β1 integrin cytoplasmic domain regulates the constitutive conformation detected by MAb 15/7, but not the ligand-induced conformation

The anti-integrin beta 1 MAb 15/7 sometimes may be a reporter of integrin activation or ligand occupancy. However, certain beta 1 tail deletions eliminate ligand binding despite inducing maximal constitutive 15/7 expression [Puzon-Mclaughlin el al. (1996): J Biol Chem 271 : 16580-16585]. Here we describe beta 1 tail mutations (e.g., double point mutations [D759L/F763L, F766L/E769L], or replacement of the beta 1 tail by the beta 5 tail) that prevent rather than induce constitutive appearance of the 15/7 epitope. Despite variable losses of constitutive 15/7 epitope, these mutants ail retained a similar inducible 15/7 epitope component as seen upon incubation with GRCDSP peptide ligand. In addition, constitutive 15/7 expression did not correlate with integrin localization into focal adhesions. In conclusion, we show for the first time for a fully functional integrin that specific mutations with in the beta 1 tail can down-regulate the constitutive appearance of an extracellular conformation defined by MAb 15/7. Because this regulation occurs away from the ligand binding site and does not correlate with responsiveness to integrin ligand, cell adhesion, or localization into focal adhesions, a novel type of conformational regulation is suggested. (C) 1998 Wiley-Liss, Inc.