DIRECT INTERACTION OF FILAMIN (ABP-280) WITH THE BETA-2-INTEGRIN SUBUNIT CD18

beta 2-Integrins mediate leukocyte extravasation into inflamed tissues , phagocytosis, and target cell killing, functions that require an int act actin cytoskeleton. Previous studies have focused on elucidating i nteractions of the cytoplasmic tails of integrins with the cytoskeleto n at focal contacts in stationary cells. As integrins are also located at other types of cell-substratum junctions, such as the leading edge of migrating cells, additional cytosolic proteins abundant at these s ites may also interact with integrins. In this study, we have identifi ed the actin-binding protein, filamin (ABP-280), as a major cytoskelet al protein that binds to the cytoplasmic tail of the beta 2-integrin s ubunit CD18. Filamin bound to cytoplasmic CD18 directly and specifical ly, co-immunoprecipitated with beta 2-integrins in detergent cell lysa te and co-immunolocalized with cross-linked beta 2-integrins in intact cells. The filamin binding site in CD18 was localized to the N-termin al (amino acids (aa) 724 to 747) but not to the C-terminal (aa 743 to 769) half of cytoplasmic CD18. Filamin did not bind to the major cr-ac tinin binding site (aa 733 to 742), however, suggesting that these two cytoskeletal proteins bind to distinct but overlapping sites. Given t he conservation of the filamin binding region among P-integrin subunit s, these findings suggest the presence of similar associations between filamin and other integrins. These associations may be important in t he spreading and extension of lamellipodia at the leading edge during cell movement and, if interrupted, may contribute to the dramatic decr ease in cell locomotion observed in genetic deletions involving filami n or integrins.