Bidirectional signals mediated by membrane-anchored ephrins and Eph receptor tyrosine kinases have important functions in cell-cell recognition events, including those that occur during axon pathfinding1, 2 and hindbrain segmentation3, 4. The reverse signal that is transduced into B-ephrin-expressing cells is thought to involve tyrosine phosphorylation of the signal's short, conserved carboxy-terminal cytoplasmic domain5, 6. The Src-homology-2 (SH2) domain proteins that associate with activated tyrosinephosphorylated B-subclass ephrins have not been identified, nor has a defined cellular response to reverse signals been described. Here we show that the SH2/SH3 domain adaptor protein Grb4 binds to the cytoplasmic domain of B ephrins in a phosphotyrosine-dependent manner. In response to B-ephrin reverse signalling, cells increase FAK catalytic activity, redistribute paxillin, lose focal adhesions, round up, and disassemble F-actin-containing stress fibres. These cellular responses can be blocked in a dominant-negative fashion by expression of the isolated Grb4 SH2 domain. The Grb4 SH3 domains bind a unique set of other proteins that are implicated in cytoskeletal regulation, including the Cb1-associated protein (CAP/ponsin), the Ab1-interacting protein-1 (Abi-1), dynamin, PAK1, hnRNPK and axin. These data provide a biochemical pathway whereby cytoskeletal regulators are recruited to Eph-ephrin bidirectional signalling complexes.
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