The major antennal chemosensory protein of red imported fire ant workers

Some chemosensory proteins (CSPs) are expressed in insect sensory appendages and are thought to be involved in chemical signalling by ants. We identified 14 unique CSP sequences in expressed sequence tag (EST) libraries of the red imported fire ant, Solenopsis invicta. One member of this group (Si-CSP1) is highly expressed in worker antennae, suggesting an olfactory function. A shotgun proteomic analysis of antennal proteins confirmed the high level of Si-CSP1 expression, and also showed expression of another CSP and two odorant-binding proteins (OBPs). We cloned and expressed the coding sequence for Si-CSP1. We used cyclodextrins as solubilizers to investigate ligand binding. Fire ant cuticular lipids strongly inhibited Si-CSP1 binding to the fluorescent dye N-phenyl-naphthylamine, suggesting cuticular substances are ligands for Si-CSP1. Analysis of the cuticular lipids showed that the endogenous ligands of Si-CSP1 are not cuticular hydrocarbons.