The fibronectin receptor, alpha5beta1, has been shown to be required for fibronectin matrix assembly and plays an important role in cell migration on fibronectin. However, it is not clear whether other fibronectin binding integrins can take the place of alpha5beta1 during matrix assembly and cell migration. To test this, we expressed the human alphav subunit in the CHO cell line CHO-B2 that lacks the alpha5 subunit. We found that the human alphav combined with CHO cell beta1 to form the integrin alphavbeta1. Cells that expressed alphavbeta1 attached to and spread well on fibronectin-coated dishes, but did so less well on vitronectin-coated dishes. This, along with other data, indicated that alphavbeta1 functions as a fibronectin receptor in CHO-B2 cells. The alphavbeta1-expressing cells failed to produce a fibronectin matrix or to migrate on fibronectin, although the same cells transfected with alpha5 do produce a matrix and migrate on fibronectin. The affinity of the alphavbeta1-expressing cells for fibronectin was fourfold lower than that of the alpha5beta1-expressing cells. In addition, alphavbeta1 was distributed diffusely throughout the cell surface, whereas alpha5beta1 was localized to focal adhesions when cells were seeded onto fibronectin-coated surfaces. Thus, of the two fibronectin receptors, alphavbeta1 and alpha5beta1, only alpha5beta1 supports fibronectin matrix assembly and promotes cell migration on fibronectin in the CHO-B2 cells. Possible reasons for this difference in the activities of alphavbeta1 and alpha5beta1 include the lower affinity of alphavbeta1 for fibronectin and the failure of this integrin to localize in adhesion plaques on a fibronectin substrate. These results show that two integrins with similar ligand specificities and cell attachment functions may be quite different in their ability to support fibronectin matrix assembly and cell motility on fibronectin.
