Institute of Molecular Physiology, Johannes Gutenberg-University of Mainz, Johann-Joachim-Becher-Weg 7, 55128, Mainz, Germany.
J Mol Evol. 2021 Feb;89(1-2):62-72. doi: 10.1007/s00239-020-09986-6. Epub 2021 Jan 13.
Hemocyanins are giant oxygen transport proteins that freely float within the hemolymph of most molluscs. The basic quaternary structure of molluscan hemocyanins is a cylindrical decamer with a diameter of 35 nm which is built of 400 kDa subunits. Previously published results, however, showed that one out of two hemocyanin subunits of Rapana venosa encompasses two polypeptides, one 300 kDa and one 100 kDa polypeptide which aggregate to typical 4 MDa and 8 MDa hemocyanin (di-)decamer molecules. It was shown that the polypeptides are bound most probably by one or more cysteine disulfide bridges but it remained open if these polypeptides were coded by one or two genes. Our here presented results clearly showed that both polypeptides are coded by one gene only and that this phenomenon can also be found in the gastropod Nucella lapillus. Thus, it can be defined as clade-specific for Muricidae, a group of the very diverse Caenogastropoda. In addition, we discovered a further deviation of this hemocyanin subunit within both species, namely a region of 340 mainly hydrophilic amino acids (especially histidines and aspartic acids) which have not been identified in any other molluscan hemocyanin, yet. Our results indicate that, within the quaternary structure, these additional amino acids most probably protrude within the inner part of didecamer cylinders, forming a large extra mass of up to 800 kDa. They presumably influence the structure of the protein and may affect the functionality. Thus, these findings reveal further insights into the evolution and structures of gastropod hemocyanins.
血蓝蛋白是一种巨大的氧气运输蛋白，可在大多数软体动物的血淋巴中自由漂浮。软体动物血蓝蛋白的基本四级结构是一个直径为 35nm 的圆柱形十聚体，由 400kDa 的亚基组成。然而，先前的研究结果表明，Rapana venosa 的两个血蓝蛋白亚基中的一个包含两个多肽，一个 300kDa 和一个 100kDa 的多肽，它们聚合形成典型的 4MDa 和 8MDa 的血蓝蛋白（二）十聚体分子。研究表明，这些多肽很可能通过一个或多个半胱氨酸二硫键结合，但这些多肽是由一个还是两个基因编码仍不清楚。我们这里的研究结果清楚地表明，这两个多肽仅由一个基因编码，这种现象也存在于腹足纲软体动物 Nucella lapillus 中。因此，它可以被定义为 Muricidae 的一个分支特有的现象，Muricidae 是一个非常多样化的 Caenogastropoda 群体。此外，我们在这两个物种中还发现了这种血蓝蛋白亚基的另一个差异，即在 340 个主要亲水氨基酸（特别是组氨酸和天冬氨酸）区域，这些氨基酸在任何其他软体动物血蓝蛋白中都没有被识别。我们的研究结果表明，在四级结构中，这些额外的氨基酸很可能在二聚体圆柱体的内部突出，形成一个高达 800kDa 的巨大额外质量。它们可能会影响蛋白质的结构，并可能影响其功能。因此，这些发现揭示了腹足纲血蓝蛋白进化和结构的进一步见解。
