The group “Structural biology of bacterial secretion” was created in October 2009 at Institut Pasteur.
Our research aims at answering a simple question. How macromolecules are transferred through the bacterial cell envelope? Indeed, we study the structure and function of several bacterial secretion systems as well as of the apparatus involved in bacterial transformation (DNA uptake and recombination).
Over the years, we have been interested in the structure of the Type 4, Type 6 and Type 8 secretion systems. These systems are involved in the transfer of proteins through the cellular envelope of Gram-negative bacteria.
- Type 4 secretion (T4S) systems mediate bacterial conjugation and are used by many bacterial pathogens to infect their host cells. As a post-doc in Gabriel Waksman’s laboratory in London (UK), Rémi Fronzes isolated the core complex of the T4S machinery (3 out of 12 proteins composing the T4SS). In tight collaboration with Waksman’s team, we deployed a tremendous effort to isolate a complete T4SS. Eventually, we managed to isolate a membrane complex of 3 MDa in which just two of the T4S components were missing. We determined the first structure of this complex using electron microscopy (Nature 2014).
- The bacterial Type 6 secretion (T6S) system is one of the key players for microbial competition, as well as an important virulence determinant during bacterial infections. It assembles a nano-crossbow-like structure that propels an arrow made of Hcp tube and VgrG spike into the cytoplasm of the attacker cell and punctures the prey’s cell wall. The nano-crossbow is stably anchored to the cell envelope of the attacker by a membrane core complex. In collaboration with Eric Cascales’ and Christian Cambillau’s laboratories in Marseille (France), we recently have shown that this membrane complex is assembled by the sequential addition of three proteins -TssJ, TssM and TssL- and presented a structure of the fully assembled complex (Nature 2015).
- Curli are functional amyloid fibers that constitute the major protein component of the extracellular matrix in pellicle biofilms formed by Bacteroidetes and Proteobacteria. They form a fitness advantage in pathogenic strains and induce a strong pro-inflammatory response during bacteremia. Curli formation requires a dedicated protein secretion machinery (Type 8 secretion or T8S) comprised of the outer membrane lipoprotein CsgG and two soluble accessory proteins, CsgE and CsgF. In collaboration with Han Remaut’s team in Brussels (Belgium), we recently reported the crystal structure of coli CsgG outer-membrane channel. We showed that the specificity factor CsgE forms a nonameric adaptor that binds and closes off the periplasmic face of the secretion channel to allow secretion (Nature 2014).
We are also very interested in understanding how DNA can be uptaken and recombined in the bacterial genome during bacterial transformation. Natural genetic transformation, first discovered in Streptococcus pneumoniae by F. Griffith in 1928, is observed in many Gram-negative and Gram-positive bacteria. This process promotes genome plasticity and adaptability. In particular, it enables many human pathogens such as Streptococcus pneumonia, Neisseria gonorrhoeae or Vibrio Cholerae to acquire resistance to antibiotics and/or to escape vaccines through the binding and incorporation of new genetic material. While it is well established that this process requires the binding, internalization of external DNA and its recombination in the bacterial genome, the molecular details of these steps are unknown. In this project, we aim at acquiring a detailed understating of each of these steps. We discovered a new appendage at the surface of S. pneumoniae cells and showed that this appendage is similar in morphology and composition to appendages called Type IV pili commonly found in Gram-negative bacteria. We demonstrated that this new pneumococcal pilus is essential for transformation and that it directly binds DNA (PLOS Pathogens 2013 and 2015). We are also actively studying the DNA translocation and recombination in tight collaboration with Patrice Polard’s team in Toulouse (France).