Temporal dissection of β₁-integrin signaling indicates a role for p130CAS-Crk in filopodia formation

Invasin-promoted spreading of β₁-integrin-deficient cells, transfected with the β_1A- or β_1B-integrin splice variants, were used to dissect early β₁-integrin signaling events. The β_1B isoform, which has a different membrane-distal part of the cytoplasmic tail from β_1A, is defective in signaling and function. When plated on surfaces coated with the high affinity ligand invasin, β_1B-integrin-expressing cells spread by forming filopodia with distinct adhesive phosphotyrosine complexes at the tips, without signs of lamellipodia. This suggested that the β _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 β_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 β_1B-integrin-expressing cells, and there was a prominent Rac1 activation. Expression of dominant negative variants of p130Cas or CrkII blocked β_1B-integrin-mediated filopodia formation, indicating that this signaling scaffold is central in this process.