Distinct Subfamilies of Odorant Binding Proteins in Locust (Orthoptera, Acrididae): Molecular Evolution, Structural Variation, and Sensilla-Specific Expression.

Xingcong Jiang,Jürgen Krieger,Heinz Breer,Pablo Pregitzer

doi:10.3389/fphys.2017.00734

Abstract

Odorant binding proteins (OBPs) play an important role in insect olfaction, facilitating transportation of odorant molecules in the sensillum lymph. While most of the researches are concentrated on Lepidopteran and Dipteran species, our knowledge about Orthopteran species is still very limited. In this study, we have investigated OBPs of the desert locust Schistocerca gregaria, a representative Orthopteran species. We have identified 14 transcripts from a S. gregaria antennal transcriptome encoding SgreOBPs, and recapitulated the phylogenetic relationship of SgreOBPs together with OBPs from three other locust species. Two conserved subfamilies of classic OBPs have been identified, named I-A and II-A, exhibiting both common and subfamily-specific amino acid motifs. Distinct evolutionary features were observed for subfamily I-A and II-A OBPs. Surface topology and interior cavity were elucidated for OBP members from the two subfamilies. Antennal topographic expression revealed distinct sensilla- and cellular- specific expression patterns for SgreOBPs from subfamily I-A and II-A. These findings give first insight into the repertoire of locust OBPs with respect to their molecular and evolutionary features as well as their expression in the antenna, which may serve as an initial step to unravel specific roles of distinct OBP subfamilies in locust olfaction.

Highlights

In insects, the process of olfactory signal processing begins in hair-like cuticle appendages, called sensilla, located mainly on the antennae and palps (Steinbrecht, 1996; Hansson and Stensmyr, 2011; Suh et al, 2014)
Toward an identification of Odorant binding proteins (OBPs) from S. gregaria and a comprehensive characterization of OBPs in locust species, we have performed a homology-based data mining of an antennal transcriptome which resulted in 14 transcripts putatively encoding SgreOBPs
The motif 1 and motif 2 appeared as common motifs in all OBPs of both subfamilies, whereas the other four motifs fit either the repertoire of subfamily I-A OBPs or the repertoire of subfamily II-A OBPs

Summary

Introduction

The process of olfactory signal processing begins in hair-like cuticle appendages, called sensilla, located mainly on the antennae and palps (Steinbrecht, 1996; Hansson and Stensmyr, 2011; Suh et al, 2014). The hydrophobic odorant molecules enter the sensillum via the porous cuticle and have to pass the aqueous lymph till reaching the chemosensory membrane of the sensory neurons (Vogt et al, 1999; Leal, 2013; Suh et al, 2014) This passage is thought to be mediated by small soluble proteins enriched in the sensilla lymph, the so called odorant binding proteins (OBPs), which are produced and secreted by accessory cells (Pelosi et al, 2006, 2017). The tertiary structure of insect OBPs consists of six α-helices forming an interior binding cavity This structure is maintained and stabilized by disulfide bridges formed by conserved C-residues (Leal et al, 1999; Scaloni et al, 1999; Sandler et al, 2000). It is proposed that the C-terminal domain that is variable in length can spatially interfere with the interior binding cavity and may affect the ligand binding mechanism (Damberger et al, 2000; Horst et al, 2001; Tegoni et al, 2004; Pelosi et al, 2017)

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Conclusion