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

Formation of the integrin alpha beta heterodimer is essential for cell surface expression and function. At the core of the alpha beta interface is a conserved Arg/Lys finger from the beta-subunit that inserts into a cup-like cage formed of two layers of aromatic residues in the alpha-subunit. We evaluated the role of this residue in heterodimer formation in an alpha A-lacking and an alpha A-containing integrin alpha V beta 3 and alpha M beta 2 (CD11b/CD18), respectively. Arg261 of beta 3 was mutated to Ala or Glu; the corresponding Lys252 of beta 2 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 alpha- and beta-subunits. The Arg261Glu (but not Arg261Ala) substitution significantly impaired cell surface expression and heterodimer formation of alpha V beta 3. 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.