Two-way communication between SecY and SecA suggests a Brownian ratchet mechanism for protein translocation.

William John Allen,Roman Tuma,Steve A Baldwin,Sheena E Radford,Robin Adam Corey,Peter Oatley,Ian Collinson,Richard Barry Sessions

doi:10.7554/elife.15598

William John Allen, Roman Tuma + Show 6 more

Open Access

https://doi.org/10.7554/elife.15598

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Journal: eLife	Publication Date: May 16, 2016
Citations: 95	License type: CC BY 4.0

Affiliation: University of Bristol, University of Leeds

Abstract

The essential process of protein secretion is achieved by the ubiquitous Sec machinery. In prokaryotes, the drive for translocation comes from ATP hydrolysis by the cytosolic motor-protein SecA, in concert with the proton motive force (PMF). However, the mechanism through which ATP hydrolysis by SecA is coupled to directional movement through SecYEG is unclear. Here, we combine all-atom molecular dynamics (MD) simulations with single molecule FRET and biochemical assays. We show that ATP binding by SecA causes opening of the SecY-channel at long range, while substrates at the SecY-channel entrance feed back to regulate nucleotide exchange by SecA. This two-way communication suggests a new, unifying 'Brownian ratchet' mechanism, whereby ATP binding and hydrolysis bias the direction of polypeptide diffusion. The model represents a solution to the problem of transporting inherently variable substrates such as polypeptides, and may underlie mechanisms of other motors that translocate proteins and nucleic acids.

Highlights

The Sec system is the main pathway for protein secretion in all forms of life
Secretion is mostly post-translational (Hartl et al, 1990): substrates are recognised by the cytosolic ATPase SecA (Lill et al, 1989), which targets them to SecYEG and drives them through the protein channel using energy derived from ATP binding and hydrolysis, and the trans-membrane proton motive force (PMF) (Brundage et al, 1990)
SecY consists of ten trans-membrane helices (TMs), which form a ‘clamshell’ structure in the membrane, with five TMs on either side of the central protein channel, blocked by a ‘plug’ and a seal comprising six hydrophobic side chains (Figure 1A)

Summary

Introduction

The Sec system is the main pathway for protein secretion in all forms of life. At the translocon core is a hetero-trimeric membrane protein complex – SecYEG in the plasma membrane of prokaryotes and Sec61agb in the ER of eukaryotes – which forms a protein channel across the membrane (Ito et al, 1983; Brundage et al, 1990; Gorlich et al, 1992). Secretion is mostly post-translational (Hartl et al, 1990): substrates are recognised by the cytosolic ATPase SecA (Lill et al, 1989), which targets them to SecYEG and drives them through the protein channel using energy derived from ATP binding and hydrolysis, and the trans-membrane proton motive force (PMF) (Brundage et al, 1990). SecY consists of ten trans-membrane helices (TMs), which form a ‘clamshell’ structure in the membrane, with five TMs on either side of the central protein channel, blocked by a ‘plug’ and a seal comprising six hydrophobic side chains (Figure 1A).

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