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![[Figure 4]](rr5172fig4mag.jpg)
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Figure 4
Structural/mechanistic models. (a) Schematic showing the scissoring mechanism proposed for zinc transfer by YiiP. The bundle of four helices (TM1, TM2, TM4 and TM5) is shown in light orange, while the bundle of two helices (TM3 and TM6) is shown in light yellow. The charge interlock (shown in grey) supports the scissoring mechanism of the CTD (shown in green) which is triggered by the release of Zn2+ ions. The N- and C-termini of the CTD are indicated in the left panel. The proposed mechanism is based on the crystal structure of EcYiiP and FRET measurements. This figure is adapted from Lu et al. (2009  ). (b) Alternating-access mechanism proposed by Coudray et al. (2013). The mechanism, based on the crystal structure of EcYiiP and the cryo-EM structure of SoYiiP, involves a proton- and zinc-bound helical bundle and inward-facing and outward-facing conformations. Major conformational changes in both helical bundles (light orange and yellow) are proposed on moving between the four states. In this model, the CTD (green) remains static and the dimer interface is mediated by four CTD-bound Zn2+ ions. (c) Model of the inward-facing and outward-facing conformations of the zinc-bound state of YiiP. Transport is driven by rearrangements of the four-helix bundle (light orange) relative to a static TM3–TM6 scaffold (light yellow). For these models, the inward-facing conformation corresponds to the cryo-EM structure of SoYiiP, whereas the outward-facing conformation corresponds to the X-ray structure of EcYiiP after adjusting the TMs to adopt a compact configuration. |
![[Figure 4]](rr5172fig4.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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