Structure of vaccinia complement protein in complex with heparin and potential implications for complement regulation

Abstract

Vaccinia virus complement control protein (VCP), a homolog of the regulators of the complement activation family of proteins, inhibits complement activation through mechanisms similar to human fluid-phase complement regulators factor H and C4b-binding protein. VCP has a heparin-binding activity that assists vaccinia in host interactions. Interaction with cell-surface polyanions like heparin is centrally important in the functioning of fluid-phase complement regulators and is the basis of host-target discrimination by the alternative pathway. We report the structure of VCP in complex with a heparin decasaccharide, which reveals changes in VCP that might be pertinent to complement regulation. Properties that VCP shares with fluid-phase complement regulators suggest that such conformational changes may be of relevance in the functioning of other complement regulators. Additionally, comparison of VCP-heparin interactions with potentially similar interactions in factor H might enable understanding of the structural basis of familial hemolytic uremic syndrome, attributed to mutational disruption of heparin and C3b binding by factor H.

Fig. 2.

Conformational change. The four SCRs of VCP-A/hepC (green), VCP-B/hepD (blue) are superimposed on those of native VCP (gold). Sidechains that might contact C3b/C4b in the native (cyan) and heparin-bound (red) conformations are also shown.

Fig. 5.

^FHSCR20 binding to heparin. A stereo pair of the modeled ^FHSCR20/heparin complex. Main-chain trace of SCR20 is shown in pink. The six basic residues that are within liganding distance of heparin are shown in blue, with sidechain positions modeled from the corresponding VCP residue. The heparin model is derived from that in this structure by extending the reducing end by one repeating unit, using the helical parameters of the experimental structure. Carbon atoms are colored green, oxygens are red, nitrogens are blue, and sulfurs are yellow.

Fig. 1.

VCP–heparin interactions. ligplot (40) representation of VCP–heparin interaction (VCP-A/hepC). Heparin bonds are colored purple; protein bonds, gold. Carbon atoms are colored black, oxygen is red, nitrogen is blue, and sulfur is yellow. Hydrogen bonds are shown as dashed green lines, and hydrophobic contacts are shown as semicircle with lines.

Fig. 6.

Residues that delimit motion. A surface representation of VCP SCR3 (light green) and SCR4 (light purple) is shown. Surface patches of residues, the mutation of which might result in altered motion, are colored magenta, with corresponding numbers of FH residues (Fig. 4) in parentheses.

Fig. 3.

Heparin binding relative to C3b binding. Each SCR is shown as an oval, cross hatched to show C3b binding and checkered to show heparin binding. FH2 and FH3 are the second and third C3b-binding sites in FH, with their associated heparin sites. Only the first four SCRs of C4b-BP of the α chain are shown. SCRs are numbered to show the polypeptide chain direction.

Fig. 4.

Sequence comparison. The sequence correspondence between ^FHSCR20 and ^VCPSCR4 is shown. Identical or conservative replacements are indicated by a period. Basic residues that are ligands to heparin in the VCP/heparin complex or in the modeled ^FHSCR20/heparin complex are colored red. Mutations in ^FHSCR20 that have been characterized in HUS patients are shown by asterisks.