Trimeric C-type lectin domains in host defence Hans-Jürgen Hoppe, Kenneth BM Reid  Structure  Volume 2, Issue 12, Pages 1129-1133 (December 1994) DOI: 10.1016/S0969-2126(94)00115-4

Figure 1 Formation and relationships of collectins. (a) In collectin formation, three polypeptide chains, each having a carboxy-terminal, C-type lectin domain (orange), are thought to associate through strong hydrophobic interactions via the α–helices forming the coiled coil part of their neck region (dark blue), whereas additional hydrophobic interactions (yellow) orient the carbohydrate-recognition domains relative to the neck region. The collagen triple helix (dark green) is formed in zipper-like fashion and a short, non-collagenous region (black) at the amino-terminal end of the polypeptide chains is thought to mediate oligomerization via disulfide bonds. (b) Schematic drawing of complement component C1q and the collectins including mannan (or mannose)-binding protein (MBP). The overall dimensions have been determined by electron microscopy and the proteins are drawn approximately to scale. The trimeric subunits of the X-shaped collectins are connected mainly by the amino-terminal regions, whereas in the bouquet-shaped molecules C1q and SP-A the collagenous regions are involved in non-covalent intersubunit associations. The polypeptide chains of SP-A, conglutinin, MBP and the A and C chains of C1q all contain an interruption in the sequence of repeating Gly–Xaa–Yaa triplets, at which point the molecules are thought to bend. Unlike the three chains of the collectins, those of C1q are not identical. The IgG molecule is drawn to illustrate the size of the collectins. The molecular weights of the chains are: SP-D, 43 kDa; conglutinin, 44 kDa; collectin-43, 43 kDa; SP-A, 32 kDa; MBP, 28 kDa; C1q A-chain, 26 kDa; C1q B-chain, 25 kDa; C1q C-chain, 23 kDa. Structure 1994 2, 1129-1133DOI: (10.1016/S0969-2126(94)00115-4)

Figure 1 Formation and relationships of collectins. (a) In collectin formation, three polypeptide chains, each having a carboxy-terminal, C-type lectin domain (orange), are thought to associate through strong hydrophobic interactions via the α–helices forming the coiled coil part of their neck region (dark blue), whereas additional hydrophobic interactions (yellow) orient the carbohydrate-recognition domains relative to the neck region. The collagen triple helix (dark green) is formed in zipper-like fashion and a short, non-collagenous region (black) at the amino-terminal end of the polypeptide chains is thought to mediate oligomerization via disulfide bonds. (b) Schematic drawing of complement component C1q and the collectins including mannan (or mannose)-binding protein (MBP). The overall dimensions have been determined by electron microscopy and the proteins are drawn approximately to scale. The trimeric subunits of the X-shaped collectins are connected mainly by the amino-terminal regions, whereas in the bouquet-shaped molecules C1q and SP-A the collagenous regions are involved in non-covalent intersubunit associations. The polypeptide chains of SP-A, conglutinin, MBP and the A and C chains of C1q all contain an interruption in the sequence of repeating Gly–Xaa–Yaa triplets, at which point the molecules are thought to bend. Unlike the three chains of the collectins, those of C1q are not identical. The IgG molecule is drawn to illustrate the size of the collectins. The molecular weights of the chains are: SP-D, 43 kDa; conglutinin, 44 kDa; collectin-43, 43 kDa; SP-A, 32 kDa; MBP, 28 kDa; C1q A-chain, 26 kDa; C1q B-chain, 25 kDa; C1q C-chain, 23 kDa. Structure 1994 2, 1129-1133DOI: (10.1016/S0969-2126(94)00115-4)

Figure 2 MBP and C1q share a common overall structure and interact in a similar fashion with the complement system. Both the globular heads of C1q and the C-type lectin domains of MBP show increased avidity upon multivalent binding to target surfaces coated with their respective ligands, carbohydrate and antibody. C1q requires the prior recognition of antigens on particles by antibody in order to bind multivalently to the Fc regions of the antibodies and to trigger effector functions, such as activation of the serine protease complex C1r–C1s, via its collagenous stalks. MBP has no requirement for antibody-mediated recognition of foreign particles, but binds to carbohydrate structures on pathogen surfaces although not to host cells. MBP appears to be directed by the orientation of the carbohydrate-binding sites, which are spaced ∼50 Å apart, as well as by collagenous stalks that separate trimers of CRDs by ∼260 Å. The MBP-associated serine protease complex (MASP) plays a similar role to that of the C1r–C1s complex in activating the complement system through a proteolytic cleavage of the C4 complement component. The collagenous stalks of both C1q and MBP can interact with membrane receptors and trigger opsonization, oxidative killing and phagocytosis. Structure 1994 2, 1129-1133DOI: (10.1016/S0969-2126(94)00115-4)

Figure 3 Wheel projection of amino acids forming the α–helical bundle of collectins. The structure of the α–helical bundles present in human SP-D, human MBP and rat MBP are established, and the equivalent regions of conglutinin, collectin-43 and SP-A are drawn for comparison. The sequences begin at the last Gly-Xaa-Yaa triplet of the collagenous region and hydrophobic residues are shown in green, positively charged residues in red and negatively charged residues in cyan. Intron positions are indicated by arrows. The residues of the α–helical coil forming the attachment part of the neck region are also shown; hydrophobic residues engaged in forming the contact point in MBP are boxed. The wheel drawings of the coiled coils of the X-shaped collectins (top row) are in excellent agreement with the proposed role of hydrophobic residues at positions ‘a ’ and ‘d ’, whereas irregularities are shown in these positions for the bouquet-shaped collectins (bottom row). Comparison of hydrophobic residues forming the contact point, as well as the presence of a ‘skip-residue’ ( S underlined in red) in the attachment part of the human MBP neck region, indicate variation in the precise molecular packing of MBP CRDs, consistent with the values of 45 Å and 53 Å for the distance between binding sites on the CRDs of human MBP and rat MBP, respectively. Structure 1994 2, 1129-1133DOI: (10.1016/S0969-2126(94)00115-4)