Mah Silvia A, Swanson Willie J, Vacquier Victor D
Center for Marine Biotechnology and Biomedicine, Scripps Institution of Oceanography, University of California San Diego, La Jolla, USA.
Mol Biol Evol. 2005 Mar;22(3):533-41. doi: 10.1093/molbev/msi037. Epub 2004 Nov 3.
A wealth of evidence shows that protein-carbohydrate recognition mediates the steps of gamete interaction during fertilization. Carbohydrate-recognition domains (CRDs) comprise a large family of ancient protein modules of approximately 120 amino acids, having the same protein fold, that bind terminal sugar residues on glycoproteins and polysaccharides. Sea urchin sperm express three suREJ (sea urchin receptor for egg jelly) proteins on their plasma membranes. suREJ1 has two CRDs, whereas suREJ2 and suREJ3 both have one CRD. suREJ1 binds the fucose sulfate polymer (FSP) of egg jelly to induce the sperm acrosome reaction. The structure of FSP is species specific. Therefore, the suREJ1 CRDs could encode molecular recognition between sperm and egg underlying the species-specific induction of the acrosome reaction. The functions of suREJ2 and suREJ3 have not been explored, but suREJ3 is exclusively localized on the plasma membrane over the sperm acrosomal vesicle and is physically associated with sea urchin polycystin-2, a known cation channel. An evolutionary analysis of these four CRDs was performed for six sea urchin species. Phylogenetic analysis shows that these CRDs were already differentiated in the common ancestor of these six sea urchins. The CRD phylogeny agrees with previous work on these species based on one nuclear gene and several mitochondrial genes. Maximum likelihood shows that positive selection acts on these four CRDs. Threading the suREJ CRDs onto the prototypic CRD crystal structure shows that many of the sites under positive selection are on extended loops, which are involved in saccharide binding. This is the first demonstration of positive selection in CRDs and is another example of positive selection acting on the evolution of gamete-recognition proteins.
大量证据表明，蛋白质 - 碳水化合物识别介导了受精过程中配子相互作用的各个步骤。碳水化合物识别结构域（CRD）由大约120个氨基酸组成的一大类古老蛋白质模块构成，具有相同的蛋白质折叠结构，可结合糖蛋白和多糖上的末端糖残基。海胆精子在其质膜上表达三种suREJ（海胆卵胶受体）蛋白。suREJ1有两个CRD，而suREJ2和suREJ3都只有一个CRD。suREJ1与卵胶的硫酸岩藻糖聚合物（FSP）结合以诱导精子顶体反应。FSP的结构具有物种特异性。因此，suREJ1的CRD可能编码了精子与卵子之间的分子识别，这是物种特异性诱导顶体反应的基础。suREJ2和suREJ3的功能尚未被研究，但suREJ3仅定位在精子顶体小泡上方的质膜上，并且与海胆多囊蛋白 - 2（一种已知的阳离子通道）存在物理关联。对六个海胆物种的这四个CRD进行了进化分析。系统发育分析表明，这些CRD在这六个海胆的共同祖先中就已经分化。CRD系统发育与之前基于一个核基因和几个线粒体基因对这些物种的研究结果一致。最大似然法表明正选择作用于这四个CRD。将suREJ的CRD与原型CRD晶体结构进行比对发现，许多受到正选择的位点位于延伸环上，这些环参与糖类结合。这是首次证明CRD中存在正选择，也是正选择作用于配子识别蛋白进化的另一个例子。
