Solution Structure of a Chemosensory Protein from the Desert Locust Schistocerca gregaria

Chem. stimuli, generally constituted by small volatile org. mols., are extremely important for the survival of different insect species. In the course of evolution, insects have developed very sophisticated biochem. systems for the binding and the delivery of specific semiochems. to their cognate membrane-bound receptors. Chemosensory proteins (CSPs) are a class of small sol. proteins present at high concn. in insect chemosensory organs; they are supposed to be involved in carrying the chem. messages from the environment to the chemosensory receptors. In this paper, we report on the soln. structure of CSPsg4, a chemosensory protein from the desert locust Schistocerca gregaria, which is expressed in the antennae and other chemosensory organs. The 3D NMR structure revealed an overall fold consisting of six α-helixes, spanning residues 13-18, 20-31, 40-54, 62-78, 80-90, and 97-103, connected by loops which in some cases show dihedral angles typical of β-turns. As in the only other chemosensory protein whose structure has been solved so far, namely, CSP from the moth Mamestra brassicae, four helixes are arranged to form a V-shaped motif; another helix runs across the two V's, and the last one is packed against the external face. Anal. of the tertiary structure evidenced multiple hydrophobic cavities which could be involved in ligand binding. In fact, incubation of the protein with a natural ligand, namely, oleamide, produced substantial changes to the NMR spectra, suggesting extensive conformational transitions upon ligand binding.