Interaction of the periplasmic chaperone SurA with the inner membrane protein secretion (SEC) machinery.
Interaction of the periplasmic chaperone SurA with the inner membrane protein secretion (SEC) machinery.
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DOI：
10.1042/bcj20220480
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发表时间：
2023-02-27
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革兰氏阴性细菌被两层富含蛋白质的膜所包围，中间夹着一层肽聚糖层。它们共同形成包膜(或细胞壁)，对能量生产、脂肪生物合成、结构完整性以及抵御物理和化学环境挑战至关重要。为了实现包膜生物发生，周质和外膜蛋白(OMP)必须通过普遍存在的SecYEG蛋白通道从细胞质中运输到内膜。突现蛋白要么在周质中折叠，要么穿过肽聚糖(PG)层向外膜插入，通过β-Barrel组装机械插入。疏水性蛋白质在周质中的运输特别危险，因为蛋白质密度很高，而且缺乏能量(三磷酸腺苷或化学渗透势)。许多分子伴侣有助于防止和恢复聚集，其中已知的这些SurA与BAM相互作用，促进外膜的输送。然而，目前还不清楚在与SurA相互作用之前，SEC机制中出现的蛋白质是如何被接收到的，并如何防止聚集和蛋白降解。通过生化分析和电子显微镜，我们证明了未被占据和底物结合的SurA与SecYEG-SecDF-YidC-又名全息转位蛋白(HTL)的内膜转位机制复合体的结合能力。在AlphaFold预测的支持下，我们提出了SecDF的周质结构域在SecYEG中蛋白质易位出口部位的伴侣招募中的作用。我们认为，这种与征募的伴侣的即时相互作用有助于防止新生包膜蛋白的聚集和降解，促进它们安全地通过周质和外膜。
Gram-negative bacteria are surrounded by two protein-rich membranes with a peptidoglycan layer sandwiched between them. Together they form the envelope (or cell wall), crucial for energy production, lipid biosynthesis, structural integrity, and for protection against physical and chemical environmental challenges. To achieve envelope biogenesis, periplasmic and outer-membrane proteins (OMPs) must be transported from the cytosol and through the inner-membrane, via the ubiquitous SecYEG protein–channel. Emergent proteins either fold in the periplasm or cross the peptidoglycan (PG) layer towards the outer-membrane for insertion through the β-barrel assembly machinery (BAM). Trafficking of hydrophobic proteins through the periplasm is particularly treacherous given the high protein density and the absence of energy (ATP or chemiosmotic potential). Numerous molecular chaperones assist in the prevention and recovery from aggregation, and of these SurA is known to interact with BAM, facilitating delivery to the outer-membrane. However, it is unclear how proteins emerging from the Sec-machinery are received and protected from aggregation and proteolysis prior to an interaction with SurA. Through biochemical analysis and electron microscopy we demonstrate the binding capabilities of the unoccupied and substrate-engaged SurA to the inner-membrane translocation machinery complex of SecYEG–SecDF–YidC — aka the holo-translocon (HTL). Supported by AlphaFold predictions, we suggest a role for periplasmic domains of SecDF in chaperone recruitment to the protein translocation exit site in SecYEG. We propose that this immediate interaction with the enlisted chaperone helps to prevent aggregation and degradation of nascent envelope proteins, facilitating their safe passage to the periplasm and outer-membrane.