The N-terminal region and the mid-region complex of the integrin β2 subunit

In the primary sequence of the integrin beta 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 beta (2) NTR and mid-region, we constructed beta (2)/beta (7) chimeras in which the NTR, I-like domain, and the mid-region of the beta (2) subunit were replaced by those of beta (7). Changing either the beta (2) NTR or mid-region, but not the I-like domain to that of beta (7) did not affect LFA-1 (alpha (L)beta (2)) formation and surface expression. Thus, the specificity of alpha (L)beta (2) pairing is conferred by the I-like domain but not the NTR or mid-region. Using these chimeras, the epitopes of six anti-beta (2) mAbs (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 beta (2) NTR 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 beta (2) NTR/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/EGTA-induced 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.