, 2010) in a side-by-side comparison. The comparison indicates a vertical motion of the S4 that is about 7–10 Å, as measured at the Cα atom of R1, though it should be noted that the estimate of vertical motion selleck screening library of the S4 helix depends on how it is defined and aligned with respect to the rest of the structure (see Experimental Procedures). Without additional information, it is prudent to envision a range of values for the vertical motion in order to reflect the uncertainty among the different models depicted in Figure 2 (see also Figure S1) and the thermal fluctuations estimated by MD simulations (see Figure S2). The displacement of S4 is considerably

larger than the 1–2 Å initially proposed on the basis of lanthanide resonance energy transfer (LRET) measurements (Cha et al., 1999). Nonetheless, the displacement of S4 in the model is consistent with the LRET results, once the molecular structures of the donor Luminespib chemical structure and acceptors with their linker are taken into account. Similarly, the motion is considerably smaller than the vertical motion typically associated

with the paddle model, originally proposed to be 20–25 Å on the basis of biotin-avidin trapping data obtained with the KvAP bacterial channel (Jiang et al., 2003). However, further analysis shows that the movement displayed by the S4 helix in the consensus model is actually consistent with the biotin-avidin trapping data (Figure 4). It is also important to keep in mind that those conformational states are dynamic Thymidine kinase and undergo significant fluctuations (Figure S2). Although the three idealized models proposed to explain voltage sensing are often contrasted by the magnitude of the S4 movements, this is clearly an oversimplification. For example, the helix-screw/sliding-helix model pictures predominantly a rigid body motion of S4. However, available X-ray structures and several independent MD simulations provide support

for the intriguing possibility that voltage sensing might be accompanied by a transformation of S4 from an α helix to a 3-10 helix in the resting state (Long et al., 2007, Clayton et al., 2008, Villalba-Galea et al., 2008, Bjelkmar et al., 2009, Khalili-Araghi et al., 2010 and Vieira-Pires and Morais-Cabral, 2010). Indeed, in our own consensus model, the S4 segment retains a portion of the 3-10 helix that was exhibited in Khalili-Araghi’s model. Similarly, one implication of the paddle model is that the helix-turn-helix S3-S4 moves together as a rigid body. Such concerted motion is observed neither in the consensus model nor in experiments. Based on the disulfide-bond pattern of cysteine pairs substituted between S3 and S4 in Shaker, Broomand and Elinder (2008) concluded that the two helices can move relative to each other. Therefore, although the consensus model recapitulates many of the suggestions embodied by the three idealized models, some specific details from those models are not supported.