Protein export and secretion among bacterial pathogens

Many bacteria secrete enzymes and toxins in the environment or into host target cells that can either be eukaryotes (animals as plants) or other bacteria. Bacterial cells are surrounded by a complex envelope including the plasma membrane and a thin peptidoglycan layer which itself is surrounded by an outer membrane containing lipopolysaccharide in Gram-negative bacteria or a thick peptidoglycan layer including teichoic acids in Gram positives. How do those exoproteins manage to cross the cell envelope? The Evolution has produced efficient and diverse mechanisms that range from a single polypeptide to sophisticated macromolecular nanomachines across the bacterial cell envelope. Gram-negatives have evolved eight secretion systems: T1SS (Type I Secretion System) to T6SS and the T9SS restricted so far to the phylum Bacteroidetes. The transport across the envelope can be a two-step process, in which exoproteins are first exported into the periplasm through the Sec or Tat export machineries, then released into the extracellular medium which corresponds to the secretion step (T2-, T5- and T9SS). The T1-, T3-, T4- and T6SS allow a one-step secretion process across the cell envelope leading to delivery of the effector in the medium (T1SS) or into host cells (T3-, T4-, and T6SS). Gram-positive bacteria proteins can be secreted by the Sec or Tat export systems or by the T7SS. Secreted proteins are particularly important in bacterial pathogenesis. Once released into the extracellular medium these effectors can act from the outside of the host cells (i.e. degradative enzymes such as proteases, cellulases, elastases) or translocate into the host cell (i.e. AB type toxins). Once within the host cell the AB type toxins as well as the injected toxins (T3-, T4-, T6SS effectors) hijack major cellular processes changing multiple signal transduction pathways that affect the actin cytoskeleton, cause inflammation… This Research Topics issue will be devoted to updating the current understanding of the various secretion machineries as well as the effectors. We will collect Original Research and Review papers on the topic, but also other article types, such as Methods and Opinions are welcome.