Polar targeting and assembly of theLegionellaDot/Icm type IV secretion system (T4SS) by T6SS-related components

Abstract

Legionella pneumophila, the causative agent of Legionnaires' disease, survives and replicates inside amoebae and macrophages by injecting a large number of protein effectors into the host cells' cytoplasm via the Dot/Icm type IVB secretion system (T4BSS). Previously, we showed that the Dot/Icm T4BSS is localized to both poles of the bacterium and that polar secretion is necessary for the proper targeting of the Legionella containing vacuole (LCV). Here we show that polar targeting of the Dot/Icm coretransmembrane subcomplex (DotC, DotD, DotF, DotG and DotH) is mediated by two Dot/Icm proteins, DotU and IcmF, which are able to localize to the poles of L. pneumophila by themselves. Interestingly, DotU and IcmF are homologs of the T6SS components TssL and TssM, which are part of the T6SS membrane complex (MC). We propose that Legionella co-opted these T6SS components to a novel function that mediates subcellular localization and assembly of this T4SS. Finally, in depth examination of the biogenesis pathway revealed that polar targeting and assembly of the Legionella T4BSS apparatus is mediated by an innovative "outside-inside" mechanism.

Evolution of the Arsenal of Legionella pneumophila Effectors To Modulate Protist Hosts

“…Intracellularly, the Dot/Icm T4SS machinery is located at the poles of the bacterium ( 111 ). Despite the potential to translocate a large number of different effectors, on average, only ∼4 Dot/Icm T4SS translocation structures are located at a pole ( 111 , 112 ). Surprisingly, nonpolar localization of the Dot/Icm structures results in failure of the pathogen to evade the lysosomes, despite translocating effectors ( 111 ).…”

“…The failure to establish the LCV could be derived from a failure to intercept ER-derived vesicles by the LCV. L. pneumophila may fail at polar delivery of Dot/Icm effectors, preventing LCV biogenesis ( 111 , 112 ). Alternatively, the host may have a unique primitive innate mechanism that L. pneumophila is not equipped to modulate.…”

The Polar Legionella Icm/Dot T4SS Establishes Distinct Contact Sites with the Pathogen Vacuole Membrane

“…Polar secretion of substrates is crucial for the establishment of the LCV and is attributed to the polar localization of the T4SS ( 16 – 18 ). In fact, upon translocation, many effector proteins are retained at the cytoplasmic face of the LCV membrane adjacent to the bacterial cell poles ( 19 – 23 ).…”

“…In fact, upon translocation, many effector proteins are retained at the cytoplasmic face of the LCV membrane adjacent to the bacterial cell poles ( 19 – 23 ). Two Icm/Dot proteins, DotU and IcmF, target the T4SS to the cell poles independently of the other Icm/Dot components ( 17 ). DotU and IcmF are homologs of TssL and TssM, which are components of the type VI secretion system (T6SS) trans-envelope complex ( 24 – 26 ).…”

Localization of EccA3 at the growing pole in Mycobacterium smegmatis

“…These include AccA3 (also identified in this study, Table S 5 ), a component of Acyl-CoA carboxylase (ACCase) which is required for lipid and mycolic acid synthesis, and the cell wall synthesis protein CwsA [ 36 – 38 ]. Interestingly, the polar localization of the type IV secretion system was also linked to cell division in L. pneumophila [ 39 ]. The co-purification of EccA 3 and polar proteins associated with polar growth (Wag31, AccA3 and ParB), was not unexpected as EccA 3 as well as proteins from ESX-1 localized to the site of active cell growth [ 12 , 18 , 37 , 40 , 41 ].…”