Figure 1.
Overall structure of CD46-4D in complex with the Ad11 knob.
Ribbon representation of the Ad11 knob trimer, with individual protomers (monomers) shown in blue, green and grey. The knob is bound to three copies of CD46-4D, shown in red. The three-fold axis of the knob lies in a vertical direction. The slightly asymmetric view was chosen to highlight the overall conformation of the CD46-4D molecule on the right hand side.
Table 1.
X-ray data collection and refinement statistics.
Figure 2.
Interactions between Ad11 knob and CD46-4D, and comparison with the structure of the knob in complex with CD46-2D.
(A) Overall contact region for one CD46 molecule (red) bound to two Ad11 knob protomers (blue and green). CD46 domains SCR1 and SCR2 contact the extensive loops of the knob protomers. The HI and DG loops are from the blue protomer, whereas the GH and IJ loops are from the green protomer. Superimposed onto the CD46-4D structure (red) is a ribbon drawing of the CD46-2D structure (grey), which was also determined in complex with Ad11 knob [32]. The superposition was performed using Ad11 knob residues only. The three main contact regions (areas 1, 2 and 3) are boxed, and are shown in atomic detail in panels (B), (C) and (D), respectively. Hydrogen bonds and salt bridges (distance<3.5 Å) present in complexes with CD46-2D and CD46-4D are represented with black dashed lines, whereas similar interactions only present in the complex with CD46-2D are shown in orange dashed lines.
Figure 3.
(A), Overall structure of CD46-4D, with domains SCR1-SCR4 shown in different colors. The protein carries glycosylation at positions Asn49 (SCR1), Asn80 (SCR2) and Asn239 (SCR4). Although only single NAG residues are visible at Asn49 and Asn80, more extensive glycosylation has been modeled to present a view of the protein that resembles its physiologic state (see Methods). (B) Structural alignment of all four repeats of CD46. The conserved cysteine and tryptophan residues, which are hallmarks of SCR domains, are highlighted in yellow and blue, respectively. The five-residue insertion of the unique CD' loop of SCR3 is shown in orange. Sites of N-linked glycosylation are highlighted in orange. Beta strands are indicated with arrows, and are labeled with letters. The alignment was performed with Modeller (http://salilab.org/modeller/modeller.html) using a gap penalty of 3. (C–E). Superpositions of domains SCR2 (yellow, panel C), SCR3 (orange, panel D) and SCR4 (red, panel E) onto SCR1 (grey). Side chains of conserved cysteine and tryptophan residues of each domain are shown in atomic detail to visualize the agreement of the core domains. Also shown as stick models are the three asparagine residues that carry glycosylation. The unique CD' loop in SCR3 is labeled.
Figure 4.
(A) Interface between domains SCR2 (yellow) and SCR3 (orange). (B) Interface between domains SCR3 (orange) and SCR4 (red). In both cases, residues that participate in contact formation are shown in atomic detail. Hydrogen bonds (distance<3.5 Å) are represented with dashed lines. The orientations of both panels are similar to that shown in Figure 3A.
Figure 5.
Comparison of CD46-4D with the structure of the N-terminal four repeats of FH.
The structures of CD46-4D (red) and FH (PDB code 2WII, blue) [42] were superimposed based on residues in SCR3 only. This yielded an r.m.s. deviation of 1.43 Å for 60 pairs of residues. Shown in grey is the C3b ligand that was crystallized in complex with FH. The individual SCR domains of CD46-4D and FH are labeled in red and blue, respectively.
Figure 6.
Ligand binding surfaces in the CD46-4D protein.
Two views of the CD46-4D structure (grey), differing by 180 degrees along a vertical axis, are shown in each case. (A) Surface representations of CD46-4D, with regions implicated in C3b- (red), C4b- (orange) and C3b + C4b-binding (blue) shown in color [35], [36], [37]. Individual residues are indicated. (B) Surface representations of CD46-4D, with regions known to interact with Ad11 and MV [34] shown in blue and green, respectively. Regions that interact with both viruses are highlighted in black. Residues predicted to contact the Streptococcus M protein (M-prot) [38] are shown in purple.
Figure 7.
Conformation of CD46 at the cell surface.
Two views of the entire CD46 protein, indicating possible orientations on the cell surface. The ribbon drawings show the CD46-4D structure, with domains SCR1-SCR4 colored as in Figure 3. Native glycosylation was modeled as described in the Methods section. The STP region (grey box) comprises about 30 amino acids that are not included in our structure. These residues carry O-linked glycosylation and likely serve as a spacer between the base of SCR4 and the membrane. Arrows indicate likely sites of interaction with C3b (panel A) and viruses (panel B).