Identification of the western tarnished plant bug (lygus hesperus) olfactory co-receptor orco: expression profile and confirmation of atypical membrane topology

Authors: Hull, Joe; Hoffmann, Eric; Perera, Omaththage; Snodgrass, Gordon

Submitted to: Archives of Insect Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2012
Publication Date: 7/26/2012
Citation: Hull, J.J., Hoffmann, E.J., Perera, O.P., Snodgrass, G.L. 2012. Identification of the western tarnished plant bug (lygus hesperus) olfactory co-receptor orco: expression profile and confirmation of atypical membrane topology. Archives of Insect Biochemistry and Physiology. 81(4):179-198.

Interpretive Summary: Similar to most insects, the ability of the western tarnished plant bug (Lygus hesperus) to perceive and interact with its environment is largely governed by its antennae. As the insect’s primary sensory organs, the antennae house the olfactory system that allow insects to detect and discriminate odors critical to identification of food sources and mating partners. A key molecular component of that system is a cell surface protein identified as the olfactory receptor co-receptor (Orco). The interactions between Orco and an odorant specific receptor results in the formation of an ion channel that opens in response to odorant binding. As a consequence, Orco is critical for olfactory receptor functionality and thus the detection and discrimination of odorants. To begin to understand the molecular basis of this system in the western tarnished plant bug, we cloned and characterized the L. hesperus Orco (LhOrco). LhOrco has many structural features shown to be functionally important in Orco proteins identified from other insect species. In addition, LhOrco adopts the atypical membrane topology characteristic of other Orco proteins and is predominantly expressed in adult antennae with lower expression in traditionally non-olfactory chemosensory tissues of the proboscis and legs. Taken together, our results strongly suggest that LhOrco functions in mediating Lygus odorant perception and discrimination. Given this role, targeted disruption of this essential gene product, either at the transcript or protein level, could present a highly efficacious means of controlling Lygus pest populations.

Technical Abstract: Lygus hesperus (western tarnished plant bug) is an agronomically important pest species of numerous cropping systems. Similar to other insects, a critical component underlying behaviors is the perception and discrimination of olfactory cues. Consequently, the molecular basis of olfaction in this species is of interest. To begin to address this issue, we utilized homology-based PCR methods to identify the Lygus olfactory receptor co-receptor (Orco) ortholog, a receptor that has been shown to be essential for olfaction. The L. hesperus Orco (LhOrco) shares significant sequence homology with known Orco proteins in other insects. Parallel experiments using the sympatric sister species L. lineolaris (tarnished plant bug) revealed that the Lygus Orco gene was completely conserved. Surprisingly, a majority of the membrane topology prediction algorithms used predicted LhOrco to have both the N terminus and C terminus intracellular. In vitro immunofluorescent microscopy experiments designed to probe the membrane topology of transiently expressed LhOrco, however, refuted those predictions and confirmed that the protein adopts the inverted topology (intracellular N terminus and an extracellular C terminus) characteristic of Orco proteins. RT-PCR analyses indicated that LhOrco transcripts are predominantly expressed in adult antennae with lower expression observed in traditionally non-olfactory chemosensory tissues of the proboscis and legs. Expression is not developmentally regulated as transcripts were detected in all nymphal stages as well as eggs. Taken together, the results suggest that LhOrco likely plays a critical role in mediating Lygus odorant perception and discrimination.