beta 2-Integrins mediate leukocyte extravasation into inflamed tissues, phagocytosis, and target cell killing, functions that require an intact actin cytoskeleton. Previous studies have focused on elucidating interactions of the cytoplasmic tails of integrins with the cytoskeleton 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 sites may also interact with integrins. In this study, we have identified the actin-binding protein, filamin (ABP-280), as a major cytoskeletal protein that binds to the cytoplasmic tail of the beta 2-integrin subunit CD18. Filamin bound to cytoplasmic CD18 directly and specifically, co-immunoprecipitated with beta 2-integrins in detergent cell lysate and co-immunolocalized with cross-linked beta 2-integrins in intact cells. The filamin binding site in CD18 was localized to the N-terminal (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 alpha-actinin binding site (aa 733 to 742), however, suggesting that these two cytoskeletal proteins bind to distinct but overlapping sites. Given the conservation of the filamin binding region among beta-integrin subunits, these findings suggest the presence of similar associations between filamin and other integrins. These associations may be important in the spreading and extension of lamellipodia at the leading edge during cell movement and, if interrupted, may contribute to the dramatic decrease in cell locomotion observed in genetic deletions involving filamin or integrins.

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