The beta 1-integrin subunit localizes to focal contacts during F9 teratocarcinoma stem cell differentiation to parietal endoderm. Concomitantly, this integrin subunit becomes dephosphorylated at serine 790, the only serine in the well-conserved cytoplasmic domain of beta 1. We hypothesized that this dephosphorylation is required for this subunit to move to a focal contact. To test this, we transfected F9 stem cells with distinct cDNAs encoding three forms of chicken beta 1: the wild-type, and two site-specific mutants possessing amino acid substitutions at residue 790 which included methionine and aspartate. The negatively charged aspartate was selected to mimic the phosphorylated serine found at position 790 in the wildtype protein but in a form that cannot be dephosphorylated upon differentiation. The chicken beta 1 subunits heterodimerized with endogenous mouse alpha subunits and, using a chicken beta 1-specific monoclonal antibody, we examined movements of these chimeric integrins to focal contacts using immunofluorescence microscopy. The chimeric integrins possessing either the wildtype or methionine mutant beta 1 subunits localized to focal contacts upon F9 differentiation. In contrast, the aspartate chimeric integrins failed to localize. These data suggest that the dephosphorylation of serine 790 is required for the beta 1 subunit to localize to focal contacts during F9 differentiation.