Mutation of a conserved asparagine in the I-like domain promotes constitutively active integrins αLß2 and αIIbß3

The leukocyte beta(2) integrins are heterodimeric adhesion receptors required for a functional immune system. Many leukocyte adhesion deficiency-1 (LAD-1) mutations disrupt the expression and function of beta(2) integrins. Herein, we further characterized the LAD-1 mutation N329S in the beta(2) inserted (I)-like domain. This mutation converted alpha(L)beta(2) from a resting into a high affinity conformer because alpha(L)beta(2)N329S transfectants adhered avidly to ligand intercellular adhesion molecule (ICAM)-3 in the absence of additional activating agent. An extended open conformation is adopted by alpha(L)beta(2)N329S because of its reactivity with the beta(2) activation reporter monoclonal antibodies MEM148 and KIM127. A corresponding mutation in beta(3) generated constitutively active alpha(IIb)beta(3) that adhered to fibrinogen. This Asn is conserved in all human beta sub-units, and it resides before the last helix of the I-like domain, which is known to be important in activation signal propagation. By mutagenesis studies and review of existing integrin structures, we conjectured that this conserved Asn may have a primary role in shaping the I- like domain by stabilizing the conformation of the alpha 7 helix and the beta 6-alpha 7 loop in the I-like domain.