The minimal essential sequence for a major cell type-specific adhesion site (CS1) within the alternatively spliced type III connecting segment domain of fibronectin is leucine-aspartic acid-valine.

Fibronectin contains at least two major domains that support cell adhesion. One is the central cell-binding domain that is recognized by a variety of cell types via the integrin alpha 5 beta 1. The second, originally identified by its ability to support melanoma cell adhesion, is located in the alternatively spliced type III connecting segment (IIICS). A dominant cell type-specific adhesion site within the IIICS has been localized to a peptide designated as CS1 comprising its amino-terminal 25 residues. The receptor for CS1 is the integrin alpha 4 beta 1. We have synthesized a variety of peptides with overlapping sequences in order to identify the minimum active amino acid sequence of this major cell adhesion site. A peptide comprising the carboxyl-terminal 8 amino acids of CS1, EILDVPST, was found to support melanoma cell spreading, while all peptides without this sequence had little or no activity. Two smaller overlapping pentapeptides, EILDV and LDVPS, were also active, whereas EILEV, containing a conservative substitution of Glu for Asp, was inactive. These data suggested that the minimum sequence for cell adhesion activity is Leu-Asp-Val, the tripeptide sequence common to both active peptides. This prediction was confirmed by the observed ability of the Leu-Asp-Val peptide itself to block spreading on fibronectin, whereas Leu-Glu-Val was inactive. Interspecies amino acid sequence comparison also supports the importance of the LDV sequence, since it is completely conserved in the IIICS regions of human, rat, bovine, and avian fibronectins.