Targeted tandem affinity purification of PSD‐95 recovers core postsynaptic complexes and schizophrenia susceptibility proteins

Abstract

The molecular complexity of mammalian proteomes demands new methods for mapping the organization of multiprotein complexes. Here, we combine mouse genetics and proteomics to characterize synapse protein complexes and interaction networks. New tandem affinity purification (TAP) tags were fused to the carboxyl terminus of PSD‐95 using gene targeting in mice. Homozygous mice showed no detectable abnormalities in PSD‐95 expression, subcellular localization or synaptic electrophysiological function. Analysis of multiprotein complexes purified under native conditions by mass spectrometry defined known and new interactors: 118 proteins comprising crucial functional components of synapses, including glutamate receptors, K+ channels, scaffolding and signaling proteins, were recovered. Network clustering of protein interactions generated five connected clusters, with two clusters containing all the major ionotropic glutamate receptors and one cluster with voltage‐dependent K+ channels. Annotation of clusters with human disease associations revealed that multiple disorders map to the network, with a significant correlation of schizophrenia within the glutamate receptor clusters. This targeted TAP tagging strategy is generally applicable to mammalian proteomics and systems biology approaches to disease. ### Synopsis Systems biology has the potential to explain physiological processes as emergent properties of sets of genes and proteins. Beyond simple cellular systems, the challenge of delivering systems biology into the intact and freely behaving animal will require new methods. Currently, the most widely used approach is immunoprecipitation of the target protein and its associate binding proteins. This method suffers from the drawbacks of single step purification strategies that include a high level of non‐specific background proteins amongst other limitations. To overcome these limitations we demonstrate that the tandem affinity purification (TAP) technology originally developed in yeast ([Rigaut et al , 1999][1]), when combined with gene targeting, can be used to efficiently isolate highly specific complexes from mouse. The ‘targeted TAP tagging’ strategy combines the two major advantages of each system. The first advantage is that the insertion of two tags into the protein of interest allows two consecutive purification steps that facilitate the recovery of protein complexes with high confidence and decreases the recovery of non‐specific proteins or weak interactors. The second advantage, conferred by targeting the endogenous gene, is that the tagged protein is expressed under its natural regulatory mechanisms. We have designed an endogenous TAP targeting strategy to isolate complexes from mouse brain excitatory synapses. The brain is the most complex organ from a cellular and molecular perspective and thus an ideal model to explore the TAP method. Post Synaptic Density 95 (PSD‐95/Dlg4) is an adaptor protein comprised of PDZ, SH3 and GK domains and is expressed in the postsynaptic terminal of excitatory synapses where it organizes signaling from neurotransmitter receptors to downstream pathways ([Kornau et al , 1995][2]; [Hunt et al , 1996][3]; [Tu et al , 1999][4]; [Husi et al , 2000][5]; [Nehring et al , 2000][6]; [Dosemeci et al , 2007][7]; [Carlisle et al , 2008][8]). Mice carrying a knockout mutation in PSD‐95 show it is essential for synaptic plasticity and a range of important behaviours ([Migaud et al , 1998][9]; [El‐Husseini et al , 2000][10]; [Beique et al , 2006][11]). Here, a new TAP tag was fused to the carboxyl terminus of PSD‐95 using gene targeting in mice. Homozygous mice showed no detectable abnormalities in PSD‐95 expression, subcellular localization or synaptic electrophysiological function ([Figure 2][12]). As a result of four independent tandem purifications and mass spectrometry analysis, we were able to define PSD‐95 core complexes with high sensitivity and reproducibility. The four purifications show an average of 125±19 proteins, having 118 proteins (94%) common in at least three of four replicates. This reproducibility rate is among the highest rate reported for systematic protein complex isolation. To further validate this interaction data we compared it to information from public datasets. Of the 118 proteins, 22% were proteins that directly bind PSD‐95 and 18% were proteins not previously found in other PSD‐95 analysis. All together, these data show robust reproducibility and sensitivity of this method for purifying synaptic complexes. These PSD‐95 core complexes comprise key functional components of synapses including the glutamate neurotransmitter receptors, K+ channels, scaffolding and signaling proteins. These complexes contain ionotropic glutamate receptors of the NMDA, AMPA and kainate subtypes as well as major K+ channels that together are the major postsynaptic constituents responsible for synaptic transmission and shaping the postsynaptic electrophysiological response to presynaptic input ([Watanabe et al , 2002][13]; [Chen et al , 2006][14]; [Kim et al , 2007][15]). We believe that this is the first method that has allowed the robust copurification of these proteins. To explore functional organization using network models, we manually curated interactions ([Pocklington et al , 2006][16]) and the UniHi database ([http://www.mdc‐berlin.de/unihi][17]) to identify 119 interactions between 50 proteins (excluding self‐interactions) of the PSD‐95 core complexes. Network clustering of the interacting proteins showed 40 out of the 50 proteins formed a large connected component (major connected component, MCC) and a modular structure that was segregated into 5 clusters referred to as cluster a (Cla) to cluster e (Cle) ([Figure 5A][18]). In addition to the 5 MCC clusters, 2 further disconnected clusters (‘Clf’ and ‘Clg’)...