Reyes-Bermudez Alejandro, Lin Zhiyi, Hayward David C, Miller David J, Ball Eldon E
ARC Centre of Excellence for Coral Reef Studies and Comparative Genomics Centre, James Cook University, Townsville, Qld, 4811, Australia.
BMC Evol Biol. 2009 Jul 29;9:178. doi: 10.1186/1471-2148-9-178.
The coral skeleton consists of CaCO3 deposited upon an organic matrix primarily as aragonite. Currently galaxin, from Galaxea fascicularis, is the only soluble protein component of the organic matrix that has been characterized from a coral. Three genes related to galaxin were identified in the coral Acropora millepora.
One of the Acropora genes (Amgalaxin) encodes a clear galaxin ortholog, while the others (Amgalaxin-like 1 and Amgalaxin-like 2) encode larger and more divergent proteins. All three proteins are predicted to be extracellular and share common structural features, most notably the presence of repetitive motifs containing dicysteine residues. In situ hybridization reveals distinct, but partially overlapping, spatial expression of the genes in patterns consistent with distinct roles in calcification. Both of the Amgalaxin-like genes are expressed exclusively in the early stages of calcification, while Amgalaxin continues to be expressed in the adult, consistent with the situation in the coral Galaxea.
Comparisons with molluscs suggest functional convergence in the two groups; lustrin A/pearlin proteins may be the mollusc counterparts of galaxin, whereas the galaxin-like proteins combine characteristics of two distinct proteins involved in mollusc calcification. Database searches indicate that, although sequences with high similarity to the galaxins are restricted to the Scleractinia, more divergent members of this protein family are present in other cnidarians and some other metazoans. We suggest that ancestral galaxins may have been secondarily recruited to roles in calcification in the Triassic, when the Scleractinia first appeared. Understanding the evolution of the broader galaxin family will require wider sampling and expression analysis in a range of cnidarians and other animals.
珊瑚骨骼由主要以文石形式沉积在有机基质上的碳酸钙组成。目前，来自束状鹿角珊瑚的 galaxin 是已从珊瑚中鉴定出的有机基质中唯一的可溶性蛋白质成分。在多孔鹿角珊瑚中鉴定出了三个与 galaxin 相关的基因。
其中一个鹿角珊瑚基因（Amgalaxin）编码一个明确的 galaxin 直系同源物，而其他基因（Amgalaxin-like 1 和 Amgalaxin-like 2）编码更大且差异更大的蛋白质。所有这三种蛋白质预计都位于细胞外，并具有共同的结构特征，最显著的是存在含有二半胱氨酸残基的重复基序。原位杂交显示这些基因在空间上有明显但部分重叠的表达模式，这与它们在钙化过程中的不同作用一致。两个 Amgalaxin-like 基因仅在钙化早期表达，而 Amgalaxin 在成体中持续表达，这与鹿角珊瑚的情况一致。
与软体动物的比较表明这两组动物在功能上有趋同现象；lustrin A/pearlin 蛋白可能是 galaxin 在软体动物中的对应物，而 galaxin 样蛋白则结合了参与软体动物钙化的两种不同蛋白质的特征。数据库搜索表明，尽管与 galaxin 高度相似的序列仅限于石珊瑚目，但该蛋白家族中差异更大的成员存在于其他刺胞动物和一些其他后生动物中。我们认为，在三叠纪石珊瑚目首次出现时，祖先的 galaxin 可能在次生过程中被招募到钙化相关的角色中。了解更广泛的 galaxin 家族的进化需要在一系列刺胞动物和其他动物中进行更广泛的样本采集和表达分析。
