A hybrid secretion system facilitates bacterial sporulation: A structural perspective

Bacteria utilize sophisticated nanomachines to transport proteins, small molecules, and DNA across membranes to the extracellular environment. These transport machineries, also known as secretion systems, are involved in various cellular functions, such as adhesion to surfaces or host cells, cell-cell communication, motility (flagella), virulence effector protein secretion, and, notably, bacterial pathogenesis (^1-5). Several of the identified protein secretion systems comprise large complexes that localize and assemble in and around the bacterial membrane(s), forming specialized channels through which the selected substrate(s) is actively delivered (^6-9). Although exhibiting significant diversity in structure, substrate, and function, the dedicated type II, III, IV, and IV-pilus secretion systems (T2SS, T3SS, T4SS, and T4PS, respectively) in didermic Gram-negative bacteria each transport a specific subset of proteins to the extracellular milieu via passage through large stacked ring-shaped channels that span the inner membrane (IM) and outer membrane (OM).