In vertebrates, glycine amidinotransferase (AGAT), guanidinoacetate methyltransferase (GAMT), and creatine transporter (CT1) are three proteins involved in creatine synthesis. To provide insight into the molecular evolution mechanism of creatine metabolism, we have cloned and identified BbAGAT, BbGAMT, and BbCT1 homologous genes in amphioxus (Branchiostoma belcheri), whose predicted proteins show high identities with AGAT, GAMT, and CT1 proteins in vertebrates. The phylogenetic analysis indicates that amphioxus AGAT, GAMT, and CT1 are branched off at the base of the vertebrate homologous clade, respectively. Genomic structures of BfAGAT, BfGAMT, and BfCT1 show their comparability with the homologs in vertebrate and original characteristic of cephalochordate, which is consistent with animal classification. To determine the expression patterns of BbAGAT, BbGAMT, and BbCT1, whole-mount and section in situ hybridizations are carried out in embryos and adults of amphioxus. During embryogenesis, they are all expressed mainly in mesendoderm and late somites, but BbCT1 is also expressed in differentiating notochord and digestive tract, as well as in the cytoplasm of zygotes and the blastomeres at cleavage stage. In adult, the transcripts of BbAGAT and BbGAMT are detected in the neural cord, gill, nephridia, endostyle, gut, and gonads, while BbCT1 is expressed mainly in the epithelium of gut. The expression pattern of these three genes is similar to their vertebrate homologs. The result reveals that AGAT, GAMT, and CT1, the primary elements of vertebrate creatine metabolism, exist in cephalochordate amphioxus, and are highly conserved during evolution. It also suggests that similar mechanism of creatine synthesis in vertebrate may occur in amphioxus.

中文翻译：

---

文昌鱼中 AGAT、GAMT 和 CT1 的表征：对无脊椎动物到脊椎动物转变过程中肌酸代谢相关分子进化保守性的影响。

在脊椎动物中，甘氨酸脒基转移酶 (AGAT)、胍基乙酸甲基转移酶 (GAMT) 和肌酸转运蛋白 (CT1) 是参与肌酸合成的三种蛋白质。为深入了解肌酸代谢的分子进化机制，我们克隆并鉴定了文蛤（Branchiostoma belcheri）中的 BbAGAT、BbGAMT 和 BbCT1 同源基因，其预测蛋白与脊椎动物中的 AGAT、GAMT 和 CT1 蛋白具有高度一致性。系统发育分析表明，amphioxus AGAT、GAMT 和 CT1 分别在脊椎动物同源进化枝的基部分支。BfAGAT、BfGAMT 和 BfCT1 的基因组结构显示它们与脊椎动物的同源物和头脊索动物的原始特征具有可比性，这与动物分类一致。为了确定 BbAGAT、BbGAMT 和 BbCT1 的表达模式，在文昌鱼的胚胎和成虫中进行了整装和切片原位杂交。在胚胎发生过程中，它们都主要在中内胚层和晚期体节中表达，但BbCT1也在分化的脊索和消化道以及卵裂期受精卵和卵裂球的细胞质中表达。在成人中，BbAGAT 和 BbGAMT 的转录物在神经索、鳃、肾、内管、肠道和性腺中检测到，而 BbCT1 主要在肠道上皮中表达。这三个基因的表达模式类似于它们的脊椎动物同源物。结果表明，脊椎动物肌酸代谢的主要元素 AGAT、GAMT 和 CT1 存在于头脊索动物中，并且在进化过程中高度保守。