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A role for the two-helix finger of the SecA ATPase in protein translocation

机译：SecA ATPase的两螺旋手指在蛋白质移位中的作用

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摘要

An important step in the biosynthesis of many proteins is their partial or complete translocation across the plasma membrane in prokaryotes or the endoplasmic reticulum membrane in eukar-yotes. In bacteria, secretory proteins are generally translocated after completion of their synthesis by the interaction of the cytoplasmic ATPase SecA and a protein-conducting channel formed by the SecY complex. How SecA moves substrates through the SecY channel is unclear. However, a recent structure of a SecA-SecY complex raises the possibility that the polypeptide chain is moved by a two-helix finger domain of SecA that is inserted into the cytoplasmic opening of the SecY channel. Here we have used disulphide-bridge cross-linking to show that the loop at the tip of the two-helix finger of Escherichia coli SecA interacts with a polypeptide chain right at the entrance into the SecY pore. Mutagenesis demonstrates that a tyr-osine in the loop is particularly important for translocation, but can be replaced by some other bulky, hydrophobic residues. We propose that the two-helix finger of SecA moves a polypeptide chain into the SecY channel with the tyrosine providing the major contact with the substrate, a mechanism analogous to that suggested for hexameric, protein-translocating ATPases.
机译：生物合成许多蛋白质的重要​​步骤是原核生物中质膜或真核生物中内质网膜的部分或全部转运。在细菌中，分泌蛋白通常在合成完成后通过细胞质ATPase SecA与SecY复合物形成的蛋白传导通道的相互作用而移位。 SecA如何通过SecY通道移动底物尚不清楚。但是，SecA-SecY复合物的最新结构增加了多肽链被插入SecY通道胞质开口的SecA的两个螺旋指结构域移动的可能性。在这里，我们使用二硫键桥联来显示大肠埃希氏菌SecA两螺旋指尖的环与SecY孔入口处的多肽链相互作用。诱变表明，环中的酪氨酸对转运非常重要，但可以被其他一些大的疏水性残基取代。我们建议SecA的两螺旋手指将多肽链移动到SecY通道中，酪氨酸提供与底物的主要接触，该机制类似于建议的六聚体蛋白转运ATPase。

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