Structure of an Odorant-Binding Protein from the Mosquito Aedes aegypti Suggests a Binding Pocket Covered by a pH-Sensitive “Lid”

Abstract

BackgroundThe yellow fever mosquito, Aedes aegypti, is the primary vector for the viruses that cause yellow fever, mostly in tropical regions of Africa and in parts of South America, and human dengue, which infects 100 million people yearly in the tropics and subtropics. A better understanding of the structural biology of olfactory proteins may pave the way for the development of environmentally-friendly mosquito attractants and repellents, which may ultimately contribute to reduction of mosquito biting and disease transmission.MethodologyPreviously, we isolated and cloned a major, female-enriched odorant-binding protein (OBP) from the yellow fever mosquito, AaegOBP1, which was later inadvertently renamed AaegOBP39. We prepared recombinant samples of AaegOBP1 by using an expression system that allows proper formation of disulfide bridges and generates functional OBPs, which are indistinguishable from native OBPs. We crystallized AaegOBP1 and determined its three-dimensional structure at 1.85 Å resolution by molecular replacement based on the structure of the malaria mosquito OBP, AgamOBP1, the only mosquito OBP structure known to date.ConclusionThe structure of AaegOBP1 ( = AaegOBP39) shares the common fold of insect OBPs with six α-helices knitted by three disulfide bonds. A long molecule of polyethylene glycol (PEG) was built into the electron-density maps identified in a long tunnel formed by a crystallographic dimer of AaegOBP1. Circular dichroism analysis indicated that delipidated AaegOBP1 undergoes a pH-dependent conformational change, which may lead to release of odorant at low pH (as in the environment in the vicinity of odorant receptors). A C-terminal loop covers the binding cavity and this “lid” may be opened by disruption of an array of acid-labile hydrogen bonds thus explaining reduced or no binding affinity at low pH.

Crystal and solution structures of an odorant-binding protein from the southern house mosquito complexed with an oviposition pheromone

“…3). It has been previously observed in the structures of AgamOBP1 and AaegOBP1 that the C-terminal carboxylate group interacts with the hydroxyl of Tyr54 and the δ-nitrogen of His23, making a potential pH-sensing triad that locks the C terminus onto helix 1 and helix 3 (25). We found that the same interaction is well conserved in the structure of the CquiOBP1·MOP complex.…”

“…Because helices 4 and 5 also form the dimeric interface of the crystal structure, the two MOP-binding tunnels of each CquiOBP1 dimeric unit meet and connect at the dimer interface. Notably, the same ligand-binding pocket was serendipitously identified in the previous structures of AgamOBP1 and AaegOBP1 by a bound PEG molecule from the crystallization solution (25,26). In those structures, a single PEG molecule 55-80 atoms long gets into one OBP molecule from an opening created by helices 1, 3, and 4; runs through the central cavities and the connected hydrophobic tunnels between helices 4 and 5; and comes out through the second OBP molecule of the dimer (Fig.…”

“…These findings suggest a different mechanism, because CquiOBP1 lacks a C-terminus region that is long enough in the moth PBPs to fold into an extra α-helix at low pH. Additionally, the structures of another two mosquito OBPs, AgamOBP1 from the malaria mosquito Anopheles gambiae and AaegOBP1 from the yellow fever mosquito Aedes aegypti, were recently solved by X-ray crystallography (25,26). Those two OBPs, closely related in sequence to each other and to CquiOBP1, reveal strikingly similar 3D structures despite remarkable difference in the chemical ecology of these three species.…”

Insect olfaction from model systems to disease control

“…Before this work, only the ortholog of Orco (153) and a small number of OBPs (154, 155) had been identified. The crystal structure of AaegOPB1 has been solved (156), and the odorant binding profile of AaegOBP22 has been characterized (157). However, the contribution that these OBPs make to odorant reception in vivo has not been determined.…”

Review An overview of odorant-binding protein functions in insect peripheral olfactory reception

“…Their affinities for ligands are also pH-dependent (Wogulis et al, 2006;Leite et al, 2009;Mao et al, 2010), whereas a C-terminal loop lacks a C-terminus region, which is also identical to Lma PBP with a direct end after the sixth helix in Leucophaea maderae (Lartigue et al, 2003) comparing with moth OBPs, folding into an extra α-helix at low pH. Hydrogen bonds involving the C-terminal loop, which forms part of the binding pocket, will be broken at low pH and make the loop open, releasing further ligands.…”