3.A.7.9.1
The Icm/Dot or Dot/Icm multicomponent protein secretion system. IcmS and IcmW form a complex that interacts with and may translocate substrate proteins (Ninio et al., 2005; De Buck et al., 2007; Cambronne and Roy, 2007). The crystal structure of the IcmR-IcmQ complex has been solved (Raychaudhury et al., 2009). Legionella pneumophila survives and replicates inside host cells by secreting ~300 effectors through the defective in organelle trafficking (Dot)/intracellular multiplication (Icm) type IVB secretion system (T4BSS). Ghosal et al. 2019, using electron cryotomography mapped the location of the core and accessory components of the Legionella core transmembrane subcomplex, revealing a well-ordered central channel that opens into a large, windowed secretion chamber with an unusual 13-fold symmetry. Immunofluorescence microscopy deciphered an early-stage assembly process that begins with the targeting of Dot/Icm components to the bacterial poles. Polar targeting of this T4BSS is mediated by two Dot/Icm proteins, DotU and IcmF, that are homologues of the T6SS membrane complex components TssL and TssM, suggesting that the Dot/Icm T4BSS is a hybrid system. Thus, the Dot/Icm complex assembles in an 'axial-to-peripheral' pattern (Ghosal et al. 2019).