Dimeric SecA Couples the Preprotein Translocation in an Asymmetric Manner

Ying Tang,Sen-Fang Sui,Xijiang Pan,Yong Chen,Phang C Tai,Vladimir Uversky

doi:10.1371/journal.pone.0016498

Abstract

The Sec translocase mediates the post-translational translocation of a number of preproteins through the inner membrane in bacteria. In the initiatory translocation step, SecB targets the preprotein to the translocase by specific interaction with its receptor SecA. The latter is the ATPase of Sec translocase which mediates the post-translational translocation of preprotein through the protein-conducting channel SecYEG in the bacterial inner membrane. We examined the structures of Escherichia coli Sec intermediates in solution as visualized by negatively stained electron microscopy in order to probe the oligomeric states of SecA during this process. The symmetric interaction pattern between the SecA dimer and SecB becomes asymmetric in the presence of proOmpA, and one of the SecA protomers predominantly binds to SecB/proOmpA. Our results suggest that during preprotein translocation, the two SecA protomers are different in structure and may play different roles.

Highlights

The general secretory pathway in bacteria involves a multipartite protein machine, Sec translocase, which is responsible for transferring unfolded newly synthesized polypeptides across the inner membrane [1]
We studied the structures of two Sec protein complexes in solution involved in tandem translocation steps by electron microscopy, providing direct structural evidence of the oligomeric state of SecA in the initiation of preprotein translocation
The most notable characteristic of the SecA/proOmpA/SecB complex is its asymmetry, which is in accordance with previous results [19], suggesting that the two SecA protomers have different binding affinities for SecB/proOmpA and only one of the SecA protomers predominantly binds with SecB/proOmpA

Summary

Introduction

The general secretory pathway in bacteria involves a multipartite protein machine, Sec translocase, which is responsible for transferring unfolded newly synthesized polypeptides across the inner membrane [1]. The nascent polypeptides are captured by the SecB chaperone in a translocation-competent state after they are released from the ribosome [2]. The complex is targeted to SecA, the translocation motor. SecA couples the stepwise translocation of preprotein across the SecYEG channel with the expenditure of metabolic energy provided by consecutive ATP binding and hydrolysis cycles [3]. In vitro translocation assays show that the ATPase activity of SecA is required for the transmembrane translocation of the amino terminus of the preprotein, including the signal sequence [4,5,6]. In the absence of a transmembrane proton-motive force, SecA is necessary for the translocation of the downstream carboxyl terminus [4]

Methods

Results

Conclusion