Structure of CD55. (a) Structure of the four SCR domains (views at 0° and 90°). The molecule shown is molecule A from crystal form A (see Table 2). The structure is colored from blue at the N terminus to red at the C terminus. All figures were drawn with aesop (M. E. M. Noble, unpublished program). (b) Comparison between eight copies of CD55₁₂₃₄ from three different crystal forms. The molecules are overlaid with α carbons from SCR domain 3 (residues 126–185) so that the lack of variation in the SCR 2/3 domain interface may be appreciated, and the molecules are colored so that the two copies from crystal forms A and B are red and green, respectively, and the four copies from crystal form C are blue. (c) Full model for CD55 in the cell membrane. This model combines the atomic coordinates for the SCR domains with models for the N- and O-linked sugars and the GPI anchor, built as described in the text. The molecule is embedded in a lipid monolayer to allow the extension of the stalk that supports the SCR domains above the membrane to be visualized.

Mapping biological function onto the structure of CD55. Two views of CD55 are shown related by a 180° rotation about the vertical axis. The view in Upper is the same view as that shown in Fig. 1a Right. (a) Charge distribution on the surface of CD55. Charges were calculated by using an amber force-field in grasp (52) and colored evenly around 0 from blue (positive) to red (negative) via white at no charge. (b) Mapping of amino acid positions where substitution is known to affect the ability of CD55 to regulate the CP convertases (10, 11). Positions where amino acid substitution is known to reduce activity to 10% or less of wild-type activity are shown in dark blue; those that reduce activity to 50% or less are shown in light blue. (c) Mapping of amino acid positions where substitution is known to affect the ability to regulate the AP convertases (10, 11). Positions where amino acid substitution is known to reduce activity to 10% or less of wild type are shown in dark green; those that reduce activity to 50% or less are shown in light green.

Model of the complex between CD55 and the von Willebrand factor type A domain (vWF-A) of factor B. b and d show two views of the complex between CD55 and the vWF-A domain of factor B generated as described in the text. CD55 is oriented as in Fig. 1a Left. b shows a surface representation, and d shows a secondary structure trace for both molecules, which are colored as follows: CD55, the hydrophobic potential was calculated by using the program grid and mapped onto the surface in aesop (43) colored from green (most favorable for hydrophobic probe interaction) via yellow to white; and vWF-A of factor B, residues implicated in CD55 binding are colored red, and those implicated in C3b binding are colored orange; positions where mutation has no effect on CD55 sensitivity are colored blue. a and c show each molecule rotated 90° away from the view shown in b to allow inspection of the surfaces buried in the interface. CD55 is shown in the same view as in that of Fig. 1a Right.