Carraher Colm, Dalziel Julie, Jordan Melissa D, Christie David L, Newcomb Richard D, Kralicek Andrew V
The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand.
Food Nutrition & Health Team, Food & Bio-based Products Group, AgResearch Private Bag 11008, Palmerston North 4442, New Zealand.
Insect Biochem Mol Biol. 2015 Nov;66:31-41. doi: 10.1016/j.ibmb.2015.09.010. Epub 2015 Sep 28.
Insects have co-opted a unique family of seven transmembrane proteins for odour sensing. Odorant receptors are believed to have evolved from gustatory receptors somewhere at the base of the Hexapoda and have expanded substantially to become the dominant class of odour recognition elements within the Insecta. These odorant receptors comprise an obligate co-receptor, Orco, and one of a family of highly divergent odorant "tuning" receptors. The two subunits are thought to come together at some as-yet unknown stoichiometry to form a functional complex that is capable of both ionotropic and metabotropic signalling. While there are still no 3D structures for these proteins, site-directed mutagenesis, resonance energy transfer, and structural modelling efforts, all mainly on Drosophila odorant receptors, are beginning to inform hypotheses of their structures and how such complexes function in odour detection. Some of the loops, especially the second extracellular loop that has been suggested to form a lid over the binding pocket, and the extracellular regions of some transmembrane helices, especially the third and to a less extent the sixth and seventh, have been implicated in ligand recognition in tuning receptors. The possible interaction between Orco and tuning receptor subunits through the final intracellular loop and the adjacent transmembrane helices is thought to be important for transducing ligand binding into receptor activation. Potential phosphorylation sites and a calmodulin binding site in the second intracellular loop of Orco are also thought to be involved in regulating channel gating. A number of new methods have recently been developed to express and purify insect odorant receptor subunits in recombinant expression systems. These approaches are enabling high throughput screening of receptors for agonists and antagonists in cell-based formats, as well as producing protein for the application of biophysical methods to resolve the 3D structure of the subunits and their complexes.
昆虫已经采用了一类独特的七跨膜蛋白用于气味感知。气味受体被认为是在六足动物基部的某个位置从味觉受体进化而来的，并且已经大幅扩展，成为昆虫纲内气味识别元件的主要类别。这些气味受体包括一个必需的共受体Orco和一个高度多样化的气味“调谐”受体家族中的一员。这两个亚基被认为以某种尚未知晓的化学计量比结合在一起，形成一个能够进行离子型和代谢型信号传导的功能复合物。虽然这些蛋白质仍然没有三维结构，但定点诱变、共振能量转移和结构建模工作（主要针对果蝇气味受体）开始为它们的结构以及此类复合物在气味检测中的功能提供假设依据。一些环，特别是被认为在结合口袋上方形成盖子的第二个细胞外环，以及一些跨膜螺旋的细胞外区域，尤其是第三个，在较小程度上还有第六个和第七个，已被认为与调谐受体中的配体识别有关。Orco和调谐受体亚基之间通过最后的细胞内环和相邻的跨膜螺旋可能存在的相互作用，被认为对于将配体结合转化为受体激活很重要。Orco第二个细胞内环中的潜在磷酸化位点和一个钙调蛋白结合位点也被认为参与调节通道门控。最近已经开发出一些新方法，用于在重组表达系统中表达和纯化昆虫气味受体亚基。这些方法能够以基于细胞的形式对受体进行高通量筛选，以寻找激动剂和拮抗剂，同时还能生产蛋白质，用于应用生物物理方法解析亚基及其复合物的三维结构。
