Figure 2. Structures of the ligand- and divalent cation-binding I domains of CD11's. Stereo pairs for crossed-eye viewing made with the program RasMol. (a) Backbone cartoon of the I domain of CD11a (atomic coordinate file 1LFA.PDB, Qu & Leahy, Proc. Natl. Acad. Sci. USA 92:10277, 1995). Mg++ is shown as a black sphere. The 5 residues of the MIDAS motif which coordinate Mg++ are shown as darkened stick representations. (Some of the coordinations are via water molecules, not shown. Mn++ was the divalent cation in the crystal from which this structure was determined.) The darkened sections of the backbone indicate the positions of the strained hydrophobic ridge, M140 plus L203LL, which is unique to CD11a and may be important in binding ICAM's. (b) Enlargement of the Mg++ from (a), with the 5 MIDAS residues identified. (c) Spacefilling representation of the putative ligand-binding face of the I domain of CD11a. Mg++ is black. Backbone and polar side chains are gray, charged side chains are white, and hydrophobic side chains are dark gray. The inset identifies the four residues which make up the strained hydrophobic ridge. (d) Spacefilling representation of the putative ligand-binding face of the I domain of CD11b, colored as in (c). Note the absence of the hydrophobic ridge pattern seen in CD11a. (Atomic coordinate file kindly provided by Amin Arnaout. J.-O. Lee, P. Rieu, M. A. Arnaout, and R. Liddington, Cell 80:631-8, 1995.) These and related images may be viewed in color at http://www.umass.edu/microbio/rasmol.