The cytoplasmic domain of the integrin α9 subunit requires the adaptor protein paxillin to inhibit cell spreading but promotes cell migration in a paxillin-independent manner

The integrin alpha9 subunit forms a single heterodimer, alpha9 beta1. The alpha9 subunit is most closely related to the alpha4 subunit, and like alpha4 integrins, alpha9 beta1 plays an important role in leukocyte migration. The A cytoplasmic domain preferentially enhances cell migration and inhibits cell spreading, effects that depend on interaction with the adaptor protein, paxillin. To determine whether the alpha9 cytoplasmic domain has similar effects, a series of chimeric and deleted alpha9 constructs were expressed in Chinese hamster ovary cells and tested for their effects on migration and spreading on an alpha9 beta1-specific ligand. Like alpha4, the alpha9 cytoplasmic domain enhanced cell migration and inhibited cell spreading. Paxillin also specifically bound the alpha9 cytoplasmic domain and to a similar level as alpha4. In paxillin(-/-) cells, alpha9 failed to inhibit cell spreading as expected but surprisingly still enhanced cell migration. Further, mutations that abolished the alpha9-paxillin interaction prevented alpha9 from inhibiting cell spreading but had no effect on alpha9-dependent cell migration. These findings suggest that the mechanisms by which the cytoplasmic domains of integrin a subunits enhance migration and inhibit cell spreading are distinct and that the alpha9 and alpha4 cytoplasmic domains, despite sequence and functional similarities, enhance cell migration by different intracellular signaling pathways.