Hijacking the Sec machinery in bacterial warfare

All cells are surrounded by membranes, made up from a double layer of fatty molecules called phospholipids. Cell membranes act as a molecular "skin", keeping the cell's insides in, and separating different biochemical reactions. The barrier needs to be breached in a controlled manner to allow transport of nutrients, waste products and for communication with the outside world; this is achieved by a wide range of membrane-inserted proteins.

We understand a great deal about the diverse biological functions that membrane proteins bestow, such as transport, respiration, photosynthesis. However, we know much less about how membranes are formed, or about the how new proteins are transported across or into membranes.

Our lab aims to understand more about how proteins are able to get in and out of the cell. Proteins, such as hormones and antibodies, are normally exported by cells via the secretory ('Sec' for short) machinery, which is essential for life. Simple bacterial cells also secrete proteins for many purposes: to form a protective cell wall, for survival and antibiotic resistance (AMR); to stick to surfaces; and to cause disease.

This proposal concerns the discovery that bacteria also produce proteins that can enter into other bacterial cells, bypassing their membranes and cell wall. This activity is particularly important during bacterial competition, and helps determine which bacteria survive in bacterial communities such as the gut, and how they respond to external factors, e.g. the arrival of disease-causing bacteria.

One of the weapons that bacteria deploy to gain the upper hand are Contact-Dependent growth Inhibitor (CDI) toxins. This project relates to a recent discovery that CDI toxins hijack the Sec system for import into rival cells.

The work will build on a current study analysing how proteins are exported. Most clinically relevant bacteria are surrounded by two membranes, each of which has their own export machinery. We have recently discovered that these machineries -Sec in the inner membrane and BAM in the outer membrane- interact directly with one another. These same two complexes are also hijacked by CDI toxins, so our hypothesis is that this interaction is important both for import as well as export.

The objectives of the project are to understand how this assembly is co-opted for toxin import through an analysis of its architecture and measurement of reversed protein transport (import) activity. To do so the project will harness complementary expertise in biochemistry and new breakthrough technologies in imaging by high-resolution electron cryo-microscopy. These scientific methods will illuminate how the CDI toxin hijacks the secretion machinery for its passage into the cytoplasm.

The results of the project will be important in terms of delivering new understanding of a fundamental process -protein trafficking- that spans the breadth of biology. Moreover, the information we gain could be further exploited. First of all, if we were able to copy and adapt the mechanism deployed by CDI toxins this would allow the delivery of bespoke proteins into bacteria -a feat that is currently very difficult to achieve.

This could be useful, for example, for the delivery of toxic proteins as a strategy to kill specific bacteria for the development of next generation antibiotics. Additionally, a more benign application could be for the import of proteins designed to bestow new synthetic activities for technical innovation useful in academic research and for commercialisation.