Kresge N, Vacquier V D, Stout C D
Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037-1093, USA.
Biochemistry. 2001 May 8;40(18):5407-13. doi: 10.1021/bi002779v.
Sp18 is an 18 kDa protein that is released from abalone sperm during the acrosome reaction. It coats the acrosomal process where it is thought to mediate fusion between sperm and egg cell membranes. Sp18 is evolutionarily related to lysin, a 16 kDa abalone sperm protein that dissolves the vitelline envelope surrounding the egg. The two proteins were generated by gene duplication followed by rapid divergence by positive selection. Here, we present the crystal structure of green abalone sp18 resolved to 1.86 A. Sp18 is composed of a bundle of five alpha-helices with surface clusters of basic and hydrophobic residues, giving it a large dipole moment and making it extremely amphipathic. The large clusters of hydrophobic surface residues and domains of high positive electrostatic surface charge explain sp18's ability as a potent fusagen of liposomes. The overall fold of sp18 is similar to that of green abalone lysin; however, the surface features of the proteins are quite different, accounting for their different roles in fertilization. This is the first crystal structure of a protein implicated in sperm-egg fusion during animal fertilization.
Sp18是一种18千道尔顿的蛋白质，在顶体反应期间从鲍鱼精子中释放出来。它覆盖在顶体突起上，据认为在那里介导精子与卵细胞的细胞膜融合。Sp18在进化上与溶素相关，溶素是一种16千道尔顿的鲍鱼精子蛋白，可溶解包围卵子的卵黄膜。这两种蛋白质是通过基因复制产生的，随后通过正选择快速分化。在此，我们展示了绿色鲍鱼Sp18的晶体结构，分辨率达到1.86埃。Sp18由一束五个α螺旋组成，表面有碱性和疏水残基簇，赋予其较大的偶极矩，使其具有极强的两亲性。大量的疏水表面残基簇和高正静电表面电荷区域解释了Sp18作为脂质体有效融合剂的能力。Sp18的整体折叠与绿色鲍鱼溶素相似；然而，这两种蛋白质的表面特征有很大不同，这解释了它们在受精过程中发挥的不同作用。这是首次报道的与动物受精过程中精卵融合相关的蛋白质晶体结构。
