News and views on protein secretion systems

The transport of large hydrophylic molecules across hydrophobic membranes is a fundamental biological principle that ensures appropriate communication between any living cell and its environment. In bacteria, this mechanism is called secretion, and years of active research have highlighted a conserved but large diversity of so-called protein secretion systems. To date, nine distinct systems, called type I to type IX secretion systems (T1SS-T9SS), have been characterized with various degrees of detail and depth. In a few cases, structural studies and cryo-electron microscopy (cryo-EM) reconstruction have revealed a huge supramolecular and hetero-multimeric complex of a size that can be measured in megadaltons. Whereas such complexity is not always needed, a common characteristic of these bacterial systems is their involvement in the transport of proteins and toxins, which is essential for many aspects of bacterial life. In particular, it includes the acquisition of basic nutriments from complex carbon sources, the interaction between the bacteria and their host (be it pathogenic or symbiotic), or the direct competition between bacterial species during colonization of a given niche.