, 2011). In place of the long α-helix that was found to block the MxiM pore, YscW only contains a α-helical turn. These results suggest either that the bound lipid in MxiM is an artifact of the crystallization process, which required detergents to be present, or that the lipid disruption mode of secretin insertion into membranes is not universally used by
Class 2 pilotins. Class 3 pilotins InvH, OutS, and PulS are predicted to be similar in size to the β-strand pilotins and to be predominantly α-helical, although they lack predicted TPRs (Fig. 1c). Structural data for this group are limited to the crystal structure of E. coli T2S GspS (PDB ID: 3SOL), an orthologue of the Class 3 pilotins that has not been functionally characterized. While the sequence identity among GspS, OutS, and PulS ranges from 30% to 36%, the sequence identity of InvH to OutS, PulS, and GspS is only 3%, 12%, and 14%, respectively. The structure of GspS is a selleck four α-helix bundle, as is predicted for OutS and PulS (Fig. 1c). One face of GspS forms a distinct groove that could provide a convenient binding surface for an interacting partner. InvH is predicted to contain shorter α-helices and a large central region without regular secondary structure. Tertiary structure predictions by Phyre2 (Kelley Palbociclib price & Sternberg, 2009) produces high confidence models (100%) for OutS and PulS
based on GspS. As InvH is significantly different from the others at the sequence level, models can only be generated for a fragment of the protein at confidence levels of 47.3% or lower, and are not templated on GspS. Accessory ID-8 proteins that have been functionally
characterized in secretin-containing systems are listed in Table 1. Accessory proteins are not always present in a particular system, nor are their functions always the same. Many accessory proteins appear to be involved in stability of the secretin or of the secretin subunit prior to assembly. Accessory proteins that have been reported to influence secretin formation include ExeA/B in A. hydrophila; GspA/B in Vibrio species and Aeromonas salmonicida; OutB in E. chrysanthemi; MxiJ in S. flexneri; PilP in Neisseria meningitidis and P. aeruginosa; FimV in P. aeruginosa; pI/pXI in filamentous phage; BfpG in E. coli; and TcpQ in V. cholerae. In T2S, GspA/B in Vibrio species and A. salmonicida (ExeA/B in A. hydrophila) has been found to be important for expression of the secretin. However, the protein pair is not universally present – or has yet to be identified – in all T2S systems (Strozen et al., 2011). GspA spans the inner membrane and has domains in both the cytoplasm and the periplasm (Schoenhofen et al., 1998; Howard et al., 2006). A surprisingly similar arrangement and orientation is predicted for the filamentous phage accessory protein, pI, which raises the possibility that the two could be evolutionarily related.