The World Health Organization ranks bacterial infections among the leading causes of illness and death worldwide. As infectious diseases continue to evolve and spread at an unprecedented rate, structural biologists are working to decipher how bacteria cause disease and evade immune responses. Using state-of-the-art technologies, across three European countries (Diamond Light Source in the UK (accessible through iNEXT and Instruct-ERIC), the Biophysical and Structural Chemistry platform (BPCS) at the IECB in France, and the ScopeM at the ETH Zürich), three teams lead by Rémi Fronzes, Eric Durand and Martin Pilhofer have studied the structure and function of the type VI secretion system (T6SS) in Gram negative bacteria. These findings will contribute to the wider effort to understand the role of microbial interactions in human health, and to exploit these interactions for the development of new therapeutics.
Recent research has uncovered a commonality within Gram-negative bacteria, in their use of effector proteins to infect host cells and to eliminate bacterial competitors. (Examples of such Gram-negative bacteria include Vibrio cholerae and Serratia marcescens, which cause cholera and meningitis, respectively). The T6SS membrane complex is a nanoscale machine that uses a contraction mechanism remarkably similar to that of the phage viruses. Acting as a spring-loaded dagger, it pierces the host cell membrane and injects toxic effector proteins.
In this study a range of structural biology techniques were employed, including cryo-electron microscopy (cryo-EM), scanning cryo-electron tomography (cryo-ET) and focused ion beam (FIB) milling, building on previous X-ray crystallographic work.
A major discovery was that the structure of the T6SS system, determined to 4.9Å resolution by Chiara Rapisarda, demonstrates the 5-fold symmetry of the complex in its native environment. The fully assembled complex forms a bell shape, composed of two rings of pillars that twist around each other (see image below). Single-particle cryo-EM uncovered a novel N-terminal fragment in the periplasmic region that forms an α‐helical domain comprising 8 helices that snake back and forth to the inner membrane, which are thought to act as a gate.
Figure. The high‐resolution structure of T6SS membrane complex in different orientations.
This high-resolution imaging has provided new detail into the structure and dynamics of the T6SS membrane complex, leading to an updated model of its assembly and function. Since effector proteins are often critical to bacterial pathogenicity, insights into their secretion could provide new targets for the treatment and prevention of disease. With antimicrobial resistance posing a serious threat to global health, an understanding of the common mechanisms of phages and bacteria might also pave the way for new phage therapies to combat antibiotic-resistant bacteria.
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