The Sec System: Protein Export in Escherichia coli.

In Escherichia coli, proteins found in the periplasm or the outer membrane are exported from the cytoplasm by the general secretory, Sec, system before they acquire stably folded structure. This dynamic process involves intricate interactions among cytoplasmic and membrane proteins, both peripheral and integral, as well as lipids. In vivo, both ATP hydrolysis and proton motive force are required. Here, we review the Sec system from the inception of the field through early 2016, including biochemical, genetic, and structural data.

Characterization of interactions between proteins using site-directed spin labeling and electron paramagnetic resonance spectroscopy.

Site-directed spin-labeling and the analysis of proteins by electron paramagnetic resonance spectroscopy provides a powerful tool for identifying sites of contact within protein complexes at the resolution of aminoacyl side chains. Here we describe the method as we have used it to study interactions of proteins involved in export via the Sec secretory system in Escherichia coli. The method is amendable to the study of most protein interactions.

Export chaperone SecB uses one surface of interaction for diverse unfolded polypeptide ligands.

SecB, a remarkable chaperone involved in protein export, binds diverse ligands rapidly with high affinity and low specificity. Site-directed spin labeling and electron paramagnetic resonance spectroscopy were used to investigate the surface of interaction on the export chaperone SecB. We examined SecB in complex with the unfolded precursor form of outer membrane protein OmpA as well as with a truncated version of OmpA that includes the transmembrane domain and lacks both the signal peptide and the periplasmic domain. In addition, we studied the binding of SecB to the unfolded mature form of galactose-binding protein, a soluble periplasmic protein. We have previously used the same strategy to map the binding surface for the precursor of galactose-binding protein. We show that for all ligands tested the patterns of contact are the same.

SecA, the motor of the secretion machine, binds diverse partners on one interactive surface.

In all living cells, regulated passage across membranes of specific proteins occurs through a universally conserved secretory channel. In bacteria and chloroplasts, the energy for the mechanical work of moving polypeptides through that channel is provided by SecA, a regulated ATPase. Here, we use site-directed spin labeling and electron paramagnetic resonance spectroscopy to identify the interactive surface used by SecA for each of the diverse binding partners encountered during the dynamic cycle of export. Although the binding sites overlap, resolution at the level of aminoacyl side chains allows us to identify contacts that are unique to each partner. Patterns of constraint and mobilization of residues on that interactive surface suggest a conformational change that may underlie the coupling of ATP hydrolysis to precursor translocation.

Sites of interaction of a precursor polypeptide on the export chaperone SecB mapped by site-directed spin labeling.

Export of protein into the periplasm of Escherichia coli via the general secretory system requires that the transported polypeptides be devoid of stably folded tertiary structure. Capture of the precursor polypeptides before they fold is achieved by the promiscuous binding to the chaperone SecB. SecB delivers its ligand to export sites through its specific binding to SecA, a peripheral component of the membrane translocon. At the translocon the ligand is passed from SecB to SecA and subsequently through the SecYEG channel. We have previously used site-directed spin labeling and electron paramagnetic resonance spectroscopy to establish a docking model between SecB and SecA. Here we report use of the same strategy to map the pathway of a physiologic ligand, the unfolded form of precursor galactose-binding protein, on SecB. Our set of SecB variants each containing a single cysteine, which was used in the previous study, has been expanded to 48 residues, which cover 49% of the surface of SecB. The residues on SecB involved in contacts were identified as those that, upon addition of the unfolded polypeptide ligand, showed changes in spectral line shape consistent with restricted motion of the nitroxide. We conclude that the bound precursor makes contact with a large portion of the surface of the small chaperone. The sites on SecB that interact with the ligand are compared with the previously identified sites that interact with SecA and a model for transfer of the ligand is discussed.

Mapping of the docking of SecA onto the chaperone SecB by site-directed spin labeling: insight into the mechanism of ligand transfer during protein export.

Export of protein into the periplasm of Escherichia coli via the general secretory system is achieved by action of a ternary complex comprising the polypeptide ligand, the chaperone SecB and SecA, a peripheral component of the membrane translocon, which is itself an ATPase. The unfolded ligand is captured initially by SecB and must be transferred to SecA and subsequently through the membrane translocon into the periplasm. We have taken the first steps in the elucidation of the mechanism of this dynamic transfer by determining the interface of interaction between SecB and SecA. Site-directed spin labeling and electron paramagnetic resonance spectroscopy were combined to identify which of the residues on SecB showed changes in spectral line shape upon addition of SecA. In all, 43% of the surface of SecB was covered by the 41 positions examined. A model of docking between SecB and SecA is proposed based on the pattern of amino acid residues on SecB shown to make contacts when in complex with SecA. This model in combination with previously published biochemical data provides insight into the transfer of the unfolded polypeptide from the chaperone SecB to SecA.

Asymmetric binding between SecA and SecB two symmetric proteins: implications for function in export.

SecB, a small tetrameric chaperone in Escherichia coli, facilitates export of precursor polypeptides from the cytoplasm to the periplasmic space. During this process, SecB displays two modes of binding. As a chaperone, it binds promiscuously to precursors to maintain them in a non-native conformation. SecB also demonstrates specific recognition of, and binding to, SecA. SecB with the precursor tightly bound enters an export-active complex with SecA and must pass the ligand to SecA at the translocon in the membrane. Here we use variants of SecA and SecB to further probe these interactions. We show that, unexpectedly, the binding between the two symmetric molecules is asymmetric and that the C-terminal alpha-helices of SecB bind in the interfacial region of the SecA dimer. We suggest that disruption of this interface by SecB facilitates conformational changes of SecA that are crucial to the transfer of the precursor from SecB to SecA.

Sites of interaction between SecA and the chaperone SecB, two proteins involved in export.

SecB, a small tetrameric cytosolic chaperone in Escherichia coli, facilitates the export of precursor poly-peptides by maintaining them in a nonnative conformation and passing them to SecA, which is a peripheral member of the membrane-bound translocation apparatus. It has been proposed by several laboratories that as SecA interacts with various components along the export pathway, it undergoes conformational changes that are crucial to its function. Here we report details of molecular interactions between SecA and SecB, which may serve as conformational switches. One site of interaction involves the final C-terminal 21 amino acids of SecA, which are positively charged and contain zinc. The C terminus of each subunit of the SecA dimer makes contact with the flat beta-sheet that is formed by each dimer of the SecB tetramer. Here we demonstrate that a second interaction exists between the extreme C-terminal alpha-helix of SecB and a site on SecA, as yet undefined but different from the C terminus of SecA. We investigated the energetics of the interactions by titration calorimetry and characterized the hydrodynamic properties of complexes stabilized by both interactions or each interaction singly using sedimentation velocity centrifugation.