来自哈佛医学院，波士顿儿童医院的研究人员发表了题为“Shape change in the receptor for gliding motility in Plasmodium sporozoites”的文章，完成了疟疾发生的重要表面蛋白的结构测定，并从中获得了功能启示。这项研究发表在《美国国家科学院院刊》（PNAS）杂志上。
文章的第一作者为哈佛医学院宋高洁博士，目前在哈佛医学院从事博士后研究。
疟疾是一种由疟原虫造成，通过疟蚊传播的全球性寄生虫传染病。疟疾仍然是全球主要的传染病和健康杀手之一，每年因患疟疾而死亡的人数达上百万。由于疟疾的生活周期极其复杂，致使其疫苗研发也变得困难。
TRAP蛋白是疟原虫孢子体穿梭和入侵肝细胞的核心元件，其通过胞外结构域连接细胞或基质的表面受体，并通过胞内区与孢子体内马达蛋白联系，从而介导孢子体的迁移,因此是重要的疫苗研究靶点。但由于目前TRAP蛋白的三维结构并不清楚，疫苗研发科学家只能工作在“黑暗”中。
在这篇文章中，基于X-射线衍射技术，研究人员解析了TRAP蛋白的胞外区的晶体结构，并发现TRAP的胞外区VWA结构域存在两种构型（分别称“开放”或“闭合”）。
在两种构型中，VWA结构域与TSR结构域相对位置发生非常显着的变化：开放构型中，整个分子呈伸展状，并产生一个新的由一对反平行β片组成的结构域（研究人员将其命名为“弹性飘带区”）。这段新的结构域，可能是新的疫苗或抗体研究的靶点。TRAP的三维结构的解析，有助于我们理解孢子体穿梭及疟疾发生的机理，并为疫苗研发打下坚实基础。(生物谷Bioon.com)
doi: 10.1073/pnas.1218581109
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Shape change in the receptor for gliding motility in Plasmodium sporozoites
Gaojie Song, Adem C. Koksal, Chafen Lu, and Timothy A. Springer1
Sporozoite gliding motility and invasion of mosquito and vertebrate host cells in malaria is mediated by thrombospondin repeat anonymous protein (TRAP). Tandem von Willebrand factor A (VWA) and thrombospondin type I repeat (TSR) domains in TRAP connect through proline-rich stalk, transmembrane, and cytoplasmic domains to the parasite actin-dependent motility apparatus. We crystallized fragments containing the VWA and TSR domains from Plasmodium vivax and Plasmodium falciparum in different crystal lattices. TRAP VWA domains adopt closed and open conformations, and bind a Mg2+ ion at a metal ion–dependent adhesion site implicated in ligand binding. Metal ion coordination in the open state is identical to that seen in the open high-affinity state of integrin I domains. The closed VWA conformation associates with a disordered TSR domain. In contrast, the open VWA conformation crystallizes with an extensible β ribbon and ordered TSR domain. The extensible β ribbon is composed of disulfide-bonded segments N- and C-terminal to the VWA domain that are largely drawn out of the closed VWA domain in a 15 Å movement to the open conformation. The extensible β ribbon and TSR domain overlap at a conserved interface. The VWA, extensible β ribbon, and TSR domains adopt a highly elongated overall orientation that would be stabilized by tensile force exerted across a ligand-receptor complex by the actin motility apparatus of the sporozoite. Our results provide insights into regulation of “stick-and-slip” parasite motility and for development of sporozoite subunit vaccines. Mosquitoes transmit malaria to humans via sporozoites. Sporozoites are important targets of pre-erythrocytic malaria vaccines. However, we know little about the structure and arrangement of the two most important vaccine targets on sporozoite surfaces, the circumsporozoite (CS) protein (1⇓–3) and thrombospondin repeat anonymous protein (TRAP) (4, 5). CS is a constitutive sporozoite surface protein and has a glycophosphatidylinositol anchor. TRAP mediates sporozoite gliding motility and cell invasion in both mosquito and vertebrate hosts (6). TRAP is mobilized from micronemes to the plasma membrane at the apical end of sporozoites, and is translocated to the posterior end during cell migration and invasion (7, 8). TRAP spans the plasma membrane, and its cytoplasmic domain connects to the actin cytoskeleton through aldolase, permitting functional cooperation between extracellular adhesive domains and the intracellular actin/myosin motor (8⇓–10). The TRAP ectodomain contains tandem von Willebrand factor A (VWA) and thrombospondin repeat (TSR) domains. A subset of VWA domains, including the inserted (I) domains in integrins, contain metal ion-dependent adhesion sites (MIDAS), with a Mg2+ ion at the center of the ligand binding site (11). Conformational change transmitted from neighboring domains regulates affinity of I domains for ligand. The TRAP VWA domain contains the sequence signature of a MIDAS. Mutations of putative TRAP VWA domain MIDAS residues and deletion of a segment of the TRAP TSR domain disrupt gliding motility and invasion of mosquito salivary glands and mammalian liver cells (12, 13). A crystal structure of a fragment of the von Willebrand factor A (VWA) domain from a Toxoplasma gondii TRAP orthologue, micronemal protein-2 (MIC-2) (5), and an NMR structure of the TRAP TSR domain (4) did not yield insights into conformational regulation or how these neighboring domains might interact in TRAP or MIC-2. In integrins, induction of the high-affinity, open conformation of both the α- and β-subunit I domains is relayed between tandem domains by axial displacement of the C-terminal α helix (14). We hypothesized that the TRAP VWA domain would undergo conformational change during ligand binding, induced by tensile force transmitted through the TSR domain by the motility apparatus. To test this hypothesis, we crystallized TRAP in multiple lattices and conformational states.
