How do bacterial proteins destined for export move from inside to outside the cell? As mBiosphere readers may know, there are a number of secretion systems that bacteria use to move materials from inside the cell to outside the cell. Some of these systems, such as the Sec secretion system, are conserved across Gram-positive and -negative bacteria. Despite decades of studies on bacterial secretion, new discoveries are still uncovering details about how these secretion systems function. Now, a recent report out from the Journal of Bacteriology makes a fundamental change in how the Sec secretion system interacts with bacterial proteins destined for the membrane.
Figure 1. Model of Bacterial Sec System. Source
The Sec secretory system is conserved across Gram-positive and -negative bacteria, as well as archaea. The current Sec system model posits its reliance on the protein pore SecYEG, which performs the translocation and the ATPase SecA, which provides the energy required for this activity. SecB is a molecular chaperone that recognizes protein substrates by their N-terminal sequences; these unfolded proteins are transferred to SecA and SecYEG, where the substrate is passed through the pore and becomes properly folded (see figure, right)*.
The research by Damon Huber and his colleagues working with senior scientist Bernd Bukau has refined the model. Previous research from this group of scientists had shown that SecA can interact directly with ribosomes. This led the researchers to hypothesize that the Sec system can interact with nascent polypeptides – proteins as they are being synthesized.
To test this model, the scientists looked at mRNAs that purified with SecA by microarray. These mRNAs are the ones being translated by ribosomes that are interacting with SecA. They found an enrichment of mRNAs coding for proteins with transmembrane domains, suggesting SecA has a higher specificity for these proteins - either the ribosome with these transcripts, or possibly to the growing peptide chain itself. Testing the interactions in a secB mutant demonstrated that these SecA interactions were independent of SecB (as well as being independent of Trigger Factor (TF), another protein that interacts with Sec substrates.)
While the previous model suggested the SecB would bind and deliver Sec protein substrates to SecA, the researchers next showed that SecA could bind directly to polypeptides as short as 116 amino acids in length. This is shorter than the approximately 180 residues required by SecB for polypeptide interactions. In fact, further genetic studies showed that the SecB interactions with polypeptides were dependent on SecA-ribosome interactions, implying the order of Sec system interactions may be different than previously described.
Figure 2. Revised Model of Bacterial Sec System Source
In their new model, the authors lay out an interaction of SecA with polypeptides from still-growing proteins as they exit the ribosome (see figure, right). Here, SecB serves to strengthen these interactions, acting as a stabilizer instead of a delivery mechanism. The results suggest that there may not be a post-translational translocation system in E. coli, in which a protein is fully translated before interacting with secretion machinery. But, as the authors themselves say in their conclusions, their results also “suggest the interplay between SecA, SecB, and TF, which is important for determining the timing of protein translation, is complex.” Future studies will help dissect these complex relationships.
The findings from this E. coli model system fundamentally shift the way bacteriologists understand protein secretion, but there are larger implications as well. In addition to bacteria and archaea, many eukaryotes have a Sec-like system for protein insertion in their endoplasmic reticulum, and plants have a similar system in the thylakoid membrane. Defining the mechanisms of protein secretion in this E. coli model may therefore have a broader impact on understanding protein secretion. And as we saw with this year’s Nobel Prize, basic science to understand fundamental cell processes can have important future impacts on many applications.
* For simplicity’s sake, this summary ignores the role of SRP in an alternative mechanism of protein translocation through SecYEG.