Laboratory of Institut de Recerca i Tecnologia Agroalimentàries-Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, 08003 Barcelona, Spain.
BMC Evol Biol. 2010 Feb 11;10:38. doi: 10.1186/1471-2148-10-38.
Aquaporins are integral membrane proteins that facilitate the transport of water and small solutes across cell membranes. These proteins are vital for maintaining water homeostasis in living organisms. In mammals, thirteen aquaporins (AQP0-12) have been characterized, but in lower vertebrates, such as fish, the diversity, structure and substrate specificity of these membrane channel proteins are largely unknown.
The screening and isolation of transcripts from the zebrafish (Danio rerio) genome revealed eighteen sequences structurally related to the four subfamilies of tetrapod aquaporins, i.e., aquaporins (AQP0, -1 and -4), water and glycerol transporters or aquaglyceroporins (Glps; AQP3 and AQP7-10), a water and urea transporter (AQP8), and two unorthodox aquaporins (AQP11 and -12). Phylogenetic analyses of nucleotide and deduced amino acid sequences demonstrated dual paralogy between teleost and human aquaporins. Three of the duplicated zebrafish isoforms have unlinked loci, two have linked loci, while DrAqp8 was found in triplicate across two chromosomes. Genomic sequencing, structural analysis, and maximum likelihood reconstruction, further revealed the presence of a putative pseudogene that displays hybrid exons similar to tetrapod AQP5 and -1. Ectopic expression of the cloned transcripts in Xenopus laevis oocytes demonstrated that zebrafish aquaporins and Glps transport water or water, glycerol and urea, respectively, whereas DrAqp11b and -12 were not functional in oocytes. Contrary to humans and some rodents, intrachromosomal duplicates of zebrafish AQP8 were water and urea permeable, while the genomic duplicate only transported water. All aquaporin transcripts were expressed in adult tissues and found to have divergent expression patterns. In some tissues, however, redundant expression of transcripts encoding two duplicated paralogs seems to occur.
The zebrafish genome encodes the largest repertoire of functional vertebrate aquaporins with dual paralogy to human isoforms. Our data reveal an early and specific diversification of these integral membrane proteins at the root of the crown-clade of Teleostei. Despite the increase in gene copy number, zebrafish aquaporins mostly retain the substrate specificity characteristic of the tetrapod counterparts. Based upon the integration of phylogenetic, genomic and functional data we propose a new classification for the piscine aquaporin superfamily.
水通道蛋白是整合膜蛋白，可促进水和小分子溶质穿过细胞膜的运输。这些蛋白质对于维持生物体内的水平衡至关重要。在哺乳动物中，已经鉴定出 13 种水通道蛋白（AQP0-12），但在鱼类等较低等的脊椎动物中，这些膜通道蛋白的多样性、结构和底物特异性在很大程度上仍是未知的。
从斑马鱼（Danio rerio）基因组中筛选和分离转录本，揭示了 18 个与四足动物水通道蛋白的四个亚家族结构相关的序列，即水通道蛋白（AQP0、-1 和 -4）、水和甘油转运体或水甘油通道蛋白（AQP3 和 AQP7-10）、水和尿素转运体（AQP8）以及两种非典型水通道蛋白（AQP11 和 -12）。核苷酸和推导氨基酸序列的系统发育分析表明，硬骨鱼和人类水通道蛋白存在双重基因平行现象。复制的斑马鱼同工型中有 3 个具有不连锁的基因座，2 个具有连锁的基因座，而 DrAqp8 在两条染色体上重复出现 3 次。基因组测序、结构分析和最大似然重建进一步揭示了存在一个假定的假基因，该假基因显示与四足动物 AQP5 和 -1 相似的混合外显子。在非洲爪蟾卵母细胞中异位表达克隆的转录本表明，斑马鱼水通道蛋白和 Glps 分别转运水或水、甘油和尿素，而 DrAqp11b 和 -12 在卵母细胞中没有功能。与人类和一些啮齿动物不同，斑马鱼 AQP8 的染色体内重复是水和尿素可渗透的，而基因组重复仅转运水。所有水通道蛋白转录本在成年组织中表达，并发现具有不同的表达模式。然而，在一些组织中，编码两个复制的同源基因的转录本似乎存在冗余表达。
斑马鱼基因组编码了功能最齐全的脊椎动物水通道蛋白，与人类同工型具有双重基因平行现象。我们的数据揭示了这些整合膜蛋白在硬骨鱼冠群的根部的早期和特异性多样化。尽管基因拷贝数增加，但斑马鱼水通道蛋白大多保留了与四足动物对应物特征性的底物特异性。基于系统发育、基因组和功能数据的整合，我们提出了一个新的鱼类水通道蛋白超家族分类。
