Reviewer #1 (Public Review):
Summary:
In this paper, Li and colleagues overcome solubility problems to determine the structure of FtsEX bound to EnvC from E. coli.
Strengths:
The structural work is well done, and the work is consistent with previous work on the structure of this complex from P. aerugionsa.
Weaknesses:
The model does not take into account all the information that the authors obtained, as well as known in vivo data.
The work lacks a clear comparison to the Pseudomonas structure highlighting new information that was obtained so that it is readily available to the reader.
The authors set out to obtain the structure of FtsEX-EnvC complex from E. coli. Previously, they were unable to do so but were able to determine the structure of the complex from P. aeruginosa. Here they persisted in attacking the E. coli complex since more is known about its involvement in cell division and there is a wealth of mutants in E. coli. The structural work is well done and recapitulates the results this lab obtained with this complex from P. aeruginosa. It would be helpful to compare more directly the results obtained here with the E. coli complex with the previously reported P. aeruginosa complex - are they largely the same or has some insight been obtained from the work that was not present in the previous complex from P. aeruginosa. This is particularly the case in discussing the symmetrical FtsX dimer binding to the asymmetrical EnvC, since this is emphasized in the paper. However, Figures 3C & D of this paper appear similar to Figures 2D & E of the P. aeruginosa structure. Presumably, the additional information obtained and presented in Figure 4 is due to the higher resolution, but this needs to be highlighted and discussed to make it clear to a general audience.
The main issue is the model (Figure 6). In the model ATP is shown to bind to FtsEX before EnvC, however, in Figure 1c, it is shown that ADP is sufficient to promote binding of FtsEX to EnvC.
The work here is all done in vitro, however, information from in vivo needs to be considered. In vivo results reveal that the ATP-binding mutant FtsE(D162N)X promotes the recruitment of EnvC (Proc Natl Acad Sci U S A 2011 108:E1052-60). Thus, even FtsEX in vivo can bind EnvC without ATP (not sure if this mutant can bind ADP).
Perhaps the FtsE protein from E. coli has to have bound nucleotides to maintain its 3D structure.
Comments after revision:
The most interesting aspect of this complex is that it has yet to be determined the order of events in the ATPase cycle as the authors acknowledge. Although the authors have responded quite well to the comments, I am still worried about the significance of the in vitro results compared to the in vivo results reported by others. In vivo ATP binding does not appear required for complex formation (of course it is possible that ADP is responsible in vivo). Have the authors tried to solve the complex with ADP since they suggested that it is sufficient to hold the complex together). If possible, it would confirm the role of ATP binding by comparing the structures. Also, it is not clear if ADP binds to any of the mutants made by the Bernhardt lab (D162N, K41M). If they do not bind ADP then FtsEX without nucleotide is able to bind EnvC as the authors indicate is the case in Pseudomonas. It is also unclear the significance of the ATPase activity of FtsEX in vitro with or without EnvC. Could the activity be some basal activity that is not relevant to the in vivo situation. If EnvC caused FtsEX to hydrolyze ATP it would be a futile cycle as FtsEX and EnvC are localized to the septum long before they are involved septal hydrolysis.

