Institute of Zoology, University of Hamburg, D-20146, Hamburg, Germany.
Instituto de Ciências Biológicas, Universidade Federal do Pará, Belém, PA, Brazil.
Sci Rep. 2019 Jan 22;9(1):280. doi: 10.1038/s41598-018-36592-w.
Globins are a classical model system for the studies of protein evolution and function. Recent studies have shown that - besides the well-known haemoglobin and myoglobin - additional globin-types occur in vertebrates that serve different functions. Globin E (GbE) was originally identified as an eye-specific protein of birds that is distantly related to myoglobin. GbE is also present in turtles and the coelacanth but appeared to have been lost in other vertebrates. Here, we show that GbE additionally occurs in lungfish, the closest living relatives of the tetrapods. Each lungfish species harbours multiple (≥5) GbE gene copies. Surprisingly, GbE is exclusively and highly expressed in oocytes, with mRNA levels that exceed that of myoglobin in the heart. Thus, GbE is the first known oocyte-specific globin in vertebrates. No GbE transcripts were found in the ovary or egg transcriptomes of other vertebrates, suggesting a lungfish-specific function. Spectroscopic analysis and kinetic studies of recombinant GbE1 of the South American lungfish Lepidosiren paradoxa revealed a typical pentacoordinate globin with myoglobin-like O-binding kinetics, indicating similar functions. Our findings suggest that the multiple copies of GbE evolved to enhance O-supply in the developing embryo of lungfish, analogous to the embryonic and fetal haemoglobins of other vertebrates. In evolution, GbE must have changed its expression site from oocytes to eyes, or vice versa.
球蛋白是研究蛋白质进化和功能的经典模式系统。最近的研究表明，除了众所周知的血红蛋白和肌红蛋白外，脊椎动物中还存在其他具有不同功能的球蛋白类型。球蛋白 E (GbE) 最初被鉴定为鸟类特有的一种眼蛋白，与肌红蛋白有较远的亲缘关系。GbE 也存在于海龟和腔棘鱼中，但似乎在其他脊椎动物中丢失了。在这里，我们表明 GbE 还存在于肺鱼中，肺鱼是四足动物的近亲。每个肺鱼物种都有多个（≥5）GbE 基因拷贝。令人惊讶的是，GbE 仅在卵母细胞中高度表达，其 mRNA 水平超过心脏中的肌红蛋白。因此，GbE 是脊椎动物中第一个已知的卵母细胞特异性球蛋白。在其他脊椎动物的卵巢或卵子转录组中未发现 GbE 转录本，这表明肺鱼具有特异性功能。对来自南美的肺鱼 Lepidosiren paradoxa 的重组 GbE1 的光谱分析和动力学研究表明，它具有典型的五配位球蛋白和类似于肌红蛋白的 O 结合动力学，表明具有相似的功能。我们的研究结果表明，GbE 的多个拷贝进化为了增强肺鱼胚胎的 O 供应，类似于其他脊椎动物的胚胎和胎儿血红蛋白。在进化过程中，GbE 必须从卵母细胞改变其表达部位到眼睛，或者反之亦然。
