# Advances in the Assembly Model of Bacterial Type IVB Secretion Systems
## 3 Mosaic Structure Model of T4 BSS
### Core Complex  
The core complex of T4 ASS, the encoding genes which are located in the IncN plasmid pKM101, consists of three proteins, VirB7, VirB9, and VirB10, in a 1:1:1 ratio. VirB7 is in the outermost layer, surrounding the complex to form a ring; VirB9 is in the middle, forming the sides of the complex; and VirB10 is oriented toward the central cavity and inserted into the outer membrane with two \(\alpha\)-helices per monomer [14]. The core complex of T4 BSS consists of five proteins, DotC-DotD-DotH-DotF-DotG, forming the skeleton structure of the whole secretion system (Figure 2b). DotC and DotD are OM lipoproteins, and their coding genes are located in the same gene cluster. Although there are few reports on DotC, previous studies showed that the ring structure of the core complex in any mutant (\(\Delta dotC\), \(\Delta dotD\) or \(\Delta dotH\)) cannot be detected by transmission electron microscopy, and the expression levels of other proteins are also reduced, indicating the importance of DotC in T4 BSS [15]. OM protein DotH binds to DotC and DotD to form a DotC-DotD-DotH OM complex [16,17]. The expression levels of these three proteins are related and crucial to the formation of circular channels in the core complex. The similarity between the N-terminal domain of VirB9 and residues 168-227 of DotH is as high as 49% (Table 2, Figure 3j). DotH structure was predicted by various programs (I-TASSER, Phyre2 and Quark), was speculated that DotH contains two or more separate domains (Figure 3c) [18]. OM-lipoprotein  
Figure 2: Protein composition of T4 ASS (the VirB/VirD4 system from _Agrobacteriumium tumefaciens_) and T4 BSS (the Dot/lcm system from _Legionella pneumophila_). (a) T4 ASS. Outer membrane components: VirB7, VirB9, and VirB10; inner membrane components: VirB3, VirB6, and VirB8; pilus components: VirB2 (major pilus component) and VirB5 (minor pilus component); lytic transglycos: VirB1; nucleoside triphosphatases: VirB4 and VirB11; coupling protein: VirD4; (b) T4 BSS. Core complex: DotC-DotD-DotH-DotF-DotG; substrate recognition protein: DotF, DotL, DotM, DotN, IcmSW and IcmS-LvgA; other functional proteins: DotE, DotV, DotP, DotI, DotJ, DotA, DotB, IcmQ, IcmR, IcmF, IcmH, DotK, and DotO; proteins with unknown functions: IcmT, IcmV, IcmX. DotD consists of a disordered N-terminal domain and a globular C-terminal N0 secretin domain [19], in which a disordered N-terminal consisting of 46 amino acid residues is homologous to the mature VirB7 (a small peptide consisting of 33 amino acid residues) [20]. Both DotG and DotF are membrane proteins with a single transmembrane helix at the N-terminus. The DotG protein spans the inner membrane and outer membrane, forming the central channel of the core complex, with some homology to VirB10 at the extreme C-terminal domain sequence (Table 2) [21; 22]. Because DotG has a variable region of about 600 residues that is rich in repeats, it tends to be longer than the VirB10 protein, as shown in Figure 3d,k [23]. In the absence of DotG, the outer diameter of the ring structure of the core complex decreases slightly, and the central hole increases (11.3 \(\pm\) 0.1 nm in diameter) by electron microscopy analysis [15]. The inner membrane (IM) protein DotF (\(\sim\)30 kDa) consists of 269 amino acid residues in three domains: the cytoplasmic domain (\(\sim\)20 amino acids), the transmembrane domain (\(\sim\)50 amino acids), and the periplasmic domain (\(\sim\)200 amino acids) [17]. [PERSON] et al. [18] speculated that the \(\beta\)-helices of DotG might have the same function in T4 BSSs as pili in T4 ASSs. Both DotF and OM protein DotD are part of the ring structure. Studies have shown that DotF is not essential for the transfer of effectors and is mainly responsible for the stabilization of the DotG channel or for triggering conformational changes [15; 18]. [PERSON] et al. [15] knocked out the _dotF_ gene in _L.