Brecht S, Carruthers V B, Ferguson D J, Giddings O K, Wang G, Jakle U, Harper J M, Sibley L D, Soldati D
Zentrum für Molekulare Biologie Heidelberg, Im Neuenheimer Feld 282, 69120 Heidelberg, Germany.
J Biol Chem. 2001 Feb 9;276(6):4119-27. doi: 10.1074/jbc.M008294200. Epub 2000 Oct 26.
The initial stage of invasion by apicomplexan parasites involves the exocytosis of the micronemes-containing molecules that contribute to host cell attachment and penetration. MIC4 was previously described as a protein secreted by Toxoplasma gondii tachyzoites upon stimulation of micronemes exocytosis. We have microsequenced the mature protein, purified after discharge from micronemes and cloned the corresponding gene. The deduced amino acid sequence of MIC4 predicts a 61-kDa protein that contains 6 conserved apple domains. Apple domains are composed of six spacely conserved cysteine residues which form disulfide bridges and are also present in micronemal proteins from two closely related apicomplexan parasites, Sarcocystis muris and Eimeria species, and several mammalian serum proteins, including kallikrein. Here we show that MIC4 localizes in the micronemes of all the invasive forms of T. gondii, tachyzoites, bradyzoites, sporozoites, and merozoites. The protein is proteolytically processed both at the N and the C terminus only upon release from the organelle. MIC4 binds efficiently to host cells, and the adhesive motif maps in the most C-terminal apple domain.
顶复门寄生虫入侵的初始阶段涉及含微线体分子的胞吐作用，这些分子有助于宿主细胞的附着和穿透。MIC4先前被描述为刚地弓形虫速殖子在微线体胞吐作用受刺激后分泌的一种蛋白质。我们对从微线体释放后纯化的成熟蛋白质进行了微量测序，并克隆了相应的基因。MIC4推导的氨基酸序列预测其为一种61 kDa的蛋白质，含有6个保守的苹果结构域。苹果结构域由6个间隔保守的半胱氨酸残基组成，这些残基形成二硫键，并且也存在于两种密切相关的顶复门寄生虫——鼠肉孢子虫和艾美耳属物种的微线体蛋白中，以及几种哺乳动物血清蛋白中，包括激肽释放酶。在这里我们表明，MIC4定位于刚地弓形虫所有侵袭性形态（速殖子、缓殖子、子孢子和裂殖子）的微线体中。该蛋白质仅在从细胞器释放时在N端和C端进行蛋白水解加工。MIC4能有效地结合宿主细胞，其黏附基序位于最C端的苹果结构域中。
