Conditions that permit the adhesion of BHK fibroblasts to a variety of surfaces after inhibition of protein synthesis and competition of any adsorbed fibronectin or vitronectin with the fibronectin cell-binding tetrapeptide, Arg-Gly-Asp-Ser (RGDS), are defined. Exposure of the cells to serum components at any stage in the preparation prevents cell attachment if cycloheximide or fibronectin tetrapeptide is present. If leupeptin is used cell adhesion and spreading occur even when all fibronectin synthesis is suppressed by cycloheximide inhibition, or fibronectin binding by tetrapeptide competition. The adhesions formed under these conditions appear by interference-reflection microscopy and by general properties to be identical to those formed by cells under normal culture conditions. The cell suspensions produced in the presence of leupeptin rather than other trypsin inhibitors show good adhesion at low temperatures, though the cells hardly spread at all. The results suggest that the role of fibronectin in cell adhesion should be reinterpreted in terms of its possible action as an activator rather than as a bonding molecule.

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