Temporal dissection of β1-integrin signaling indicates a role for p130Cas-Crk in filopodia formation

Invasin-promoted spreading of B-1-integrin-deficient cells, transfected with the X-1A- or beta(1B)-integrin splice variants, were used to dissect early beta(1)-integrin signaling events. The X-1B isoform, which has a different membrane-distal part of the cytoplasmic tail from beta(1A), is defective in signaling and function. When plated on surfaces coated with the high affinity ligand invasin, beta(1B)-integrin-expressing cells spread by forming filopodia with distinct adhesive phosphotyrosine complexes at the tips, without signs of lamellipodia. This suggested that the beta(1B)-integrin mediated a partial signaling sufficient for formation of filopodia but insufficient for lamellipodia formation. When screening for proteins present in the distal filopodial phosphotyrosine complexes of beta(1B) cells, p130Cas and the filopodia proteins vasodilator-stimulated phosphoprotein and talin were found, whereas the typical focal complex proteins focal adhesion kinase, paxillin, and vinculin were not. Invasin-promoted adhesion induced complex formation of p130Cas and the adapter Crk. Moreover, Crk together with Dock180 were present at the filopodial tips of beta(1B)-integrin-expressing cells, and there was a prominent Rac1 activation. Expression of dominant negative variants of p130Cas or CrkII blocked beta(1B)-integrin-mediated filopodia formation, indicating that this signaling scaffold is central in this process.