Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada M5G 2M9.
Proc Natl Acad Sci U S A. 2012 Feb 21;109(8):2908-12. doi: 10.1073/pnas.1115743109. Epub 2012 Feb 2.
The saposins are small, membrane-active proteins that exist in both soluble and lipid-bound states. Saposin A has roles in sphingolipid catabolism and transport and is required for the breakdown of galactosylceramide by β-galactosylceramidase. In the absence of lipid, saposin A adopts a closed monomeric apo conformation typical of this family. To study a lipid-bound state of this protein, we determined the crystal structure of saposin A in the presence of detergent to 1.9 Å resolution. The structure reveals two chains of saposin A in an open conformation encapsulating 40 internally bound detergent molecules organized in a highly ordered bilayer-like hydrophobic core. The complex provides a high-resolution view of a discoidal lipoprotein particle in which all of the internalized acyl chains are resolved. Saposin A lipoprotein discs exhibit limited selectivity with respect to the incorporated lipid, and can solubilize phospholipids, sphingolipids, and cholesterol into discrete, monodisperse particles with mass of approximately 27 kDa. These discs may be the smallest possible lipoprotein structures that are stabilized by lipid self-assembly.
类脂运载蛋白是小的、膜活性蛋白，以可溶性和脂结合状态存在。类脂运载蛋白 A 在鞘脂分解代谢和转运中发挥作用，是β-半乳糖苷脂酶分解半乳糖脑苷脂所必需的。在没有脂质的情况下，类脂运载蛋白 A 采取封闭的单体脱辅基构象，这是该家族的典型特征。为了研究该蛋白的脂结合状态，我们在去污剂存在的情况下测定了类脂运载蛋白 A 的晶体结构，分辨率达到 1.9Å。结构显示，两条类脂运载蛋白 A 链以开放构象存在，包埋 40 个内部结合的去污剂分子，这些分子以高度有序的双层状疏水性核心排列。该复合物提供了一个高分辨率的盘状脂蛋白颗粒的视图，其中所有内化的酰基链都得到解析。类脂运载蛋白 A 脂蛋白盘对所包含的脂质表现出有限的选择性，并且可以将磷脂、鞘脂和胆固醇溶解到离散的、单分散的颗粒中，其质量约为 27 kDa。这些盘可能是通过脂质自组装稳定的最小可能的脂蛋白结构。
