9% or greater). ATP hydrolysis by these domains is necessary for both secretion and phage assembly (Russel, 1995; Schoenhofen et al., 2005), suggesting they may be involved in priming the secretin for activity. The periplasmic portion of GspA, but not pI, is predicted to contain a three-helix-bundle-type

peptidoglycan (PG)-binding domain that is well modeled by Phyre2 (Kelley & Sternberg, 2009). Despite the resemblance of pI to GspA, the similarity is not maintained in the second accessory component in these systems, pXI and GspB, respectively. GspB is encoded separately from GspA, while pXI is formed by an alternate translation start site within the pI transcript and plays a different role (Haigh & Webster, 1999). The Erwinia Out system contains a GspB homolog, OutB, but oddly, lacks a GspA equivalent. Phyre2

buy Metformin (Kelley & Sternberg, 2009) is able to generate only partial models of ExeB, GspB, OutB, and pXI and all are of low confidence. Secondary structure predictions also show significant variations between the proteins. MxiJ is an accessory protein involved in S. flexneri T3S secretin formation (Schuch & Maurelli, 2001). A structure of MxiJ is not available but it can be well modeled on its homologs, S. typhimurium PrgH and E. coli EscJ. PrgH and EscJ are integral proteins involved in T3S and are thought to form 24-membered rings in the inner membrane (Yip et al., 2005; Schraidt & Marlovits, 2011). While the MxiJ homolog is a common component of T3S systems, find protocol the consequences

of mutating this protein are inconsistent across T3S systems. The presence of either MxiJ or the pilotin, MxiM, is sufficient for secretin assembly (Schuch & Maurelli, 2001). In the absence of YscJ in Y. enterocolitica, the secretin formed by YscC appears normal (Diepold et al., 2010). However, without E. coli EscJ or P. aeruginosa PscJ, secretion is abolished, although whether this PTK6 is attributable to a malformed secretin has not been demonstrated (Ogino et al., 2006; Burns et al., 2008). To date, these systems have not been shown to require a MxiM-like pilotin. Structures of T4bP accessory proteins TcpQ and BfpG have yet to be determined, but in both cases Phyre2 (Kelley & Sternberg, 2009) predicts the C-terminal half of the protein to adopt a VirB7-like fold. VirB7, together with VirB9 and VirB10, is involved in forming the outer membrane pore in type IV secretion systems and resembles the N0 domain found in secretins (Souza et al., 2011) although none of the Vir proteins contains a ‘secretin domain’. The presence of an N0-like domain in this non-secretin protein family suggests that Gram-negative bacteria have adopted a common protein fold to allow communication between components of membrane-spanning systems.