PURIFICATION AND STRUCTURAL CHARACTERIZATION OF THE CD11B CD18 INTEGRIN ALPHA-SUBUNIT-I DOMAIN REVEALS A FOLDED CONFORMATION IN SOLUTION/

The alpha subunits of the leukocyte CD11/CD18 integrins contain a simi lar to 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 re cognition site for several adhesion ligands, including iC3b, fibrinoge n, factor,X, and ICAM-1. The I domain from the Mac-1 alpha subunit has been expressed in Escherichia coli as a soluble GST-fusion protein co ntaining a factor X(a) sensitive cleavage site. Analytical characteriz ation of the purified I domain reveals that it is obtained in very hig h quality at high yields. CD and NMR spectra indicate that I domain ad opts a predominantly folded structure in solution, independent of the remainder of the alpha subunit. addition of Ca2+ and Mg2+ did not sign ificantly perturb the structural conformation.