Chimeras of the Integrin β Subunit Mid-Region Reveal Regions Required for Heterodimer Formation and for Activation

A central region of the β2 integrin subunit, RN (residues D300 to C459), was replaced by the equivalent sequences from β1 and β7 to give the chimeras β2RN1 and β2RN7. Whilst the former construct failed to form heterodimer at the cell surface with αL, the later of these could be expressed together with the αL subunit to form a variant LFA-1. Based on recent modelling work, the RN region consists of two parts, one is the C-terminal end of the putative A-domain (RB, residues D300 to A359), and the other the mid-region (BN, residues Y360 to C459). Chimeras exchanging the two component regions were made. Of the four resultant chimeras, only the β2RB1 chimera failed to support LFA-1 expression. Thus the β1 specific residues of this region affect the interaction with the αL subunit. Whereas the αLβ2RB7 LFA-1 variant is wildtype like with respect to ICAM-1 adhesion, the αLβ2BN1 and αLβ2BN7, as well as the αLβ2BN7, variants are more adhesive than the wildtype. These results suggest that an authentic β2 mid-region is, in part, required for maintaining the LFA-1 in a resting state.