By J. Andrew Alexander, PhD Candidate, Strynadka Lab
How do pathogenic bacteria manipulate host cells and cause disease? What molecular structures allow them to accomplish such feats? Here, at the Centre for Blood Research, Dr Natalie Strynadka’s laboratory and colleagues have used cryo-electron microscopy (cryo-EM) to visualize molecular details of the “nano-syringe” Salmonella uses to inject virulence proteins into eukaryotic host cells and cause disease. Their findings have recently been published in Nature.
The molecular “syringe and needle”, or Type 3 Secretion System (T3SS), is used by many pathogenic Gram-negative bacteria, including Salmonella, to transfer proteins directly from the bacterial cell to their host cell. This feat requires penetration through the two bacterial membranes and the host membrane to deliver the proteins essential for virulence. A detailed understanding of how the T3SS functions could facilitate the development of much needed therapeutics to treat Salmonella infections.
The Centre for Disease Control estimates that Salmonella infections caused over 650,000 deaths in 2010, with Africa having the highest incidence of disease. In the United States alone, the CDC estimates that Salmonella causes approximately 1 million illnesses, 19,000 hospitalisations, and 380 deaths per annum.
The T3SS has long been an area of intense study, given the impact of this deadly pathogen, along with other Gram negative bacteria that rely on it for pathogenicity. Because the T3SS is so large and composed of multiple proteins, most previous detailed structural information has resulted from the study of individual components. This work provides the first high resolution look at the molecular details of the assembled structure of the T3SS basal body – the “syringe” portion of the system.
Dr Liam Worrall, the co-first author of the paper says, “Structural biology can be typically a rather reductionist pursuit where we take things apart to discover how they work … although this often leaves large gaps in our understanding of how things work. The really exciting thing we have done here was to show many proteins assembled together, giving us a much better idea of how things actually function in the pathogen.”
Liam explained that the level of detail of the T3SS structure was made possible by several key advances in cryo-EM hardware and software, which allowed the structure of the T3SS to be resolved down to positions of the amino acid side chains. Liam was quick to highlight the highly collaborative effort involving colleagues in Dr Strynadka’s and Dr Finlay’s laboratories at UBC and the Janelia Research Campus in the US that made this paper possible.
Although this paper presents a milestone structure in the field, the Strynadka lab is still seeking to understand and visualize other parts of the T3SS. Future studies will focus on the assembled T3SS “export apparatus”, which feeds partially folded proteins into the basal body, and the “translocon”, which forms the pore in the host cell membrane.
Excitingly, UBC has recently acquired an FEI Titan Krios, the cryo-electron microscope at the heart of this “resolution revolution”. Now that we have a cutting-edge cryo-EM system right here at UBC, who knows what more we will be able to accomplish!