The formation of neuronal synapses and the dynamic regulation of their efficacy depend on the assembly of the postsynaptic neurotransmitter receptor apparatus. Receptor recruitment to inhibitory GABAergic and glycinergic synapses is controlled by the scaffold protein gephyrin and the adaptor protein collybistin. We derived new insights into the structure of collybistin and used these to design biochemical, cell biological, and genetic analyses of collybistin function. Our data define a collybistin-based protein interaction network that controls the gephyrin content of inhibitory postsynapses. Within this network, collybistin can adopt open/ active and closed/inactive conformations to act as a switchable adaptor that links gephyrin to plasma membrane phosphoinositides. This function of collybistin is regulated by binding of the adhesion protein neuroligin-2, which stabilizes the open/active conformation of collybistin at the postsynaptic plasma membrane by competing with an intramolecular interaction in collybistin that favors the closed/inactive conformation. By linking trans-synaptic neuroligin-dependent adhesion and phosphoinositide signaling with gephyrin recruitment, the collybistin-based regulatory switch mechanism represents an integrating regulatory node in the formation and function of inhibitory postsynapses.

The Drosophila gene mushroom bodies tiny (mbt)encodes a putative p21-activated kinase (PAK), a family of proteins that has been implicated in a multitude of cellular processes including regulation of the cytoskeleton, cell polarisation, control of MAPK signalling cascades and apoptosis. The mutant phenotype of mbt is characterised by fewer neurones in the brain and the eye, indicating a role of the protein in cell proliferation, differentiation or survival. We show that mutations inmbt interfere with photoreceptor cell morphogenesis. Mbt specifically localises at adherens junctions of the developing photoreceptor cells. A structure-function analysis of the Mbt protein in vitro and in vivo revealed that the Mbt kinase domain and the GTPase binding domain, which specifically interacts with GTP-loaded Cdc42, are important for Mbt function. Besides regulation of kinase activity, another important function of Cdc42 is to recruit Mbt to adherens junctions. We propose a role for Mbt as a downstream effector of Cdc42 in photoreceptor cell morphogenesis.

The p21 activated kinase (Pak) family of protein kinases are involved in many cellular functions like re-organisation of the cytoskeleton, transcriptional control, cell division, and survival. These pleiotropic actions are reflected in a plethora of known interacting proteins and phosphorylation substrates. Yet, the integration of a single Pak protein into signalling pathways controlling a particular developmental process are less well studied. For two of the three known Pak proteins in Drosophila melanogaster, D-Pak and Mbt, distinct functions during eye development have been established. In this study we undertook a genetic approach to identify proteins acting in the Mbt signalling pathway during photoreceptor cell morphogenesis. The genetic screen identified the actin depolymerisation factor Twinstar/Cofilin as one target of Mbt signalling. Twinstar/Cofilin becomes phosphorylated upon activation of Mbt. However, biochemical and genetic experiments question the role of the LIM domain protein kinase (Limk) as a major link between Mbt and Twinstar/Cofilin as it has been suggested for other PAK proteins. Constitutive activation of Mbt not only disturbs the actin cytoskeleton but also affects adherens junction organisation indicating a requirement of the protein in cell adhesion dependent processes during photoreceptor cell differentiation.

The regulatory protein collybistin (CB) recruits the receptorscaffolding protein gephyrin to mammalian inhibitory glycinergic and GABAergic postsynaptic membranes in nerve cells. CB is tethered to the membrane via phosphoinositides. We developed an in vitro assay based on solid-supported 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine membranes doped with different phosphoinositides on silicon/silicon dioxide substrates to quantify the binding of various CB2 constructs using reflectometric interference spectroscopy. Based on adsorption isotherms, we obtained dissociation constants and binding capacities of the membranes. Our results show that full-length CB2 harboring the N-terminal Src homology 3 (SH3) domain (CB2 SH3؉ ) adopts a closed and autoinhibited conformation that largely prevents membrane binding. This autoinhibition is relieved upon introduction of the W24A/E262A mutation, which conformationally "opens" CB2 SH3؉ and allows the pleckstrin homology domain to properly bind lipids depending on the phosphoinositide species with a preference for phosphatidylinositol 3-monophosphate and phosphatidylinositol 4-monophosphate. This type of membrane tethering under the control of the release of the SH3 domain of CB is essential for regulating gephyrin clustering.The function of neuronal synapses and the dynamic regulation of their efficacy depend on the assembly of the postsynaptic neurotransmitter receptor apparatus. The main scaffolding protein of inhibitory glycinergic and GABAergic postsynapses in mammals is gephyrin (1, 2), whose recruitment to the postsynaptic membrane is controlled by the adaptor protein collybistin (CB) 2 (3). Loss of CB results in a strong reduction of gephyrin and GABA A receptor clusters in several regions of the forebrain, which demonstrates the essential role of CB in the assembly and maintenance of GABAergic postsynaptic structures (4). CB belongs to the Dbl family of guanine nucleotide exchange factors. In mouse, four differently spliced CB mRNAs are present (CB1 SH3ϩ , CB2 SH3Ϫ , CB2 SH3ϩ , and CB3 SH3ϩ ). All four mRNAs encode a Dbl homology (DH) and a pleckstrin homology (PH) domain. The three major variants (CB1 SH3ϩ , CB2 SH3ϩ , and CB3 SH3ϩ ) encode CBs with an additional N-terminal Src homology 3 (SH3) domain but differ in their C termini. A fourth variant (CB2 SH3Ϫ ) encodes a CB2 isoform lacking the SH3 domain ( Fig. 1) but is very rare (5) as its protein product is not detectable in mouse brain (6). Importantly, the PH domain of the different CBs is required for proper function as indicated by the fact that its deletion abolishes the plasma membrane targeting of gephyrin-CB complexes when cotransfected in HEK293 cells and causes a loss of dendritic gephyrin clusters in dissociated rat cortical neurons (5).In vitro binding studies utilizing a variety of inositol headgroups, soluble phosphoinositide analogs, and liposomes containing phosphoinositides showed that PH domains bind phosphoinositides with a broad range of selectivity and affinity (7-10). An early membrane acti...

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