Role of the β-subunit arginine/lysine finger in integrin heterodimer formation and function

Formation of the integrin αβ heterodimer is essential for cell surface expression and function. At the core of the αβ interface is a conserved Arg/Lys "finger" from the β-subunit that inserts into a cup-like "cage" formed of two layers of aromatic residues in the α-subunit. We evaluated the role of this residue in heterodimer formation in an αA-lacking and an αA-containing integrin αVβ3 and αMβ2 (CD11b/CD18), respectively. Arg261 of β3 was mutated to Ala or Glu; the corresponding Lys252 of β2 was mutated to Ala, Arg, Glu, Asp, or Phe; and the effects on heterodimer formation in each integrin examined by ELISA and immunoprecipitation in HEK 293 cells cotransfected with plasmids encoding the α- and β-subunits. The Arg261Glu (but not Arg261Ala) substitution significantly impaired cell surface expression and heterodimer formation of αVβ3. Although Lys252Arg, and to a lesser extent Lys252Ala, were well tolerated, each of the remaining substitutions markedly reduced cell surface expression and heterodimer formation of CD11b/CD18. Lys252Arg and Lys252Ala integrin heterodimers displayed a significant increase in binding to the physiologic ligand iC3b. These data demonstrate an important role of the Arg/Lys finger in formation of a stable integrin heterodimer, and suggest that subtle changes at this residue affect the activation state of the integrin.