Lipids Activate SecA for High Affinity Binding to the SecYEG Complex

Sabrina Koch,Janny G De Wit,Iuliia Vos,Jan Peter Birkner,Pavlo Gordiichuk,Andreas Herrmann,Antoine M Van Oijen,Arnold J.M Driessen

doi:10.1074/jbc.m116.743831

Sabrina Koch, Janny G De Wit + Show 6 more

Open Access

https://doi.org/10.1074/jbc.m116.743831

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Abstract

Protein translocation across the bacterial cytoplasmic membrane is an essential process catalyzed predominantly by the Sec translocase. This system consists of the membrane-embedded protein-conducting channel SecYEG, the motor ATPase SecA, and the heterotrimeric SecDFyajC membrane protein complex. Previous studies suggest that anionic lipids are essential for SecA activity and that the N terminus of SecA is capable of penetrating the lipid bilayer. The role of lipid binding, however, has remained elusive. By employing differently sized nanodiscs reconstituted with single SecYEG complexes and comprising varying amounts of lipids, we establish that SecA gains access to the SecYEG complex via a lipid-bound intermediate state, whereas acidic phospholipids allosterically activate SecA for ATP-dependent protein translocation.

Highlights

25–30% of bacterial proteins are embedded in the cytoplasmic membrane or carry out their distinct functions outside the cell
We further show that the SecA N terminus that interacts with acidic phospholipids is important to tether SecA to the membrane and that this binding event induces a conformational change of SecA that promotes its interactions with SecYEG
We propose a new mechanism of protein translocation, whereby SecA first binds acidic phospholipids in the membrane whereupon the lipid bound SecA intermediate interacts with SecYEG with high affinity

Summary

Lipids Activate SecA for High Affinity Binding to the SecYEG Complex*

In the SecASecY structure, the helical amphipathic N terminus of SecA is positioned away from where the membrane would be located, and a major conformational change involving a 30 Å translational movement would be required to allow this region to deeply penetrate the membrane, which could potentially impact the SecY binding mode and SecA function This N-terminal displacement of SecA suggests a tethering function of the N terminus and a key role function in conformational activation of SecA upon lipid binding. We propose a new mechanism of protein translocation, whereby SecA first binds acidic phospholipids in the membrane whereupon the lipid bound SecA intermediate interacts with SecYEG with high affinity

Results

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Discussion

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