Fig. 3. Distribution of FN7-10 monomers and oligomers bound to fibroblasts. Soluble biotinylated proteins at 3 µM (subunit concentration) were added to fibroblasts grown overnight on a laminin substrate. After 2.5 minutes the cells were fixed and stained with fluorescent avidin to localize bound FN. Two examples are given for each protein. For the tandem dimer and pentamer, the focal plane of the right hand images was set above the substrate to optically section at the level of the cell body, illustrating the diffuse membrane staining for the tandem dimer and the punctate staining for the pentamer. For all other images the focus was on the upper surface of the lamellapodia. Control TN trimer (lacking the FN7-10 segment) does not bind the cell surface (bottom left). A competition experiment is shown in the bottom-right panel, where cells were treated with 3 µM biotinylated FN7-10 trimer in the presence of 30 µM unbiotinylated FN7-10 monomer. Binding of trimer is efficiently competed by excess monomer.