(A) Ribbon diagram of one monomeric unit of the intermediate-affinity αL I domain (gold) complex with ICAM-1 domains 1-2 (cyan). The Mg²⁺ ion is shown as a magenta sphere. I domain MIDAS and ICAM-1 Glu-34 side chains are shown as ball-and-stick with red oxygen atoms. The interacting β strands C, D, and F of ICAM-1 are labeled. N-acetyl glucosamine residues of ICAM-1 are shown with silver bonds.
(B) The two ICAM-1-I domain complexes in the crystallographic asymmetric unit are shown with the I domains colored gold and the two ICAM-1 molecules colored cyan and green. The 2-fold axis between the ICAM-1 molecules is in the vertical direction, normal to the predicted membrane plane. Positions of the magnesium ions (magenta spheres) and Glu-34 of ICAM-1 (CPK) are shown for reference. The ICAM-1 Val-51 residues at the center of the ICAM-1 dimer interface are shown as gray CPK models.
(C) A view of domains 1 of the ICAM-1 dimer (cyan and green), rotated about 90° from the view in (B), with domain 1 of the uncomplexed ICAM-1 molecule A dimer () superimposed using domain 1 of one of the ICAM-1 molecules (black), whereas the other one is colored gray. Figures 1– are prepared with programs GLR (provided by L. Esser), Molscript (), Bobscript (), Raster3D (), GRASP (), and Povray (The Povray Team, http://www.povray.org).

(A) Residues in the I domain:ICAM-1 interface in an open-book representation of the protein surface with the book folded open horizontally, with ICAM-1 above and the I domain below. Interacting residues are colored green and labeled, except for Glu-34 in ICAM-1 and the MIDAS Mg²⁺ ion in ICAM-1, which are magenta and unlabeled.
(B) The electrostatic surface of the complex in the same open-book representation. The color scale for the charge distribution extends from −10 kT/e⁻ (red) to +10 kT/e⁻ (blue) calculated using GRASP (). The position of bulges in the interface are marked by arrows, as are the salt-bridged residues Lys-39 in ICAM-1 and Glu-241 in the I domain and Glu-34 in ICAM-1 and the Mg²⁺ ion in the I domain.
(C) Stereo view of interface details with ICAM-1 in cyan and I domain in gold. The view is about the same as in . The metal ion, oxygens, and nitrogens are represented as magenta, red, and blue spheres, respectively. Metal coordination and hydrogen bonds are represented by solid black lines and gray dotted lines, respectively. Contacting residues are shown as ball-and-stick models. For clarity, MIDAS residues are omitted.

Structures are from the I domain:ICAM-1 complex (A), the pseudo-liganded high-affinity I domain (B), the unliganded high-affinity I domain (C), the unliganded intermediate-affinity I domain (D), and the wild-type I domain (E) (1LFA) (). The keys to the color scheme are shown below, with the ICAM-1 or ligand mimetic molecule colored cyan in (A) and (B). The metal ions are colored blue, water molecule and ligating side chain oxygen atoms are colored red, and the chloride ion from the wild-type I domain structure is colored orange. The MIDAS residues and Glu-34 from ICAM-1 in (A) and Glu-272 from a lattice mate I domain in (B) are shown as ball-and-stick models. Metal coordination and hydrogen bonds are represented by solid black lines and gray dotted lines, respectively.

(A–C) The effect of ligand binding (A) and remodeling of the β6-α7 loop in absence of ligand binding (B and C) are compared on I domain structures near the MIDAS (A and B) and the α7-α1 interface (C). Views of (A) and (B) are similar and (C) is approximately orthogonal to (B). Key residues in conformational change are shown in ball-and-stick and labeled, except Gly-240 is unlabeled and shown as a large Cα atom sphere and MIDAS residues Ser-139 and Ser-141 are shown but not labeled. Metal ions are shown as large spheres with the same color as the backbone of the corresponding I domain. The position of the missing metal ion in the unliganded high-affinity structure is simulated by superimposing the metal ion from the pseudo-liganded high-affinity structure and is shown smaller than the other metals. Side chain oxygen atoms are shown as red; water oxygens are omitted for clarity. The directions of major shifts from closed to open conformations are shown with arrows. A portion of ICAM-1 domain-1 containing the metal-coordinating residue E34 is shown in cyan. In (C), the side chain bonds in the designed disulfide bridge (Cys-287-Cys-294) are shown in gray.
(D) The hydrophobic pocket that acts as a detent for the rachet-like movement of the β6-α7 loop. The backbone of the β6-α7 loop and the three residues that occupy the same hydrophobic pocket in the three different conformational states are shown in the same color key as in (A)–(C). The pocket is shown as a GRASP van der Waals surface using the wild-type 1LFA structure with the residues from 287 to the C terminus deleted. The upper hydrophobic pocket is also shown, which is occupied only in the closed conformation (by F292 which is shown in the wild-type structure along with L295). For the closed conformation, the β6-α7 main chain trace is broken between F292 and L295 for clarity. The view is about the same as in (C). On the otherwise gray GRASP surface, residues Ile-150, Phe-153, Ile-237, Ile-259, and Ile-261 are colored yellow to show the hydrophobic pockets.

(A) The three unliganded αL I domain backbones are shown superimposed and viewed centered on the MIDAS. Regions of the backbones that differ structurally are labeled and color keyed; other backbone regions are gray. The metal ions at the MIDAS and the atoms in disulfide-linked cysteines are shown in the same colors as the backbone differences; the cysteine side chain bonds are yellow. As in , the position of the missing metal ion in the high-affinity structure is simulated with a smaller sphere. β6-α7 loop and α7 are shown in gray for clarity; differences in these regions are shown in (B).
(B) The C-terminal fragments encompassing the β6-α7 loop for the three unligated αL conformations and open and closed α2 and αM I domain structures (color keys on bottom left and middle). The side chain bonds of Cys-287 and Cys-294 in the designed disulfide bridge in the high-affinity mutant are shown in yellow; the Cα atom of Cys-299 in the designed disulfide of the intermediate mutant is shown as a green sphere.
(C) The C-terminal fragments encompassing the β6-α7 loop for four different wild-type αL structures in the closed conformation (color keys on bottom right).