Structure and mechanism in salivary proteins from blood-feeding arthropods

Abstract

The saliva of blood-feeding arthropods contains rich mixtures of ligand binding proteins targeted at inhibiting hemostasis and inflammation in the host. Since blood feeding has evolved many times, different taxonomic groups utilize completely different families of proteins to perform similar tasks. Structural studies performed on a number of these proteins have revealed biologically novel and sophisticated mechanisms used to perform their functions. Here, the results of these structural and mechanistic studies are reviewed.

Fig 1

Structures of nitrophorins. A: Ribbon diagram of the NP4-ammonia complex (PDB accession number: 1D2U, (Roberts, et al., 2001)). The lipocalin β-barrel is colored green: heme is a stick diagram with carbon colored white, oxygen colored red, nitrogen colored blue, and iron colored orange. Coordination bond of the proximal imidazole of histidine (His, in the same color scheme as heme) with the heme iron is shown. Ammonia occupies the distal pocket. The side chain of Leu 30 is colored red, and is shown in its position in the absence of NO. B: The NO-complex of NP4 (1ERX (Weichsel, et al., 2000)). In this case the lipocalin structure is colored cyan. The rest of the complex is colored as in A. Note the conformational changes in loops surrounding the binding pocket and resulting in a shift in the position of Leu 130. These changes act to bury the NO ligand (in the distal pocket) in a hydrophobic environment. C: Detail of the NP4-histamine complex (1IKE, (Roberts, et al., 2001)). Carbon atoms from protein residues are shown in cyan, from heme are shown in white, and from histamine are shown in red. Atom colors are as in A, and hydrogen bonds/ionic bonds are shown as red dashed lines. D: Structure of Cimex lectularis nitrophorin (1YJH, (Weichsel, et al., 2005)). The protein structure is shown in cyan, with heme as a stick diagram. Heme coloring is the same as in A.

Fig. 2

Structure of the triabin-thrombin complex (1AVG, (Fuentes-Prior, et al., 1997)). A: Ribbon diagram of the triabin-thrombin complex, with thrombin (light) and triabin (dark) labeled. B: Detail of the triabin fold showing rearrangement of strands A-D from the up-down-up-down arrangement of a true lipocalin to up-up-down-down.

Fig. 3

Structures of tick lipocalins. A: Ribbon diagram of the Ra-HBP2 structure with bound histamine ligands shown as a stick diagram (1QFT, (Paesen, et al., 1999)). The position of histamine bound in the low-affinity site is marked L and that in the high affinity site is marked H. B: For comparison, a ribbon diagram of the structure of monotonin with histamine ligand (Hist) bound (3BU1, (Mans, et al., 2008)). Note that the position of the ligand corresponds to the L site in Ra-HBP2. C: Ribbon diagram of the OmC1 structure with the ligand (R)-12-hydroxy-cis-9-octadecenoic acid (Ric) bound (2CM4, (Roversi, et al., 2007)). D: Detail of histamine binding in monotonin. Amino acid residues are labeled. Histamine is shaded darker than the protein. Water molecules are indicated by spheres, and hydrogen / ionic bonds are indicated by dashed lines. E: Serotonin binding in AM182 (3BRN, (Mans, et al., 2008)). Stick diagram is colored as in D. Note the similarity in positioning of the ligand between D and E, as well as the conserved nature of interactions involving the ammonium group. F:Detail of the interaction of (R)-12-hydroxy-cis-9-octadecenoic acid with OmC1 (Roversi, et al., 2007). Hydrogen bonding / ionic interactions of the carboxyl and hydroxyl groups of the fatty acid ligand are shown as dashed lines.

Fig. 4

Structures of D7 type proteins from mosquitoes. A: Ribbon diagram of the AeD7-leukotriene E₄ complex (3DZT, (Calvo, et al., 2009)). The protein structure is shown in cyan, and the fatty acid portion of LTE4 is shown as a stick diagram with carbon colored blue, and oxygen red. Note that the C-terminal portion of the C-terminal domain is a partially disordered coil. B: Ribbon diagram of the AeD7-norepinephrine complex (3DYE, (Calvo, et al., 2009)). The protein portion is colored green and norepinephrine is shown as a stick diagram with carbon and oxygen colored red and nitrogen colored blue. Note that norepinephrine binding causes the C-terminal coil to order into an eighth α-helix, helix H₂. C: Ribbon diagram of the D7R4-serotonin complex (2QEH, (Mans, et al., 2007). The protein portion of the molecule is shown in white and the serotonin ligand is shown as a stick diagram with carbon and oxygen colored red and nitrogen blue. Disulfide cysteines are shown in yellow. The helices are labeled A-H with A being the most N-terminal. Helix H occupies a nearly identical position to helix H₂ in the norepinephrine complex of AeD7. D: Stick diagram showing binding pocket detail of the D7R4 complex. Protein residues are shown with carbon in green, oxygen in red and nitrogen in blue. Serotonin is shown with carbon and oxygen in red and nitrogen in blue. The large number of hydrogen bonds and ionic interactions stabilizing the ligand are shown as dashed red lines. E: Stick diagram showing binding pocket detail of the AeD7-LTE₄ complex. Protein residues are shown as in D. LTE4 is shown with carbon as blue and oxygen as red. Hydrogen bonds and ionic interactions are shown as in D.