Length analyses of Drosophila odorant receptors

Odorant receptors comprise a unique family of G-protein-coupled seven-transmembrane receptors both in mammals and insects. In the fruit fly Drosophila melanogaster, all 61 candidate odorant receptor genes have been identified based on the complete genome sequence, and their expression patterns have been examined. A given odorant receptor is expressed in the antenna or maxillary palp, or not expressed at all. Here we have applied a set of statistical analyses to the length of the extra- and intracellular loops and terminals (LTs) of Drosophila odorant receptors to examine possible inter- and intramolecular relations at the population level. We have first provided some useful statistical information such as mean length values and length histograms to depict a general nature of Drosophila odorant receptors at the population level, after focusing on discrepancy on assigning transmembrane domains between researchers. In a preferable transmembrane assignment, all extracellular LTs, especially the second extracellular loops, were relatively large in length, suggesting their functional significance. Somewhat surprisingly, principle component analysis (PCA) indicated that the maxillary palp receptors were almost as diverse as the antenna receptors despite their much smaller population size. PCA together with histograms also revealed that receptors with an abnormal length configuration tended not to be expressed, suggesting that LT length deviations are related to transcriptional silencing of odorant receptor genes. Rank transformation tests pointed out possible LTs that could have different length between differently expressed receptors at the population level. Taken together, length analyses provide us with a general picture, i.e. "length configuration," of Drosophila odorant receptors at the population level that could point out putatively important functional sites for experimental studies.