# Advances in the Assembly Model of Bacterial Type IVB Secretion Systems
## 3 Mosaic Structure Model of T4 BSS
### Other Functional Proteins  
Both DotE and DotV are IM proteins composed of four-transmembrane helix bundles that functionally resemble TraQ in I-type conjugation systems. Similar to DotE and DotV, DotP has a cleavable signal sequence at the N-terminus in addition to two transmembrane helices. The genes encoding these three proteins are downstream of the DotF-encoding gene and are arranged in the order of _dotF_, _dotE_, _dotV_, and _dotP_[11]. DotI is also an IM protein with a molecular weight of 23 kDa. The amino acid sequence is homologous to that of the TraM protein in the plasmid R64 conjugative mobilization system, and the C-terminal protein structure is structural homologs to that of VirB8 in T4 ASS (Table 2, Figure 3f). Both DotI and VirB8 form a stack of oligomeric rings in vivo [38; 39] (Figure 3f). The expression levels of DotI are related to its partial homolog DotJ and the protein forms an IM hetero-complex with DotJ at a weight ratio of 1.87:1, which is closely related to the integrity of the inner proteins but does not appear to be associated with the core complex of T4 BSS. Contrary to DotJ, VirB8 has an indispensable role in T4 ASS. The _dotI_ and _dotJ_ genes, coding for DotI and DotJ, are upstream of the core complex DotF, DotG, and DotH protein-encoding genes. DotJ forms a ring around the substrate translocation channel, and its role may be related to the IM-associated ATPase DotO [21; 38]. DotI is present in all known T4 BSS, whereas DotJ is currently only found in bacteria of the order Legionellales, such as \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Protein of T4 BSS**} & \multicolumn{4}{c}{**Homologous Component**} & \multirow{2}{*}{**Protein Type**} \\ \cline{2-2} \cline{4-6}  & **T4 ASS** & **T2 SS** & **T3 SS** & **T6 SS** \\ \hline DotB & VirB11 & GspE & — & — & IM-associated ATPase a \\ DotD & VirB7 & GspD & EscC & — & OM lipoprotein b \\ DotC & — & — & — & — & OM lipoprotein b \\ DotH/IcmK & VirB9 & — & — & — & OM protein b \\ DotG/IcmE & VirB10 & — & — & — & IM protein a \\ DotO/IcmB & VirB4 & — & — & — & IM-associated ATPase a \\ DotCHFG & VirB7, 9, 10 & — & — & — & Core complex \\ DotI & VirB8 & — & — & — & IM protein a \\ DotU/IcmH & — & — & — & TssL & IM protein a \\ IcmF & — & — & — & TssM & IM protein a \\ \hline \hline \end{tabular}  
* a IM: inner membrane; b OM: outer membrane.  
\end{table} 
Table 2: Homologous proteins in T4 BSS and other secretion systems. genus _Legionella_ and _Ricketsiella_. There is a certain similarity between the N-terminal sequences of DotJ and DotJ, although DotJ does not have a periplasmic domain [39]. Similar to VirB4 of T4 ASS, DotO contains Walker A and B motifs (Table 2) [11], is also an IM-associated ATPase, and both of them are the most conserved components of the T4 SS superfamily (Figure 3e) [40]. DotA is an IM protein consisting of eight membrane-spanning domains, two large periplasmic domains (approximately 503 and 73 amino acids), and a small C-terminal cytoplasmic domain (122 amino acids) [41]. [PERSON] et al. reported that DotA and the periplasmic IcmX are in a T4 BSS-dependent manner [42]. The periplasmic domain of DotA is located at the top of the central channel, which can stabilize the channel wall or be released [41]. DotA is only found in bacteria of the genus _Legionella_, forming a cyclic oligomer in the extracellular domain and binding to a protein of unknown function with 46 kDa [11]. When the bacterial _dotA_ gene is deleted, the mutant strain is unable to infect the host cell through the T4 BSS secreting effector protein [43]. DotB is an ATPase mostly present in the cytoplasm, which is similar to ATPases of T2 SS and T4 SS, all of which form hexameric rings. DotB is essential to the normal function of T4 BSS. [PERSON] et al. [44] studied the structure and function of DotB by constructing a series of _dotB_ allele mutants, and found that DotB is composed of an N-terminal domain interacting with cell membranes, an ATPase domain, and a C-terminal domain of unknown function.