Department of Neuroscience, Functional Pharmacology, Uppsala University, Biomedical Center, Box 593, 75 124, Uppsala, Sweden.
Department of Biotechnology, Indian Institute of Technology Madras, Chennai, 600036, India.
BMC Evol Biol. 2014 Dec 21;14:270. doi: 10.1186/s12862-014-0270-4.
G protein-coupled receptors (GPCRs) play a central role in eukaryotic signal transduction. However, the GPCR component of this signalling system, at the early origins of metazoans is not fully understood. Here we aim to identify and classify GPCRs in Amphimedon queenslandica (sponge), a member of an earliest diverging metazoan lineage (Porifera). Furthermore, phylogenetic comparisons of sponge GPCRs with eumetazoan and bilaterian GPCRs will be essential to our understanding of the GPCR system at the roots of metazoan evolution.
We present a curated list of 220 GPCRs in the sponge genome after excluding incomplete sequences and false positives from our initial dataset of 282 predicted GPCR sequences obtained using Pfam search. Phylogenetic analysis reveals that the sponge genome contains members belonging to four of the five major GRAFS families including Glutamate (33), Rhodopsin (126), Adhesion (40) and Frizzled (3). Interestingly, the sponge Rhodopsin family sequences lack orthologous relationships with those found in eumetazoan and bilaterian lineages, since they clustered separately to form sponge specific groups in the phylogenetic analysis. This suggests that sponge Rhodopsins diverged considerably from that found in other basal metazoans. A few sponge Adhesions clustered basal to Adhesion subfamilies commonly found in most vertebrates, suggesting some Adhesion subfamilies may have diverged prior to the emergence of Bilateria. Furthermore, at least eight of the sponge Adhesion members have a hormone binding motif (HRM domain) in their N-termini, although hormones have yet to be identified in sponges. We also phylogenetically clarified that sponge has homologs of metabotropic glutamate (mGluRs) and GABA receptors.
Our phylogenetic comparisons of sponge GPCRs with other metazoan genomes suggest that sponge contains a significantly diversified set of GPCRs. This is evident at the family/subfamily level comparisons for most GPCR families, in particular for the Rhodopsin family of GPCRs. In summary, this study provides a framework to perform future experimental and comparative studies to further verify and understand the roles of GPCRs that predates the divergence of bilaterian and eumetazoan lineages.
G 蛋白偶联受体（GPCRs）在真核信号转导中起着核心作用。然而，在后生动物最早的起源中，这种信号系统的 GPCR 组成部分还不完全清楚。在这里，我们的目标是鉴定和分类腔肠动物（海绵）中的 GPCR，腔肠动物是最早分化的后生动物谱系（多孔动物门）的成员。此外，海绵 GPCR 与真后生动物和两侧对称动物 GPCR 的系统发育比较对于我们理解后生动物进化根源的 GPCR 系统至关重要。
我们从最初的 282 个预测 GPCR 序列数据集中排除了不完整的序列和假阳性序列后，在海绵基因组中鉴定并分类了 220 个 GPCR。系统发育分析表明，海绵基因组包含属于五个主要 GRAFS 家族中的四个家族的成员，包括谷氨酸（33）、视紫红质（126）、黏附（40）和卷曲（3）。有趣的是，海绵视紫红质家族序列与真后生动物和两侧对称动物谱系中的同源序列没有同源关系，因为它们在系统发育分析中分别聚类形成海绵特有的组。这表明海绵视紫红质与其他基底后生动物的视紫红质有很大的分歧。一些海绵黏附蛋白聚类位于大多数脊椎动物中常见的黏附亚家族的基础上，这表明一些黏附亚家族可能在两侧对称动物出现之前就已经分化了。此外，海绵的至少 8 个黏附蛋白在其 N 末端具有激素结合基序（HRM 结构域），尽管海绵中的激素尚未被鉴定。我们还通过系统发育分析澄清了海绵与其他后生动物基因组的代谢型谷氨酸（mGluR）和 GABA 受体具有同源物。
我们对海绵 GPCR 与其他后生动物基因组的系统发育比较表明，海绵包含一组明显多样化的 GPCR。这在大多数 GPCR 家族的家族/亚家族水平比较中显而易见，特别是视紫红质家族的 GPCR。总之，本研究为进一步验证和理解 GPCR 的作用提供了一个框架，这些 GPCR 的作用可以追溯到两侧对称动物和真后生动物谱系的分化之前。
