Wilms' tumor 1-associating protein (WTAP) is a putative splicing regulator that is thought to be required for cell cycle progression through the stabilization of cyclin A2 mRNA and mammalian early embryo development. To further understand how WTAP acts in the context of the cellular machinery, we identified its interacting proteins in human umbilical vein endothelial cells and HeLa cells using shotgun proteomics. Here we show that WTAP forms a novel protein complex including Hakai, Virilizer homolog, KIAA0853, RBM15, the arginine/serine-rich domain-containing proteins BCLAF1 and THRAP3, and certain general splicing regulators, most of which have reported roles in post-transcriptional regulation. The depletion of these respective components of the complex resulted in reduced cell proliferation along with G₂/M accumulation. Double knockdown of the serine/arginine-rich (SR)-like proteins BCLAF1 and THRAP3 by siRNA resulted in a decrease in the nuclear speckle localization of WTAP, whereas the nuclear speckles were intact. Furthermore, we found that the WTAP complex regulates alternative splicing of the WTAP pre-mRNA by promoting the production of a truncated isoform, leading to a change in WTAP protein expression. Collectively, these findings show that the WTAP complex is a novel component of the RNA processing machinery, implying an important role in both posttranscriptional control and cell cycle regulation.

Although regulators of the Wnt/planar cell polarity (PCP) pathway are widely expressed in vertebrate nervous systems, their roles at synapses are unknown. Here, we show that Vangl2 is a postsynaptic factor crucial for synaptogenesis and that it coprecipitates with N-cadherin and PSD-95 from synapse-rich brain extracts. Vangl2 directly binds N-cadherin and enhances its internalization in a Rab5-dependent manner. This physical and functional interaction is suppressed by β-catenin, which binds the same intracellular region of N-cadherin as Vangl2. In hippocampal neurons expressing reduced Vangl2 levels, dendritic spine formation as well as synaptic marker clustering is significantly impaired. Furthermore, Prickle2, another postsynaptic PCP component, inhibits the N-cadherin-Vangl2 interaction and is required for normal spine formation. These results demonstrate direct control of classic cadherin by PCP factors; this control may play a central role in the precise formation and maturation of cell-cell adhesions at the synapse.