The structure of the Drosophila melanogaster sex peptide: Identification of hydroxylated isoleucine and a strain variation in the pattern of amino acid hydroxylation

In Drosophila melanogaster mating triggers profound changes in the behaviour and reproductive physiology of the female. Many of these post-mating effects are elicited by sex peptide (SP), a 36-mer pheromone made in the male accessory gland and passed to the female in the seminal fluid. The peptide comprises several structurally and functionally distinct domains, one of which consists of five 4-hydroxyprolines and induces a female immune response. The SP gene predicts an isoleucine (Ile14) sandwiched between two of the hydroxyprolines of the mature secreted peptide, but the identity of this residue was not established by peptide sequencing and amino acid analysis, presumably because of modification of the side chain. Here we have used matrix-assisted laser desorption ionisation mass spectrometry together with Fourier-transform ion cyclotron resonance mass spectrometry to show that Ile14 is modified by oxidation of the side chain - a very unusual post-translational modification. Mass spectrometric analysis of glands from different geographical populations of male D. melanogaster show that SP with six hydroxylated side chains is the most common form of the peptide, but that a sub-strain of Canton-S flies held at Leeds only has two or three hydroxylated prolines and an unmodified Ile14. The D. melanogaster genome has remarkably 17 putative hydroxylase genes that are strongly and almost exclusively expressed in the male accessory gland, suggesting that the gland is a powerhouse of protein oxidation. Strain variation in the pattern of sex peptide hydroxylation might be explained by differences in the expression of individual hydroxylase genes.