bind to any given PDZ domain.     

Binding of PTEN to specific PDZ domains contributes to PTEN protein stability and phosphorylation by microtubule-associated serine/threonine kinases

The tumor suppressor phosphatase PTEN is a key regulator of cell growth and apoptosis that interacts with PDZ domains from regulatory proteins, including MAGI-1/2/3, hDlg, and MAST205. Here we identified novel PTEN-binding PDZ domains within the MAST205-related proteins, syntrophin-associated serine/threonine kinase and MAST3, characterized the regions of PTEN involved in its interaction with distinctive PDZ domains, and analyzed the functional consequences on PTEN of PDZ domain binding. Using a panel of PTEN mutations, as well as PTEN chimeras containing distinct domains of the related protein TPTE, we found that the PTP and C2 domains of PTEN do not affect PDZ domain binding and that the C-terminal tail of PTEN (residues 350-403) provides selectivity to recognize specific PDZ domains from MAGI-2, hDlg, and MAST205. Binding of PTEN to the PDZ-2 domain from MAGI-2 increased PTEN protein stability. Furthermore, binding of PTEN to the PDZ domains from microtubule-associated serine/threonine kinases facilitated PTEN phosphorylation at its C terminus by these kinases. Our results suggest an important role for the C-terminal region of PTEN in the selective association with scaffolding and/or regulatory molecules and provide evidence that PDZ domain binding stabilizes PTEN and targets this tumor suppressor for phosphorylation by microtubule-associated serine/threonine kinases.     

Conserved tertiary couplings stabilize elements in the PDZ fold,leading to characteristic patterns of domain conformational flexibility

Single‐domain allostery has been postulated to occur through intramolecular pathways of signaling within a protein structure. We had previously investigated these pathways by introducing a local thermal perturbation and analyzed the anisotropic propagation of structural changes throughout the protein. Here, we develop an improved approach, the Rotamerically Induced Perturbation (RIP), that identifies strong couplings between residues by analyzing the pathways of heat‐flow resulting from thermal excitation of rotameric rotations at individual residues. To explore the nature of these couplings, we calculate the complete coupling maps of 5 different PDZ domains. Although the PDZ domain is a well conserved structural fold that serves as a scaffold in many protein–protein complexes, different PDZ domains display unique patterns of conformational flexibility in response to ligand binding: some show a significant shift in a set of α‐helices, while others do not. Analysis of the coupling maps suggests a simple relationship between the computed couplings and observed conformational flexibility. In domains where the α‐helices are rigid, we find couplings of the α‐helices to the body of the protein, whereas in domains having ligand‐responsive α‐helices, no couplings are found. This leads to a model where the α‐helices are intrinsically dynamic but can be damped if sidechains interact at key tertiary contacts. These tertiary contacts correlate to high covariation contacts as identified by the statistical coupling analysis method. As these dynamic modules are exploited by various allosteric mechanisms, these tertiary contacts have been conserved by evolution.     

Formation of a native-like beta-hairpin finger structure of a peptide from the extended PDZ domain of neuronal nitric oxide synthase in aqueous solution.

Neuronal nitric oxide synthase (nNOS) is targeted to the cell membrane via interactions of its extended PDZ domain with PDZ domains of membrane-associated proteins including PSD-95 and alpha1-syntrophin. The formation of heterodimers between the nNOS PDZ domain and the PDZ domains of nNOS-binding proteins requires a stretch of continuous amino-acid residues C-terminal to the canonical nNOS PDZ domain. In this work, we show that a 27-residue peptide comprising the C-terminal extension of the extended nNOS PDZ domain is capable of binding to PSD-95. The structure of the 27-residue peptide in aqueous solution was determined using multidimensional NMR-spectroscopic techniques. The free peptide adopts a native-like beta-hairpin finger structure in aqueous solution. The results indicate that the C-terminal extension peptide of the nNOS PDZ domain may represent a relatively independent structural unit in the mediation of the interaction between nNOS and PDZ domain-containing proteins including PSD-95 and alpha1-syntrophin.     

Structural Insights into Neutrophilic Migration Revealed by the Crystal Structure of the Chemokine Receptor CXCR2 in Complex with the First PDZ Domain of NHERF1

Neutrophil plays an essential role in host defense against infection, but uncontrolled neutrophilic infiltration can cause inflammation and severe epithelial damage. We recently showed that CXCR2 formed a signaling complex with NHERF1 and PLC-2, and that the formation of this complex was required for intracellular calcium mobilization and neutrophilic transepithelial migration. To uncover the structural basis of the complex formation, we report here the crystal structure of the NHERF1 PDZ1 domain in complex with the C-terminal sequence of CXCR2 at 1.16 Å resolution. The structure reveals that the CXCR2 peptide binds to PDZ1 in an extended conformation with the last four residues making specific side chain interactions. Remarkably, comparison of the structure to previously studied PDZ1 domains has allowed the identification of PDZ1 ligand-specific interactions and the mechanisms that govern PDZ1 target selection diversities. In addition, we show that CXCR2 can bind both NHERF1 PDZ1 and PDZ2 in pulldown experiments, consistent with the observation that the peptide binding pockets of these two PDZ domains are highly structurally conserved. The results of this study therefore provide structural basis for the CXCR2-mediated neutrophilic migration and could have important clinical applications in the prevention and treatment of numerous neutrophil-dependent inflammatory disorders.