# Advances in the Assembly Model of Bacterial Type IVB Secretion Systems
## 4 Comparison of T4 BSS with Other Secretion Systems  
In the currently studied Gram-negative bacterial T1 SS to T6 SS, T1 SS, T3 SS, T4 SS, and T6 SS are one-step Sec-independent secretion systems [54], that is, the target protein is directly secreted from the cell to the extracellular environment, not into the periplasmic space. In contrast, T2 SS and T5 SS are two-step Sec-dependent secretion systems [55; 56; 57]: The effectors are transported by substrate recognition proteins to the transmembrane channel, which is then translocated across the IM to the periplasmic space, forming its periplasmic intermediate form. Subsequently, the periplasmic intermediate form is transported to the extracellular space through a channel protein across the OM (Figure 4). T1 SS, also known as ATP-binding cassette (ABC) secretion system, is found in all bacterial genomes that have been fully sequenced. It is mainly used for translocating virulence factors for extracellular transport [58]. The composition of the T1 SS secretion system is very simple, mainly including three functional proteins: ABC translocating enzyme that provides energy for effector secretion in the IM, membrane fusion protein (MFP, IM protein) for movement across the periplasmic space, and OM protein (OMP) [59], corresponding to HlyB, HlyD, and TolC in a typical \(\alpha\)-haemolysin secretion system (_E. coli_), respectively. Taking _Klebsiella axytoca_ as an example, the main components of T2 SS include a series of IM proteins referred to as PullGFKN, that are intracellular secretory proteins transported across the membrane using energy provided by the ATPase PulE. The molecular chaperone PulC that binds to secreted proteins in the periplasmic space, the channel protein PulD inserted into the OM, and the substrate recognition protein PulS [60]. Similar to T2 SS, T3 SS also includes chaperones, translocators, and effector proteins. Unlike other secretory systems, the secretion of effector proteins by T3 SS is dependent on the 5\({}^{\prime}\) end of the mRNA, the N-terminus of the secreted protein, and the binding of secretory proteins to corresponding chaperone proteins [61]. T5 SS just consists of a secreted moiety and a putative membrane translocator domain and is the simplest secretory device. It is proposed that these virulence factors are autonomous secretion systems [62; 63]. Currently, T5 SSs are classified into five classes (Types Va-Ve) based on their domain architecture [64; 65]. The genes for T6 SS are often arranged together to form a gene cluster, which encode 13 to 25 proteins that form core and auxiliary components [66]. Figure 4: Protein components of T1 SS to T6 SS. T1 SS: haemolysin A (HlyA) secretion system in _Escherichia coli_; T2 SS: pullulanase secretion system in _Klebsiella axytoca_; T3 SS: yop secretion system in _Yersinia_; T4 ASS: VirB/VirD4 secretion system in _Agrobacteriumium tumefaciens_; T4 BSS: Dot/Icm system in _Legionella pneumophila_; T5 SS: Type Va (autotransporter system); Type Vb (two-partner secretion systems); Type Vc (trimeric autotransporters); Type Vd (encoding a patatin-like passenger domain fused to a C-terminal \(\beta\)-barrel domain that resembles TpsB proteins); Type Ve (inverse autotransporters and two-partner inverse autotransporters); and T6 SS: secretion system in _Vibrio cholera_. T4 BSS is obviously different from other secretory systems in its special mosaic structure and different constituent proteins, which are homologous with different secretory system components. IcmS and IcmW of T4 BSS perform the same function as the coupling proteins of T3 SS. Loss of these proteins affects intracellular growth but has no significant effect on cytotoxicity [67]. The function of the amorphous N-terminal of the core complex protein DotD is similar to that of VirB7 in T4 ASS (Figure 3i) [68]. In addition, the spherical structure domain of the amorphous N-terminal of DotD shares also homology with the N0/T3S domain of the OM proteins GspD (T2 SS) and EscC (T3 SS) [19] (Table 2, Figure 3b).