title: Two Minus-C odorant binding proteins from Helicoverpa armigera display higher ligand binding affinity at acidic pH than neutral pH.
abstract: Odorant-binding proteins (OBPs), widely and abundantly expressed in insect olfactory organs, are important for insect olfaction. In this study, two novel Minus-C OBP genes (HarmOBP17 and HarmOBP18) were characterized from the cotton bollworm, Helicoverpa armigera, one of the most important pests in cotton and other crops. Quantitative RT-PCR results indicate that the expression levels of the two genes differed greatly in regard to developmental stages, tissues, and genders. Binding properties of the two OBPs with plant volatiles were investigated at different pH levels using a competitive binding assay. Both OBPs displayed the highest binding affinities (Ki<13 μM) with Beta-ionone among the 85 tested compounds. Surprisingly, the two OBPs (HarmOBP17 in particular) displayed higher binding at pH 5.0 than at pH 7.4 and 10.0. This is the first reported Lepidopteran OBPs showing a contrary pH-dependent ligand binding behavior to other OBPs, implying a different mechanism of ligand binding and release. In addition, binding assays of the mutant OBPs (with the C-terminus eliminated) showed that the C-terminus was important for the ligand binding, but this importance varied with different ligands and OBPs, suggesting that their binding properties depend on the specific interactions between OBP and ligand.
authors.affiliation: Education Ministry Key Laboratory of Integrated Management of Crop Diseases and Pests, College of Plant Protection, Nanjing Agricultural University, Nanjing 210095, China.
authors.forename: Zhao-Qun
authors.initials: ZQ
authors.lastname: Li
authors.forename: Shuai
authors.initials: S
authors.lastname: Zhang
authors.forename: Jun-Yu
authors.initials: JY
authors.lastname: Luo
authors.forename: Jin-Jie
authors.initials: JJ
authors.lastname: Cui
authors.forename: Yan
authors.initials: Y
authors.lastname: Ma
authors.forename: Shuang-Lin
authors.initials: SL
authors.lastname: Dong
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005260:Female; D006863:Hydrogen-Ion Concentration; D019476:Insect Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008297:Male; D008969:Molecular Sequence Data; D009036:Moths / Q000145:classification / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D010802:Phylogeny; D011485:Protein Binding; D018035:Receptors, Odorant / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D016415:Sequence Alignment
chemical_list: D019476:Insect Proteins; D008024:Ligands; D018035:Receptors, Odorant; C045648:odorant-binding protein