Graphic by Matthieu Ng Fuk Chong.

One and a half million: The estimated number of deaths caused by mosquito-borne diseases every year across the world. Prevention is fundamental to curbing these diseases, and doctors recommend the use of nets and vaccines, as well as repellants, which deter mosquitoes by interfering with their sense of smell. Scientists at the National Centre for Biological Sciences (NCBS) have now discovered 110 new proteins in three mosquito species - proteins belonging to a group critical to mosquito olfaction. Repellants targetting this larger group of proteins may function better than existing ones, decreasing instances of dreaded diseases such as dengue and malaria.

Mosquitoes transmit a number of life-threatening diseases to humans when these disease-carrying insects feed on human blood. But how do they find their prey? Like most insects, mosquitoes have an extraordinary sense of smell, facilitated by odorant binding proteins (OBPs) located in hair-like sense organs on their antennae. OBPs are critical to odor detection, for they transport odor molecules (chemicals that each distinct smell is made up of) to odor receptors, in the first step of olfaction. Scientists have identified three sub-classes of OBPs in mosquitoes: classic, atypical and plus-C. Each of these differ in structure, caused by differing levels of the amino acid cysteine, which plays an important role in protein formation. Some of these proteins are present in varying levels across species. This diversity of OBPs and their varying levels of expression increase the number of odors that can be transported by them, helping insects like mosquitoes distinguish scents as they draw near food sources. And that's where mosquito repellants strike. Repellants interfere with the function of these proteins. They inhibit the expression of these proteins, preventing mosquitoes from finding their prey.

In a study that could fuel research in increasing the target proteins of such repellants, Malini Manoharan and Professor Ramanathan Sowdhamini at the NCBS, together with their colleagues in Professor Bernard Offmann's laboratory at the Université de la Réunion and Université de Nantes (France), have discovered 110 new OBPs in the genomes of three disease-transmitting mosquitoes: Anopheles gambiae, Aedes aegypti and Culex quinquefasciatus - notorious carriers of diseases such as malaria, dengue, chikungunya and filariasis. Their research adds 27 new classic, 57 atypical, and 26 novel plus-C OBPs to these mosquito genomes. Their discovery also includes very structurally-different "minus-C" OBPs in Culex. This is the first time minus-C OBPs have been reported from any mosquito genome (but occur in the mosquito's closest relative, the common fruit fly Drosophila melanogaster). This study additionally records the presence of the first ever atypical OBPs (26 of them) in C. quinquefasciatus, thought to play a role in recognising chemical stimuli in larvae and adults.

The study, published in the journal Genome Biology and Evolution, compared the genomes of the three mosquitoes using a bioinformatics approach. The scientists first compiled existing protein sequences from open access databases and built phylogenetic trees that helped establish how each protein is related to others genetically. They mapped the positions of OBP-producing genes on the mosquito genomes. They also compared proteins using bioinformatic algorithms that bring out differences in protein structure, which helped them categorize each OBP into its correct class. This in-depth characterization of OBPs allowed the team to delineate the structure of complex proteins such as atypical OBPs. In such proteins, amino acids cluster into two or more semi-independent parts called domains. These play an important role in protein evolution, due to their ability to be exchanged within a single protein or between many, to form new proteins. Manoharan et al's study confirms that atypical OBPs are composed of two domains. This bit of structural information is vital to studying processes of protein formation. In the case of moquitoes, this information could have huge implications in the field of host recognition.

Apart from deciphering structures of individual proteins, the scientists also noticed very interesting patterns in the genomes of the three mosquitoes. "The genome size increases in Aedes and Culex, when compared to Anopheles. There is some kind of evolution going on in these genomes: the former species carry more OBPs and this could be because they need to adapt to different environmental conditions. It is the OBPs that contribute to this increase in genome size," says lead author Manoharan, who is currently a post-doctoral fellow working with Sowdhamini at NCBS.

Manoharan hopes to publish more of her work on mosquito OBPs soon. "It will be interesting to see exactly what atypical OBPs look like - their detailed structure. We have done a bit of work on predicting the structure of the protein; it is still ongoing. If we get to know the structure, then we can get to the function of the proteins, which would be very interesting."

Getting there could mean getting one step closer to creating more effective mosquito repellants. With the discovery of the structurally-different minusC OBPs, hitherto-unknown in mosquitoes, and 110 novel OBPs, their research has increased the range of protein structures that preventive measures like repellants could target. "The discovery of 110 new OBPs provide a handle to understand the huge reportoire of domains within this family and how diverse odorants may be recognised by mosquitoes. The availability of three-dimensional structures for these domains could lead to the design of novel repellants and thereby help in combating diseases transmitted by mosquitoes," says Sowdhamini, Professor at NCBS and Manoharan's mentor for her doctoral work. "At a more basic level, it exemplifies how simple computational tools like domain finding and sequence search algorithms could be effective in providing clues about the structure and function of newer gene products," she adds.

And has this work proved to be worthy of more attention? "Surely," says Sowdhamini. "Apart from classical OBPs, we will model other domains in this family. They are more challenging to model and this will form one of our future directions."

Click here to access the paper "Comparative Genomics of Odorant Binding Proteins in Anopheles gambiae, Aedes aegypti, and Culex quinquefasciatus" online.