Moth chemosensory protein exhibits drastic conformational changes and cooperativity on ligand binding

Abstract

Chemosensory proteins (CSPs) have been proposed to transport hydrophobic chemicals from air to olfactory or taste receptors. They have been isolated from several sensory organs of a wide range of insect species. The x-ray structure of CSPMbraA6, a 112-aa antennal protein from the moth Mamestra brassicae (Mbra), was shown to exhibit a novel type of ␣-helical fold. We have performed a structural and binding study of CSPMbraA6 to get some insights into its possible molecular function. Tryptophan fluorescence quenching demonstrates the ability of CSPMbraA6 to bind several types of semio-chemicals or surrogate ligands with M Kd. Its crystal structure in complex with one of these compounds, 12-bromo-dodecanol, reveals extensive conformational changes on binding, resulting in the formation of a large cavity filled by three ligand molecules. Furthermore, binding cooperativity was demonstrated for some ligands, suggesting a stepwise binding. The peculiar rearrangement of CSPMbraA6 conformation and the cooperativity phenomenon might trigger the recognition of chemicals by receptors and induce subsequent signal transduction.

Structure of a specific alcohol-binding site defined by the odorant binding protein LUSH from Drosophila melanogaster

“…There are other chemosensory proteins implicated in insect olfaction including those typified by CSPMbraA6 from the moth Mamestra brassicae. These proteins share no sequence homology with the OBP family and have completely different structures 25,26 .…”

Comparative genomics of chemosensory protein genes reveals rapid evolution and positive selection in ant-specific duplicates

“…The positively selected sites at the cavity opening could possibly affect which chemicals are bound. It has also been suggested that conformational changes in CSPs would trigger recognition in ORs (Campanacci et al, 2003). We can thus speculate that the positively selected sites may also have a role in the interaction between CSPs and ORs.…”

“…HsalCSP11 consists of six a-helices and has a cavity with several openings (Figure 3), like the M. brassicae CSP. Three residues in M. brassicae CSP (Y26, W81 and W94) were suggested to be crucial for binding substrates (Campanacci et al, 2003). These three residues are highly conserved in the ant sequences, two of them showing no variation and one (MbraCSP W94) being mutated only in two sequences.…”

“…These three residues are highly conserved in the ant sequences, two of them showing no variation and one (MbraCSP W94) being mutated only in two sequences. In M. brassicae, the amino acids located at the openings of the binding pocket were suggested to have a role in substrate binding (Campanacci et al, 2003). None of these residues are among the positively selected amino acids inferred in ants, but also the openings to the binding pocket in the ant model are on different locations.…”

“…This did not change the results. To map the positively selected sites to the protein model, we performed a search in the Protein Data Bank and found that ant CSPs are sufficiently similar to known CSP structure of Mamestra brassicae (Campanacci et al, 2003).…”