Frontiers in Cellular Neuroscience Cellular Neuroscience Insect Taste Receptors Evolution of Insect Gustatory Receptors
Frontiers in Cellular Neuroscience Cellular Neuroscience Insect Taste Receptors Evolution of Insect Gustatory Receptors
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Buck和Axel（1991）对嗅觉g蛋白偶联受体（GPCR）基因大家族的发现引发了对味觉GPCR基因的分子生物学研究。因此，当果蝇基因组计划接近完成时，通过计算机算法从果蝇基因组中发现了一个新的候选味觉GPCR基因家族，该基因家族位于7跨膜结构域（Clyne et al., 2000）。他们发现，共有40多个GR基因属于一个新的家族，与果蝇气味受体（OR）基因共享一个特征基序，这些基因在味觉组织中特异性表达。后来对整个果蝇基因组的分析预测了总共68个GR基因（Robertson et al., 2003；表1）。随后的分子和功能研究表明，它们大多数编码gr。这些基因被命名为“Gr”（味觉受体），后跟一个染色体位点编号。当GR基因串联聚集时，以“a”开头的字母被附加在末尾，如Gr5a或Gr10b。果蝇GR基因中，6个位于X染色体上，38个位于果蝇第二染色体上，24个位于果蝇第三染色体上。每个GR基因编码一个长度约为350-550个氨基酸残基的七跨膜受体蛋白。整体序列差异很大，随机选择的两个gr之间的同源性平均低15-25%，明显低于or。然而，GRs和ORs在第七跨膜和C末端结构域共享一个共同的氨基残基基序，表明它们是从祖先的化学受体家族进化而来的。基因组分析表明，GR/ OR基因的扩增只发生在昆虫纲中。秀丽隐杆线虫是一个例外，因为这种动物携带一些功能性GR基因，这将在后面讨论。秀丽隐杆线虫的大部分化学感受器、海胆（Strongylocentrotus purpuratus）的所有化学感受器或脊椎动物的所有味觉和嗅觉感受器都不具有GR基因。
Identification of a large family of olfactory G-protein coupled receptor (GPCR) genes by Buck and Axel (1991) provoked searches for taste GPCR genes by molecular biology. A novel family of candidate taste GPCR genes was thus found from the Drosophila genome by a computer algorithm to hit seven-transmembrane domain when the Drosophila genome project was nearly completed (Clyne et al., 2000). They showed that a total of more than 40 GR genes that belong to a novel family and share a signature motif with Drosophila odorant receptor (OR) genes are expressed specifically in taste tissues. Later analysis of the whole Drosophila genome predicted a total of 68 GR genes (Robertson et al., 2003; Table 1). Subsequent molecular and functional studies showed that most of them encode GRs. The genes are given the name " Gr " (gustatory receptor) followed by a chromo-somal locus number. When GR genes are tandemly clustered, an alphabet starting from " a " was appended at the end like Gr5a or Gr10b. Among Drosophila GR genes, 6 are located on the X chromosome while 38 and 24 are found on the second and the third Drosophila chromosome, respectively. Each GR gene encodes a seven-transmembrane receptor protein of about 350–550 amino residues in length. The overall sequences are very divergent with homologies between two randomly chosen GRs as low as 15–25% on the average, which is significantly lower than those for ORs. However, GRs and ORs share a common amino residue motif in the seventh transmembrane plus C terminal domain, indicating that they have evolved from an ancestral chemoreceptor family. Genome analysis suggests that a robust expansion of GR/ OR genes has occurred only in the class Insecta. The nematode Caenorhabditis elegans is an exception since the animal carries a few functional GR genes as will be discussed later. The majority of chemoreceptors in C. elegans, all chemoreceptors in sea urchin (Strongylocentrotus purpuratus) or all gustatory and olfactory receptors in vertebrates do not have GR genes.