Instituto Nacional de Ciência e Tecnologia em Bioimagens e Biologia Estrutural, Centro de Ciências da Saúde, Universidade Federal do Rio de Janeiro, Cidade Universitária, Ilha do Fundão, 21941-590 Rio de Janeiro, Brazil.
Laboratório de Genômica Estrutural, Instituto de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, 21941-590 Rio de Janeiro, Brazil.
Microbiology (Reading). 2011 Oct;157(Pt 10):2818-2830. doi: 10.1099/mic.0.049247-0. Epub 2011 Jul 14.
Crithidia deanei is a trypanosomatid protozoan that harbours a symbiotic bacterium. The partners maintain a mutualistic relationship, thus constituting an excellent model for studying metabolic exchanges between the host and the symbiont, the origin of organelles and cellular evolution. According to molecular analysis, symbionts of different trypanosomatid species share high identity and descend from a common ancestor, a β-proteobacterium of the genus Bordetella. The endosymbiont is surrounded by two membranes, like Gram-negative bacteria, but its envelope presents special features, since phosphatidylcholine is a major membrane component and the peptidoglycan layer is highly reduced, as described in other obligate intracellular bacteria. Like the process that generated mitochondria and plastids, the endosymbiosis in trypanosomatids depends on pathways that facilitate the intensive metabolic exchanges between the bacterium and the host protozoan. A search of the annotated symbiont genome database identified one sequence with identity to porin-encoding genes of the genus Bordetella. Considering that the symbiont outer membrane has a great accessibility to cytoplasm host factors, it was important to characterize this single porin-like protein using biochemical, molecular, computational and ultrastructural approaches. Antiserum against the recombinant porin-like molecule revealed that it is mainly located in the symbiont envelope. Secondary structure analysis and comparative modelling predicted the protein 3D structure as an 18-domain β-barrel, which is consistent with porin channels. Electrophysiological measurements showed that the porin displays a slight preference for cations over anions. Taken together, the data presented herein suggest that the C. deanei endosymbiont porin is phylogenetically and structurally similar to those described in Gram-negative bacteria, representing a diffusion channel that might contribute to the exchange of nutrients and metabolic precursors between the symbiont and its host cell.
德氏锥虫是一种原生动物，体内寄生着一种共生细菌。这对共生伙伴维持着互利共生关系，因此成为研究宿主和共生体之间代谢交换、细胞器起源和细胞进化的绝佳模型。根据分子分析，不同的原生动物种属的共生体具有高度的同源性，它们都起源于一个共同的祖先，即博德特氏菌属的β-变形菌。内共生体被两层膜所包围，就像革兰氏阴性菌一样，但它的包膜具有特殊的特征，因为磷脂酰胆碱是主要的膜成分，肽聚糖层高度减少，这在其他专性细胞内细菌中也有描述。与产生线粒体和质体的过程一样，原生动物中的内共生依赖于促进细菌和宿主原生动物之间密集代谢交换的途径。在注释的共生体基因组数据库中搜索，发现了一个与博德特氏菌属的孔蛋白编码基因具有同源性的序列。考虑到共生体的外膜对宿主细胞质因子具有很大的可及性，因此使用生化、分子、计算和超微结构方法来对这种单一的孔蛋白样蛋白进行特征描述是很重要的。针对重组孔蛋白样分子的抗血清表明，它主要位于共生体包膜中。二级结构分析和比较建模预测该蛋白的 3D 结构为 18 个结构域的β-桶，与孔蛋白通道一致。电生理测量表明，该孔蛋白对阳离子的偏好略高于阴离子。综上所述，本文提供的数据表明，C. deanei 内共生体孔蛋白在系统发育和结构上与革兰氏阴性菌中描述的孔蛋白相似，代表了一种扩散通道，可能有助于共生体与其宿主细胞之间营养物质和代谢前体的交换。
