Dab1 consists of an aminoterminal phosphotyrosine binding domain followed by a stretch of tyrosyl residues and a large carboxyl-terminal domain. At least two of the tyrosine residues are phosphorylated in vivo by the Src family kinase Fyn (2, 3) in response to Reelin. Tyrosine phosphorylation of Dab1 at these residues is essential for the transduction of the Reelin signal (4) and requires binding of Reelin to its receptors, apolipoprotein E receptor 2 (apoER2) and VLDL receptor, two members of the low density lipoprotein (LDL) receptor gene family (reviewed in Ref. 5). Tyrosinephosphorylated Dab1 triggers the activation of class I phosphatidylinositol 3 (PI3)-kinase (6, 7), which has been shown to be necessary for the formation of a normal cortical plate (8).The conserved phosphotyrosine binding domain mediates the interaction of Dab1 with unphosphorylated NPXY tetraamino acid sequences in the cytosolic domains of transmembrane receptors, including members of the LDL receptor family (9, 10), and with phosphoinositides at the inner leaflet of the plasma membrane (10 -12). The function of the carboxyl-terminal sequence is less well defined. It is not required for the tyrosine phosphorylation of Dab1 or the activation of Src family kinases and PI3-kinase by Reelin (7). Mice expressing a carboxyl-terminal truncated Dab1 protein (p45) display normal brain development (13), although haploinsufficiency of the p45 allele causes a restricted phenotype characterized by positioning defects of late-born cortical plate neurons and hippocampal CA1 pyramidal cells. Interestingly, a serine residue near the carboxyl terminus of Dab1 is phosphorylated by Cdk5 in vivo (14), a proline-directed serine/threonine kinase, which is also essential for the correct positioning of postmitotic neurons (reviewed in Ref. 15), but the functional significance of this biochemical cross-talk remains unclear.How the Reelin signal in migrating neurons is terminated is not known. The Reelin-induced tyrosine phosphorylation of Dab1 declines only after prolonged incubation periods (16). Treatment of neurons with sodium orthovanadate did not significantly increase Dab1 tyrosine phosphorylation, suggesting that phosphotyrosine phosphatases do not play a major role in limiting the response to Reelin (16). Up-regulation of Dab1 protein levels is a common feature in mice carrying dysfunctional mutations of genes that encode key Reelin signaling molecules, namely Reelin itself (17), its receptors apoER2 and VLDLR (18), and its kinase Fyn (2, 3), as well as in mice that * This work was supported in part by National Institutes of Health Grants HL20948, HL63762, and NS43408, the Alzheimer's Association, the Wolfgang-Paul Award of the Humboldt Foundation, the Perot Family Foundation (to J. H.), and an Emmy Noether fellowship of the Deutsche Forschungsgemeinschaft (to P. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Secti...

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The peptide transporter (PTR) family represents a group of proton-coupled secondary transporters responsible for bulk uptake of amino acids in the form of di-and tripeptides, an essential process employed across species ranging from bacteria to humans. To identify amino acids critical for peptide transport in a prokaryotic PTR member, we have screened a library of mutants of the Escherichia coli peptide transporter YdgR using a high-throughput substrate uptake assay. We have identified 35 single point mutations that result in a full or partial loss of transport activity. Additional analysis, including homology modeling based on the crystal structure of the Shewanella oneidensis peptide transporter PepT so , identifies Glu 56 and Arg 305 as potential periplasmic gating residues. In addition to providing new insights into transport by members of the PTR family, these mutants provide valuable tools for further study of the mechanism of peptide transport.

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