Expression pattern of a 'Plus-C' class odorant binding protein in the antenna of the malaria vector Anopheles gambiae

In the malaria mosquito Anopheles gambiae (Ag), olfaction plays a crucial role in various behaviours, most strikingly in the seeking of females after a blood meal. The first step of odorant recognition in antennal sensilla involves soluble odorant binding proteins (OBPs), which transfer odorous compounds to olfactory receptors (ORs) in the dendritic membrane of sensory neurons. A particular OBP subtype of the 'Plus-C' class, called AgOBP48, is abundantly transcribed in female antennae and partially down-regulated after a blood meal, suggesting a possible role in host detection. In the present study, we have identified the AgOBP48-expressing cells, explored their antennal topography and determined their position relative to cells that express the 'classic' AgOBP1, the AgOR co-receptor (AgOrco) and the receptor AgOR1. By means of two-colour whole-mount fluorescence in situ hybridization it was found that AgOBP48 was expressed in cells, which are closely associated with AgOrco-expressing sensory neurons. Furthermore, AgOBP48 was not expressed in the same cells as AgOBP1, but subpopulations of AgOBP48- and of AgOBP1-expressing cells were found closely associated and adjacent to sensory neurons expressing AgOR1. Together, the results indicate that cells that express either AgOBP48, AgOBP1 or AgOR1 are housed together in distinct olfactory sensilla and that an interplay of the proteins may contribute to the specific responsiveness of the sensillum to distinct odorants.