Purification and structural characterization of the CD11b/CD18 integrin α subunit I domain reveals a folded conformation in solution

The α subunits of the leukocyte CD11/CD18 integrins contain a ∼200 amino acid ‘inserted’ or I domain. The I domain of the cell-surface Mac-1(CD11b/CD18) integrin has been shown to be the major recognition site for several adhesion ligands, including iC3b, fibrinogen, factor X, and ICAM-1. The I domain from the Mac-1 α subunit has been expressed in Escherichia coli as a soluble GST-fusion protein containing a factor X a sensitive cleavage site. Analytical characterization of the purified I domain reveals that it is obtained in very high quality at high yields. CD and NMR spectra indicate that I domain adopts a predominantly folded structure in solution, independent of the remainder of the α subunit. Addition of Ca 2+ and Mg 2+ did not significantly perturb the structural conformation.