Abstract
Insect gustatory receptors are predicted to have a seven-transmembrane structure and are distantly related to insect olfactory receptors, which have an inverted topology compared with G-protein coupled receptors, including mammalian olfactory receptors. In contrast, the topology of insect gustatory receptors remains unknown. Except for a few examples from Drosophila, the specificity of individual insect gustatory receptors is also unknown. In this study, the total number of identified gustatory receptors in Bombyx mori was expanded from 65 to 69. BmGr8, a silkmoth gustatory receptor from the sugar receptor subfamily, was expressed in insect cells. Membrane topology studies on BmGr8 indicate that, like insect olfactory receptors, it has an inverted topology relative to G protein-coupled receptors. An orphan GR from the bitter receptor family, BmGr53, yielded similar results. We infer, from the finding that two distantly related BmGrs have an intracellular N-terminus and an odd number of transmembrane spans, that this is likely to be a general topology for all insect gustatory receptors. We also show that BmGr8 functions independently in Sf9 cells and responds in a concentration-dependent manner to the polyalcohols myo-inositol and epi-inositol but not to a range of mono- and di-saccharides. BmGr8 is the first chemoreceptor shown to respond specifically to inositol, an important or essential nutrient for some Lepidoptera. The selectivity of BmGr8 responses is consistent with the known responses of one of the gustatory receptor neurons in the lateral styloconic sensilla of B. mori, which responds to myo-inositol and epi-inositol but not to allo-inositol.
Highlights
Despite detailed morphological and physiological knowledge of gustation in insects [1,2,3,4,5], this understanding does not extend to the molecular level
Phylogenetic analysis of B. mori gustatory receptors In order to select the receptors to study we performed a phylogenetic analysis of the B. mori GR family
The sequences of all 69 BmGrs together with those of the putative sugar receptors from nine other insect species can be found at the silkworm database
Summary
Despite detailed morphological and physiological knowledge of gustation in insects [1,2,3,4,5], this understanding does not extend to the molecular level. The recent identification of gustatory receptor (GRs) sequences from whole genome sequencing has enabled investigations of molecular gustation in insects. Insect GRs and olfactory receptors (ORs) were originally classified as a large subfamily of G protein-coupled receptors [6,7,8,9]. Subsequent studies have shown that insect ORs have an inverted transmembrane structure and can signal independently of heterotrimeric G proteins in vitro [10,11,12,13,14], olfactory signal transduction has been shown to involve heterotrimeric G-proteins [15]. The transmembrane topology and signalling mechanisms of insect GRs remain unknown there is evidence of both a G-protein independent pathway and a G-protein dependent pathway [16,17,18,19]. DmGr64f is required for detection of a range of sugars and it is thought to act as a co-receptor for other sugar receptors [24]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
