In the primary sequence of the integrin β subunit, the N-terminal region (NTR) and mid-region are separated by the I-like domain. To determine the spatial relationship and functional properties of the integrin β2NTR and mid-region, we constructed β2/β7chimeras in which the NTR, I-like domain, and the mid-region of the β2subunit were replaced by those of β7. Changing either the β2NTR or mid-region, but not the I-like domain to that of β7did not affect LFA-1 (αLβ2) formation and surface expression. Thus, the specificity of αLβ2pairing is conferred by the I-like domain but not the NTR or mid-region. Using these chimeras, the epitopes of six anti-β2mAbs (H52, 7E4, AZN-L18, AZN-L27, KIM202, and MEM-148) were mapped. All except H52 require both the NTR and mid-region for epitope expression. Since these mAbs have distinct properties in terms of epitope expression and effect on LFA-1 binding to ICAM-1, we conclude that the β2NTR and mid-region interact extensively. Although the I-like domain is located between the NTR and mid-region, its removal does not affect the folding of the β2NTR/mid-region complex because this complex alone can be expressed as a soluble protein and precipitated by the appropriate mAbs. Finally, the mAbs H52 and 7E4, abrogated KIM185- but not Mg/EGTAinduced LFA-1/ICAM-1 binding and the epitope of MEM-148 is expressed on Mg/EGTA-activated but not resting LFA-1. These results suggest that the NTR/mid-region complex is involved in the regulation of LFA-1 function.
