Export of autotransported proteins proceeds through an oligomeric ring shaped byC-terminal domains

An investigation was made into the oligomerization, the ability to form pores and the secretion-related properties of the 45 kDa C-terminal domain of the IgA protease (C-IgAP) from Neisseria gonorrhoeae. This protease is the best studied example of the autotransporters (ATs), a large family of exoproteins from Gram-negative bacteria that includes numerous virulence factors from human pathogens. These proteins contain an N-terminal passenger domain that em bodies the secreted polypeptide, while the C-domain inserts into the outer membrane (OM) and trans locates the linked N-module into the extracellular medium. Here we report that purified C-IgAP forms an oligomeric complex of ∼500 kDa with a ring-like structure containing a central cavity of ∼2 nm diameter that is the conduit for the export of the N-domains. These data overcome the previous model for ATs, which postulated the passage of the N-module through the hydrophilic channel of the β-barrel of each monomeric C-domain. Our results advocate a secretion mechanism not unlike other bacterial export systems, such as the secretins or fimbrial ushers, which rely on multimeric complexes assembled in the OM.

本研究针对淋病奈瑟菌（Neisseria gonorrhoeae）来源的IgA蛋白酶（IgA protease）45 kDa C端结构域（C-IgAP）的寡聚化、成孔能力及分泌相关特性展开探究。该蛋白酶是自动转运蛋白（autotransporters, ATs）领域研究最为深入的范例；自动转运蛋白是革兰氏阴性菌分泌蛋白的一个大家族，涵盖人类致病菌的多种毒力因子。这类蛋白包含一段N端乘客结构域，其承载分泌型多肽；而C端结构域插入外膜（outer membrane, OM），并将相连的N端模块转运至细胞外环境中。本研究发现，纯化的C-IgAP可形成分子量约500 kDa的寡聚复合体，该复合体呈环状结构，拥有直径约2 nm的中央空腔，此空腔即为N端结构域输出的通道。本研究数据推翻了此前的自动转运蛋白模型——该模型曾假设N端模块通过每个单体C端结构域的β桶状结构（β-barrel）亲水性通道进行转运。本研究结果支持一种与其他细菌分泌系统（如分泌素（secretins）或菌毛引导蛋白（fimbrial ushers））类似的分泌机制，这类系统均依赖于外膜中组装的多聚复合体。