Functional beta 1-integrins release the suppression of fibronectin matrix assembly by vitronectin

beta 1-null GD25 fibroblasts adherent to vitronectin fail to bind the N-ter minal 70-kDa matrix assembly domain of fibronectin or to assemble fibronect in (Sakai, T,, Zhang, Q,, Fassler, R., and Mosher, D, F, (1998) J. Cell Bio l. 141, 527-538), We have made four observations that extend this finding. First, the presence of vitronectin on a substrate that otherwise can suppor t fibronectin assembly has a dominant-negative effect on assembly. Sec end, the dominant-negative effect is lost when active beta 1A is expressed. Thi rd, beta 1A containing the extracellular D130A inactivating mutation has a dominant-negative effect on fibronectin assembly. Fourth, beta 1-null cells adherent to vitronectin are flat and lack filopodia, whereas beta 1-null c ells adherent to fibronectin or beta 1A-expressing cells adherent to either vitronectin or fibronectin are contracted and exhibit numerous filopodia. These results reveal, therefore, that GD25 cells adherent to vitronectin ca n only assume a shape suitable for assembly of fibronectin when there is a countervailing signal from functional beta 1-integrins.