Tonkin Christopher J, Roos David S, McFadden Geoffrey I
Plant Cell Biology Research Centre, The School of Botany, The University of Melbourne, Parkville, Melbourne, Victoria 3010, Australia.
Mol Biochem Parasitol. 2006 Dec;150(2):192-200. doi: 10.1016/j.molbiopara.2006.08.001. Epub 2006 Aug 28.
The non-photosynthetic plastid - or apicoplast - of Toxoplasma gondii and other apicomplexan parasites is an essential organelle and promising drug target. Most apicoplast proteins are encoded in the nucleus and targeted into the organelle through the apicoplast's four membranes courtesy of a bipartite N-terminal leader sequence comprising of an endomembrane signal peptide followed by a plastid transit peptide. Apicoplast transit peptides, like plant plastid transit peptides, have no primary consensus, are variable in length and may be distinguishable only by a relative depletion of negative charged residues and consequent enrichment in basic residues. In this study we examine the role of charged residues within an apicoplast transit peptide in T. gondii by point mutagenesis. We demonstrate that positive charged residues, combined with the absence of negatively charged amino acids, are essential for apicoplast transit peptide fidelity, as also observed in P. falciparum. Furthermore, we show that positive charge is more important at the transit peptide's N-terminus than its C-terminus, and that the nature of the positive residue and the exact position of the N-terminal positive charge are not important. These results suggest that a simple, rule-based prediction for T. gondii transit peptides, similar to that successfully implemented for P. falciparum should help to identify apicoplast proteins and facilitate the identification of drug targets in this important human pathogen.
刚地弓形虫和其他顶复门寄生虫的非光合质体——即顶质体——是一种重要的细胞器，也是很有前景的药物靶点。大多数顶质体蛋白由细胞核编码，并通过一个由内膜信号肽和质体转运肽组成的二分N端前导序列，穿过顶质体的四层膜进入该细胞器。顶质体转运肽与植物质体转运肽一样，没有主要的共有序列，长度可变，可能只能通过负电荷残基的相对减少和随之而来的碱性残基富集来区分。在本研究中，我们通过点突变研究了刚地弓形虫顶质体转运肽中带电残基的作用。我们证明，正如在恶性疟原虫中观察到的那样，带正电荷的残基与不带负电荷的氨基酸相结合，对顶质体转运肽的保真度至关重要。此外，我们表明，正电荷在转运肽的N端比C端更重要，并且正残基的性质和N端正电荷的确切位置并不重要。这些结果表明，类似于成功应用于恶性疟原虫的基于规则的简单预测方法，应该有助于识别刚地弓形虫的顶质体蛋白，并促进在这种重要的人类病原体中识别药物靶点。
