SecY-SecA fusion protein retains the ability to mediate protein transport

A unifying mechanism for protein transport through the core bacterial Sec machinery

Encapsulation and compartmentalization are fundamental to the evolution of cellular life, but they also pose a challenge: how to partition the molecules that perform biological functions-the proteins-across impermeable barriers into sub-cellular organelles, and to the outside. The solution lies in the evolution of specialized machines, translocons, found in every biological membrane, which act both as gate and gatekeeper across and into membrane bilayers. Understanding how these translocons operate at the molecular level has been a long-standing ambition of cell biology, and one that is approaching its denouement; particularly in the case of the ubiquitous Sec system. In this review, we highlight the fruits of recent game-changing technical innovations in structural biology, biophysics and biochemistry to present a largely complete mechanism for the bacterial version of the core Sec machinery. We discuss the merits of our model over alternative proposals and identify the remaining open questions. The template laid out by the study of the Sec system will be of immense value for probing the many other translocons found in diverse biological membranes, towards the ultimate goal of altering or impeding their functions for pharmaceutical or biotechnological purposes.

The SecA ATPase motor protein binds to Escherichia coli liposomes only as monomers

The essential SecA motor ATPase acts in concert with the SecYEG translocon to secrete proteins into the periplasmic space of Escherichia coli. In aqueous solutions, SecA exists largely as dimers, but the oligomeric state on membranes is less certain. Crystallographic studies have suggested several possible solution dimeric states, but its oligomeric state when bound to membranes directly or indirectly via the translocon is controversial. We have shown using disulfide crosslinking that the principal solution dimer, corresponding to a crystallographic dimer (PDB 1M6N), binds only weakly to large unilamellar vesicles (LUV) formed from E. coli lipids. We report here that other soluble crosslinked crystallographic dimers also bind weakly, if at all, to LUV. Furthermore, using a simple glutaraldehyde crosslinking scheme, we show that SecA is always monomeric when bound to LUV formed from E. coli lipids.

Structural Basis of the Sec Translocon and YidC Revealed Through X-ray Crystallography

Protein translocation and membrane integration are fundamental, conserved processes. After or during ribosomal protein synthesis, precursor proteins containing an N-terminal signal sequence are directed to a conserved membrane protein complex called the Sec translocon (also known as the Sec translocase) in the endoplasmic reticulum membrane in eukaryotic cells, or the cytoplasmic membrane in bacteria. The Sec translocon comprises the Sec61 complex in eukaryotic cells, or the SecY complex in bacteria, and mediates translocation of substrate proteins across/into the membrane. Several membrane proteins are associated with the Sec translocon. In Escherichia coli, the membrane protein YidC functions not only as a chaperone for membrane protein biogenesis along with the Sec translocon, but also as an independent membrane protein insertase. To understand the molecular mechanism underlying these dynamic processes at the membrane, high-resolution structural models of these proteins are needed. This review focuses on X-ray crystallographic analyses of the Sec translocon and YidC and discusses the structural basis for protein translocation and integration.

GREACE-assisted adaptive laboratory evolution in endpoint fermentation broth enhances lysine production by Escherichia coli

BACKGROUND

Late-stage fermentation broth contains high concentrations of target chemicals. Additionally, it contains various cellular metabolites which have leaked from lysed cells, which would exert multifactorial stress to industrial hyperproducers and perturb both cellular metabolism and product formation. Although adaptive laboratory evolution (ALE) has been wildly used to improve stress tolerance of microbial cell factories, single-factor stress condition (i.e. target product or sodium chloride at a high concentration) is currently provided. In order to enhance bacterial stress tolerance to actual industrial production conditions, ALE in late-stage fermentation broth is desired. Genome replication engineering assisted continuous evolution (GREACE) employs mutants of the proofreading element of DNA polymerase complex (DnaQ) to facilitate mutagenesis. Application of GREACE coupled-with selection under stress conditions is expected to accelerate the ALE process.

RESULTS

In this study, GREACE was first modified by expressing a DnaQ mutant KR5-2 using an arabinose inducible promoter on a temperature-sensitive plasmid, which resulted in timed mutagenesis introduction. Using this method, tolerance of a lysine hyperproducer E. coli MU-1 was improved by enriching mutants in a lysine endpoint fermentation broth. Afterwards, the KR5-2 expressing plasmid was cured to stabilize acquired genotypes. By subsequent fermentation evaluation, a mutant RS3 with significantly improved lysine production capacity was selected. The final titer, yield and total amount of lysine produced by RS3 in a 5-L batch fermentation reached 155.0 ± 1.4 g/L, 0.59 ± 0.02 g lysine/g glucose, and 605.6 ± 23.5 g, with improvements of 14.8%, 9.3%, and 16.7%, respectively. Further metabolomics and genomics analyses, coupled with molecular biology studies revealed that mutations SpeB^A302V, AtpB^S165N and SecY^M145V mainly contributed both to improved cell integrity under stress conditions and enhanced metabolic flux into lysine synthesis.

CONCLUSIONS

Our present study indicates that improving a lysine hyperproducer by GREACE-assisted ALE in its stressful living environment is efficient and effective. Accordingly, this is a promising method for improving other valuable chemical hyperproducers.

Investigating the stability of the SecA-SecYEG complex during protein translocation across the bacterial membrane

During posttranslational translocation in Escherichia coli, polypeptide substrates are driven across the membrane through the SecYEG protein-conducting channel using the ATPase SecA, which binds to SecYEG and couples nucleotide hydrolysis to polypeptide movement. Recent studies suggest that SecA is a highly dynamic enzyme, able to repeatedly bind and dissociate from SecYEG during substrate translocation, but other studies indicate that these dynamics, here referred to as "SecA processivity," are not a requirement for transport. We employ a SecA mutant (PrlD23) that associates more tightly to membranes than WT SecA, in addition to a SecA-SecYEG cross-linked complex, to demonstrate that SecA-SecYEG binding and dissociation events are important for efficient transport of the periplasmic protein proPhoA. Strikingly however, we find that transport of the precursor of the outer membrane protein proOmpA does not depend on SecA processivity. By exchanging signal sequence and protein domains of similar size between PhoA and OmpA, we find that SecA processivity is not influenced by the sequence of the protein substrate. In contrast, using an extended proOmpA variant and a truncated derivative of proPhoA, we show that SecA processivity is affected by substrate length. These findings underscore the importance of the dynamic nature of SecA-SecYEG interactions as a function of the preprotein substrate, features that have not yet been reported using other biophysical or in vivo methods.

Single-Unit Imaging of Membrane Protein-Embedded Nanodiscs from Two Oriented Sides by High-Speed Atomic Force Microscopy

Membrane proteins play important roles in various cellular functions. To analyze membrane proteins, nanodisc technology using membrane scaffold proteins allows single membrane protein units to be embedded into the lipid bilayer disc without detergents. Recent advancements in high-speed atomic force microscopy (HS-AFM) have enabled us to monitor the real-time dynamics of proteins in solution at the nanometer scale. In this study, we report HS-AFM imaging of membrane proteins reconstituted into nanodiscs using two membrane protein complexes, SecYEG complex and MgtE dimer. The observed images showed single particles of membrane protein-embedded nanodiscs in an end-up orientation whereby the membrane was fixed parallel to the supporting solid surface and in a side-on orientation whereby the membrane plane was vertically fixed to the solid surface, enabling the elucidation of domain fluctuations in membrane proteins. This technique provides a basic method for the high-resolution imaging of single membrane proteins by HS-AFM.