One protein, many functions: SslE, the E. coli mucin degrading enzyme that facilitates biofilm formation

Like Comment
Read the paper

Escherichia coli is a Gram-negative bacterium that colonises the intestine of warm-blooded animals. Many E. coli strains are beneficial to gut health, however, virulent strains can cause severe diarrhoeal and extraintestinal diseases such as urinary tract infection, pneumonia, and sepsis1. E. coli is also found in environmental water, for example lakes and irrigation streams, where it can attach to vegetation. To survive and thrive in all these different environments, E. coli strains have developed many tools to cope with stress and use various mechanisms to respond to ever changing conditions2.

In our work, we are interested in a protein that influences intestinal colonisation called SslE (secreted and surface exposed lipoprotein of Ecoli). A vast range of pathogenic and commensal E. coli strains secrete this protein, and it is a major determinant of infection in virulent strains. SslE interacts with mucosal membranes in the host’s intestine where it can degrade mucins. In commensal strains this provides nutrients for growth, but in pathogenic E. coli it also aids bacterial penetration of the gut mucosa and the subsequent delivery of toxins to host cells. This lipoprotein has also been shown to promote biofilm maturation which can give these strains an advantage during colonisation of the gut. Reverse vaccinology studies have also appointed SslE as a strong immunogenic vaccine against E. coli related diseases3,4

When we began studying SslE we did not understand how this protein could perform all these functions, so we first determined the low-resolution structure of SslE using electron microscopy and small angle X-ray scattering. We determined that SslE is a monomer in solution and although we could only see its overall shape, we observed two globular domains at its N-terminus (NT1 and NT2 domains), and a central doughnut shaped core composed of a region we called NT3 and its M60-family peptidase domain5. Using various biophysical and biochemical techniques, we could also see that the NT1 and NT2 domains are dynamic, and we think this is probably important for the recognition and/or breakdown of mucins. However, we also observed that both recombinant and SslE secreted by different E. coli strains, with varying levels of pathogenicity, can form a unique aggregate with amyloid-like properties in response to acidification in biofilms. Looking again at the overall structure of purified SslE we could see that in acidic conditions it undergoes a conformational change and becomes more rigid, and this then leads to its polymerisation into a fibrous material.  

It is widely known that biofilms are complex structures, and components of the biofilm matrix interact with one another to provide stability and access to common goods such as nutrition and water6. We thought that SslE might bind to a component of the matrix and after screening many different polymers, we determined that SslE monomers and aggregates can both bind to DNA in vitro and SslE co-localises with extracellular DNA (eDNA) in mature biofilms. We think SslE could act as a bridge to link different components of the matrix with bacteria, but this is still unclear. We are now looking forward to working out which form of SslE is functional in binding eDNA in the biofilm as we still do not know whether it is the monomeric or aggregated form that is more relevant. If SslE aggregates are not functional here, this then asks the question of what are they doing?  We now need to dig much deeper, but in this work, we have taken a big step forward in understanding the role of SslE in biofilms.

 

References

1          Kaper, J. B., Nataro, J. P. & Mobley, H. L. Pathogenic Escherichia coli. Nat Rev Microbiol 2, 123-140, doi:10.1038/nrmicro818 (2004).

2          Gonzales-Siles, L. & Sjoling, A. The different ecological niches of enterotoxigenic Escherichia coli. Environ Microbiol 18, 741-751, doi:10.1111/1462-2920.13106 (2016).

3          Moriel, D. G. et al. Identification of protective and broadly conserved vaccine antigens from the genome of extraintestinal pathogenic Escherichia coli. Proc Natl Acad Sci U S A 107, 9072-9077, doi:10.1073/pnas.0915077107 (2010).

4          Thorsing, M. et al. Linking inherent O-Linked Protein Glycosylation of YghJ to Increased Antigen Potential. Front Cell Infect Microbiol 11, 705468, doi:10.3389/fcimb.2021.705468 (2021).

5          Noach, I. et al. Recognition of protein-linked glycans as a determinant of peptidase activity. Proc Natl Acad Sci U S A 114, E679-E688, doi:10.1073/pnas.1615141114 (2017).

6          Garnett, J. A. & Matthews, S. Interactions in bacterial biofilm development: a structural perspective. Curr Protein Pept Sci 13, 739-755, doi:10.2174/138920312804871166 (2012).

Paula

Research Associate, Kings College London