Molecular characterization of ligand selectivity of the sex peptide receptors of Drosophila melanogaster and Aedes aegypti.

Drosophila melanogaster sex peptide receptor (DrmSPR) is a G protein-coupled receptor (GPCR) with 'dual ligand selectivity' towards sex peptide (SP) and myoinhibitory peptides (MIPs), which are only remotely related to one another. SPR is conserved in almost all the sequenced lophotrochozoan and ecdysozoan genomes. SPRs from non-drosophilid taxa, such as those from the mosquitoes Aedes aegypti (AeaSPR), Anopheles gambiae (AngSPR), and the sea slug Aplysia californica (ApcSPR), are highly sensitive to MIP, but not to SP. To understand how Drosophila SPRs evolved their SP sensitivity while maintaining MIP sensitivity, we examined ligand selectivity in a series of chimeric GPCRs that combine domains from the SP-sensitive DrmSPR and the SP-insensitive AeaSPR. We found replacement of Pro 238 (P238) in DrmSPR with the corresponding residue from AeaSPR (L310) reduced its SP sensitivity 2.7 fold without altering its MIP sensitivity. The P238 residue located in the third extracellular loop (ECL3) is conserved in Drosophila SPRs and in SPR from the moth Bombyx mori (BomSPR), which is considerably more sensitive to SP than AeaSPR, AngSPR, or ApcSPR. We found, however, that rather than improving AeaSPR's sensitivity to SP, replacement of L310 in AeaSPR with Pro significantly reduces its MIP sensitivity. Thus, our identification of a single amino acid residue critical for SP sensitivity, but not for MIP sensitivity is an important step in clarifying how DrmSPR evolved the ability to detect SP.
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