, 2010, Eden et al., 2002, Kobayashi et al., 1998, Schenck et al., 2003 and Steffen et al., 2004). Rearrangements of the Regorafenib clinical trial actin cytoskeleton strongly influence the formation, retraction, motility, stability, and shape of the dendritic spines (Tada and Sheng, 2006), and genetic ablation of WRC components affects spine morphology and excitability (Grove et al., 2004, Kim et al., 2006, Soderling et al., 2007 and Wiens et al., 2005). However, the interplay of this process with other events regulating

spine function, such as local translation, is still unknown. Here, we demonstrate that active Rac1 changes the equilibrium between two distinct CYFIP1 complexes, activating the translation of mRNAs important Fulvestrant for synaptic structure and function, such as Arc/Arg3.1 mRNA. This switch occurs through a conformational change in CYFIP1, detectable by Förster resonance energy transfer (FRET). Knockdown of Cyfip1 or mutations in the regions interacting with eIF4E or WRC produce dendritic spine defects resembling those found in FXS and other synaptopathies.

These findings shed light on the molecular mechanisms that tune the balance between translational control and actin remodeling at synapses. The identification of interaction partners of CYFIP1 suggests that neurological disorders characterized by spine dysmorphogenesis might be due to perturbations in the balance between these two CYFIP1 interconnected pathways. To dissect the CYFIP1 function and its possible crosstalk with the FMRP-eIF4E translational complex and the actin-regulatory complex WRC, we investigated the structural organization of the two CYFIP1 complexes. According

to the crystal structure of the WRC that includes CYFIP1 (Chen et al., 2010), NCKAP1 interacts with CYFIP1 over a large surface (Figure 1A, upper panel); the lysine critical for the binding to eIF4E L-NAME HCl (Lys743) (Napoli et al., 2008) is covered by NCKAP1 and therefore is not accessible to solvent when CYFIP1 is in the WRC (Figure 1A, bottom panels, Table S1). These structural data indicate that the same CYFIP1 molecule cannot simultaneously interact with the WRC and eIF4E. Synapses are severely affected in FXS and other neurological disorders (Fiala et al., 2002, Penzes et al., 2011 and Valnegri et al., 2012). Electron microscopy (EM) and biochemical studies revealed that CYFIP1, at synapses, is enriched in postsynaptic compartments (Figure S1 available online). In mouse cortical synaptoneurosomes, CYFIP1 coimmunoprecipitates with FMRP, eIF4E, NCKAP1, and WAVE1 (Figure 1B). Furthermore, immunoprecipitation of NCKAP1 revealed the presence of CYFIP1 but not eIF4E, whereas immunoprecipitation of the eIF4E complex detected CYFIP1 but not NCKAP1 (Figure 1C). We conclude that CYFIP1 engages in two distinct complexes. Synaptic activity leads to an increase of protein synthesis as well as actin remodeling (Bramham, 2008).