Structural variations of Drosophila tubular lipid-binding proteins, and secretion in antenna and palps

Insect olfactory neurons are inside fluid-filled sensilla that project from the antenna and other chemosensory tissues, thus requiring transporters for water-insoluble volatiles to reach the olfactory receptors. Odorant-binding proteins (OBPs) were assumed to be the transporters, but recent knock-out experiments have re-opened the question of volatile transport.

We previously found many isoforms of extracellular tubular lipid-binding proteins (TULIPs) in proteomic analyses of a variety of insect chemosensory tissues. These proteins are known to carry out a wide range of intracellular and extracellular functions. TULIPs have been overlooked in insect olfaction research, partly because of their high sequence variability, and partly because their genes are frequently mis-annotated in databases. In a published antennal transcriptome from Drosophila melanogaster, we found 24 different TULIPs expressed. We carried out a computational analysis of the antennal TULIPs, using A.I. structural models, and the results show interior hydrophobic pockets with highly variable sizes, shapes, and amino acid compositions.

We examined the expression of CG2016, a D. melanogaster antennal TULIP available as a Gal4/UAS construct expressing yellow fluorescent protein (YFP). We observed strong YFP fluorescence from the antenna and also the maxillary palps. Spinning disc confocal images show that the cells expressing CG2016 are adjacent to the olfactory receptor neurons in basiconic sensilla. These support cells are known to be active in secretion of proteins into the sensillar fluid.

Our results suggest a role for TULIPs in insect olfaction, either in delivery of odorants to olfactory receptor neurons, or in clearing odorants from the neurons after signaling.