A novel functional role for the highly conserved α-subunit KVGFFKR motif distinct from integrin αIIbβ3 activation processes

Background: The highly conserved integrin alpha-subunit membrane-proximal motif KVGFFKR plays a decisive role in modulating the activation of integrin alpha(IIb)beta(3). Previously, we have shown that a platelet permeable palmityl (pal)-peptide with this seven amino acid sequence can directly activate alpha(IIb)beta(3) leading to platelet aggregation. Objectives: To investigate further the role of the KVGFFKR motif in integrin alpha(IIb)beta(3) function. Methods: We used two sequence-specific complementary model systems, palmityl pal-peptides in platelets, and mutant alpha(IIb)beta(3)-expressing Chinese Hamster Ovary (CHO) cell lines. Results: In platelets we show that the two phenylalanine amino acids in pal-KVGFFKR (pal-FF) peptide are critical for stimulating platelet aggregation. Pal-FF peptide treatment of platelets also gives rise to a tyrosine phosphorylation signal despite the presence of inhibitors of fibrinogen binding. In CHO cells, a double alanine substitution, alpha(IIb)(F992A, F993A)beta(3), induces constitutive integrin activation but prevents actin stress fiber formation upon adhesion to fibrinogen, suggesting that alpha(IIb)beta(3)-mediated cytoskeletal reorganization is also dependent on F-992 and F-993. This further highlights a critical role for the two phenylalanine residues in both of these alpha(IIb)beta(3)-mediated processes. Conclusion: In addition to regulating integrin alpha(IIb)beta(3) activation state, the KVGFFKR motif also influences cytoskeletal reorganization. This activity is critically determined by F-992 and F-993 within the seven amino acid sequence.