Fig. 2. Structure of the ttHsp40−PhoA complex.

(A) Solution structure of the ttHsp40−PhoA. ttHsp40 is shown as a space-filling model in grey. The PhoA sites recognized by ttHsp40 are shown as space-filling models and colored per the color code in the graphic in panel B. The flexible regions of PhoA are shown as a pink ribbon. (B) Cartoon representation of the complex between ttHsp40 and PhoA as determined by NMR is shown in the middle of the panel. The strong binding sites in PhoA are colored distinctly, whereas the weak binding sites are colored uniformly in light blue. The inter-subunit distance between the two CBD1s is ~60 Å and the distance between CBD1 and CBD2 within the same subunit is ~35 Å. The red arrows denote a dynamic equilibrium of the indicated PhoA sites between ttHsp40-bound and dissociated state. Expanded views of the ttHsp40−PhoA complex highlighting the binding details and contacts that mediate recognition of the seven strong PhoA sites (a through g) by ttHsp40 are shown. ttHsp40 in the expanded views is shown as grey ribbon and residues contacting PhoA are displayed as grey ball-and-stick. As discussed in the main text and in Material and Methods, the structure of the complex shown here displays one of the several similar structures that are in dynamic equilibrium. For example, PhoA^b may be bound to CBD1 instead of PhoA^a, and PhoA^e or PhoA^g may be bound to CBD2 instead of PhoA^f. Simultaneous binding of all four ttHsp40 CBDs by PhoA results in the most stable complex (fig. S10).