The Type IVB secretion system: an enigmatic chimera.

Type IV secretion systems (T4SSs) are transporters that span the bacterial inner and outer membranes and deliver substrate biomolecules, including proteins and DNAs, into cells. Recent progress in structural analyses of conjugative plasmid-encoded type IVA secretion systems (T4ASSs) has revealed a unique molecular architecture. The essential virulence system, the Dot/Icm Type IVB secretion system (T4BSS) encoded by the intracellular bacterial pathogen Legionella pneumophila is distantly related to T4ASSs. Molecular and structural analyses of the Dot/Icm T4BSS have provided insights into the mechanisms of assembly and function. Here we highlight the chimeric quality of T4BSSs that possess components with similarity to many bacterial secretion systems.