Rojas-Lopez Maricarmen, Zorgani Mohamed A, Kelley Lawrence A, Bailly Xavier, Kajava Andrey V, Henderson Ian R, Polticelli Fabio, Pizza Mariagrazia, Rosini Roberto, Desvaux Mickaël
Université Clermont Auvergne, INRA, UMR454 MEDiS, Clermont-Ferrand, France.
GSK, Siena, Italy.
Front Microbiol. 2018 Jan 5;8:2607. doi: 10.3389/fmicb.2017.02607. eCollection 2017.
Autotransporters (ATs) belong to a family of modular proteins secreted by the Type V, subtype a, secretion system (T5aSS) and considered as an important source of virulence factors in lipopolysaccharidic diderm bacteria (archetypical Gram-negative bacteria). While exported by the Sec pathway, the ATs are further secreted across the outer membrane via their own C-terminal translocator forming a β-barrel, through which the rest of the protein, namely the passenger, can pass. In several ATs, an autochaperone domain (AC) present at the C-terminal region of the passenger and upstream of the translocator was demonstrated as strictly required for proper secretion and folding. However, considering it was functionally characterised and identified only in a handful of ATs, wariness recently fells on the commonality and conservation of this structural element in the T5aSS. To circumvent the issue of sequence divergence and taking advantage of the resolved three-dimensional structure of some ACs, identification of this domain was performed following structural alignment among all AT passengers experimentally resolved by crystallography before searching in a dataset of 1523 ATs. While demonstrating that the AC is indeed a conserved structure found in numerous ATs, phylogenetic analysis further revealed a distribution into deeply rooted branches, from which emerge 20 main clusters. Sequence analysis revealed that an AC could be identified in the large majority of SAATs (self-associating ATs) but not in any LEATs (lipase/esterase ATs) nor in some PATs (protease autotransporters) and PHATs (phosphatase/hydrolase ATs). Structural analysis indicated that an AC was present in passengers exhibiting single-stranded right-handed parallel β-helix, whatever the type of β-solenoid, but not with α-helical globular fold. From this investigation, the AC of type 1 appears as a prevalent and conserved structural element exclusively associated to β-helical AT passenger and should promote further studies about the protein secretion and folding via the T5aSS, especially toward α-helical AT passengers.
自转运蛋白（ATs）属于由V型a亚型分泌系统（T5aSS）分泌的模块化蛋白家族，被认为是脂多糖双膜细菌（典型革兰氏阴性菌）中致病因子的重要来源。虽然ATs通过Sec途径输出，但它们通过自身的C端转运体进一步分泌穿过外膜，形成一个β桶，蛋白质的其余部分，即乘客结构域，可以通过该β桶。在几种自转运蛋白中，位于乘客结构域C端区域且在转运体上游的自伴侣结构域（AC）被证明是正确分泌和折叠所必需的。然而，考虑到它仅在少数自转运蛋白中具有功能特征并被鉴定出来，最近人们对T5aSS中这种结构元件的共性和保守性产生了疑虑。为了规避序列差异问题并利用一些AC的解析三维结构，在对1523个自转运蛋白数据集进行搜索之前，通过晶体学实验解析的所有自转运蛋白乘客结构域之间进行结构比对，从而鉴定该结构域。虽然证明AC确实是在众多自转运蛋白中发现的保守结构，但系统发育分析进一步揭示其分布在根深蒂固的分支中，从中出现了20个主要簇。序列分析表明，在大多数自缔合自转运蛋白（SAATs）中可以鉴定出AC，但在任何脂肪酶/酯酶自转运蛋白（LEATs）或某些蛋白酶自转运蛋白（PATs）和磷酸酶/水解酶自转运蛋白（PHATs）中均未鉴定出。结构分析表明，无论β螺线管的类型如何，在呈现单链右手平行β螺旋的乘客结构域中存在AC，但α螺旋球状折叠的乘客结构域中不存在。从这项研究来看，1型AC似乎是一种普遍且保守的结构元件，专门与β螺旋自转运蛋白乘客结构域相关，应该会促进关于通过T5aSS进行蛋白质分泌和折叠的进一步研究，特别是针对α螺旋自转运蛋白乘客结构域的研究。
