Chiswell Brian, Lovelace Leslie L, Brannen Charity, Ortlund Eric A, Lebioda Lukasz, Sodetz James M
Department of Chemistry and Biochemistry and School of Medicine, University of South Carolina, Columbia, SC 29208, USA.
Biochim Biophys Acta. 2007 May;1774(5):637-44. doi: 10.1016/j.bbapap.2007.03.004. Epub 2007 Mar 20.
Human C8 is one of five components of the cytolytic membrane attack complex of complement. It contains three subunits (C8alpha, C8beta, C8gamma) arranged as a disulfide-linked C8alpha-gamma heterodimer that is noncovalently associated with C8beta. C8gamma has the distinction of being the only lipocalin in the complement system. Lipocalins have a core beta-barrel structure forming a calyx with a binding site for a small hydrophobic ligand. A natural ligand for C8gamma has not been identified; however previous structural studies indicate C8gamma has a typical lipocalin fold that is suggestive of a ligand-binding capability. A distinctive feature of C8gamma is the division of its putative ligand binding pocket into a hydrophilic upper portion and a large hydrophobic lower cavity. Access to the latter is restricted by the close proximity of two tyrosine side chains (Y83 and Y131). In the present study, binding experiments were performed using lauric acid as a pseudoligand to investigate the potential accessibility of the lower cavity. The crystal structure of a C8gamma.laurate complex revealed that Y83 and Y131 can move to allow penetration of the hydrocarbon chain of laurate into the lower cavity. Introducing a Y83W mutation blocked access but had no effect on the ability of C8gamma to enhance C8 cytolytic activity. Together, these results indicate that the lower cavity in C8gamma could accommodate a ligand if such a ligand has a narrow hydrophobic moiety at one end. Entry of that moiety into the lower cavity would require movement of Y83 and Y131, which act as a gate at the cavity entrance.
人C8是补体溶细胞性膜攻击复合物的五个组分之一。它包含三个亚基（C8α、C8β、C8γ），排列成一个通过二硫键连接的C8α-γ异二聚体，该异二聚体与C8β非共价结合。C8γ是补体系统中唯一的脂质运载蛋白。脂质运载蛋白具有一个核心β-桶状结构，形成一个带有小疏水配体结合位点的杯状结构。尚未鉴定出C8γ的天然配体；然而，先前的结构研究表明C8γ具有典型的脂质运载蛋白折叠结构，提示其具有配体结合能力。C8γ的一个显著特征是其假定的配体结合口袋被分为一个亲水的上部和一个大的疏水下部腔。后者的通道受到两个酪氨酸侧链（Y83和Y131）的紧密接近的限制。在本研究中，使用月桂酸作为假配体进行结合实验，以研究下部腔的潜在可及性。C8γ-月桂酸复合物的晶体结构表明，Y83和Y131可以移动，以使月桂酸的烃链渗透到下部腔中。引入Y83W突变会阻止通道进入，但对C8γ增强C8溶细胞活性的能力没有影响。总之，这些结果表明，如果C8γ的下部腔有一个配体，该配体一端具有狭窄的疏水部分，则下部腔可以容纳该配体。该部分进入下部腔需要Y83和Y131的移动，它们在腔入口处起门的作用。
