(A) A model of human plasma FN showing two heterologous subunits (A and B), which are linked at their carboxy-termini by two disulfide bonds. Each subunit contains a series of type I (rectangles), type II (circles), and type III (squares) homologous repeats. The type III repeats which contain the classical RGDS cell adhesion sequence (▧) and heparin-binding cell adhesion sequences () are shown. The alternatively spliced type III connecting segment (IIICS), which contains two known cell adhesion sequences is indicated (▪). The location of the chymotryptic fragments used in these studies, along with their approximate molecular weights, are shown by solid bars. Antibody recognition and fibrin-binding data suggest that the 42-kD fragment originates from the B chain, while the 30-/35-kD fragments are from the A chain. (B) Amino acid sequence of the type IIICS region, which spans amino acids 1961-2080 of the A chain of human plasma FN. Synthetic peptides used in these studies (CS-1, CS-2, CS-3, and PREDVDY) are indicated by solid lines. The two sequences known to be active in cell adhesion are indicated by closed circles (LDV) and open circles (REDV). Arrowheads indicate putative splice sites in the IIICS region.