BETA-1 INTEGRIN-DEPENDENT AND INTEGRIN-INDEPENDENT POLYMERIZATION OF FIBRONECTIN

The mouse cell line GD25, which lacks expression of the beta 1 family of integrin heterodimers due to disruption of the beta 1 integrin subu nit gene, was used for expression of full-length cDNA coding for splic e variant A of the mouse beta 1 integrin subunit. In a stably transfor med clone (GD25-beta 1A), the expressed protein was found to form func tional heterodimeric receptors together with the subunits alpha 3, alp ha 5, and alpha 6. Both GD25 and GD25-beta 1A attached to fibronectin and formed focal contacts which contained alpha v beta 3, but no detec table alpha 5 beta 1A, The presence of GRGDS peptide allowed alpha 5 b eta 1A to locate to focal contacts of GD25-beta 1A cultured on fibrone ctin, while the beta 1-null GD25 cells were unable to attach under the se conditions. Affinity chromatography revealed that alpha 5 beta 1A a nd alpha v beta 3 could bind to a large cell-binding fragment of fibro nectin, alpha 5 beta 1A strongly promoted polymerization of fibronecti n into a fibrillar network on top of the cells. Whereas little alpha v beta 3 was colocalized with the fibronectin fibrils in GD25-beta 1A c ells, this integrin was able to support fibronectin fibril polymerizat ion in GD25 cells, However, the alpha v beta 3-induced polymerization was less efficient and occurred mainly in dense cultures of the GD25 c ells. Thus, while both alpha 5 beta 1A and alpha v beta 3 are able to support adhesion to fibronectin, alpha v beta 3 dominates in the forma tion of focal contacts, and alpha 5 beta 1A has a prime function in fi bronectin matrix assembly, This is the first report on fibronectin mat rix assembly in the absence of beta 1 integrins.