Interaction between Dab1 and CrkII is promoted by Reelin signaling

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Abstract
Reelin-induced Dab1 tyrosine phosphorylation has been implicated in the regulation of neuronal positioning during brain development. The downstream consequences of Dab1 tyrosine phosphorylation are not fully understood, however. Here we identify CrkII, CrkL and Dock1 in complexes bound to tyrosine-phosphorylated Dab1, through mass spectrometry. The CrkII-Dab1 interaction requires tyrosine phosphorylation of Dab1 at residues 220 or 232 and is promoted by Reelin treatment of embryonic forebrain neurons. Unlike other CrkII binding proteins, such as paxillin and p130Cas, expression of Dab1 interfered with CrkII-dependent cell migration of Nara Bladder Tumor II (NBT-II) cells, in a tyrosine phosphorylation-site dependent manner. Overexpression of CrkIIGFP rescued the migration of these cells, suggesting that Dab1 makes Crk a limiting factor for migration. The Dock1-Dab1 association is indirect and requires CrkII. In organisms such as Drosophila melanogaster and Caenorhabditis elegans, signaling complexes, which contain Crk and Dock1 family members are conserved and act through Rac. We show that a rough-eye phenotype in Drosophila caused by exogenous expression of tyrosine-phosphorylated mouse Dab1RFP is partially rescued by a loss-of-function mutation in myoblast city, a Dock1-like gene in Drosophila. We propose a model that tyrosine-phosphorylated Dab1 engages the conserved Crk-Dock1-Rac signaling cassette, but when bound to Dab1 this signaling complex does not support migration.