Rathore D, McCutchan T F
Growth and Development Section, Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, 9000 Rockville Pike, Bethesda, MD 20892-0425, USA.
Proc Natl Acad Sci U S A. 2000 Jul 18;97(15):8530-5. doi: 10.1073/pnas.140224597.
Various pathogenic bacteria, viruses, and protozoan bind to glycosaminoglycan-based receptors on host cells and initiate an infection. Sporozoites of Plasmodium predominantly express circumsporozoite (CS) protein on their surface, which binds to heparan sulfate proteoglycans on liver cell surface that subsequently leads to malaria. Here we show that the interaction of free heparin with this parasite ligand has the potential to be a critical component of invasion. CS protein of P. falciparum contains four cysteines at positions 361, 365, 396, and 401. In this study, all four cysteine residues were mutagenized to alanine both individually and in different combinations. Conversion of cysteine 396 to alanine (protein CS3) led to a 10-fold increase in the binding activity of the protein to HepG2 cells. Replacement of cysteines at positions 361, 365, and 401 either alone or in different combinations led to a near total loss of binding. Surprisingly, activity in these inactive mutants could be effectively restored in the presence of submolar concentrations of heparin. Heparin also up-regulated binding of CS3 at submolar concentrations with respect to the protein but down-regulated binding when present in excess. Given the significantly different concentrations of heparin in different organs of the host and the in vitro results described here one can consider in vivo ramifications of this phenomenon for pathogen targeting of specific organs and for the functional effects of antigenic variation on receptor ligand interaction.
多种致病细菌、病毒和原生动物会与宿主细胞上基于糖胺聚糖的受体结合并引发感染。疟原虫的子孢子在其表面主要表达环子孢子（CS）蛋白，该蛋白与肝细胞表面的硫酸乙酰肝素蛋白聚糖结合，随后导致疟疾。在此我们表明，游离肝素与这种寄生虫配体的相互作用有可能成为入侵的关键组成部分。恶性疟原虫的CS蛋白在361、365、396和401位含有四个半胱氨酸。在本研究中，所有四个半胱氨酸残基均被单独或不同组合地突变为丙氨酸。将396位半胱氨酸转化为丙氨酸（蛋白CS3）导致该蛋白与HepG2细胞的结合活性增加了10倍。单独或以不同组合替换361、365和401位的半胱氨酸导致结合几乎完全丧失。令人惊讶的是，在亚摩尔浓度的肝素存在下，这些无活性突变体的活性可以有效恢复。相对于该蛋白，肝素在亚摩尔浓度下也上调了CS3的结合，但过量存在时则下调了结合。鉴于宿主不同器官中肝素浓度存在显著差异以及此处描述的体外结果，人们可以考虑这种现象对病原体靶向特定器官以及对抗原变异对受体配体相互作用的功能影响的体内影响。
