Consistent with this suggestion, Ahern and Horn, 2004 and Ahern and Horn, 2005 found that the membrane field acting on R1 falls over a distance of about 4 Å by using functional measurements with tethered charges attached to the S4 segment. Similarly, Phillips and Swartz (2010) concluded that by using a tarantula toxin, the voltage-sensing S3-S4 helix-turn-helix traverses no more than

the outer leaflet of the membrane bilayer during activation (Phillips et al., 2005b). Fluorescent resonance energy transfer measurements between a donor attached to the voltage sensor and the lipophilic ion dipicrylamine, which serves as the acceptor and is located in the bilayer membrane, indicated that the S4 segment

does PD0325901 order not translocate across the whole lipid bilayer during the voltage-gating transition (Chanda et al., 2005). Luminescence resonance energy transfer used to measure distances between the voltage sensors and a pore-bound scorpion toxin indicated that the VSD segments do not undergo significant transmembrane translation in functional Shaker Kv channels (Posson et al., 2005). A high degree of structural complementarity within the S3-S4 helix-turn-helix paddle motif is not required for the voltage sensing, because it is possible to delete much of S3b while retaining functional channels (Xu et al., 2010). Moreover, the S3b segment is not seen to change accessibility during gating in Shaker channels (Gonzalez et al., 2005). Therefore, although VX-770 datasheet the exact position of the R1 side chain in the resting-state conformation is uncertain and could be either close to E1 or E2, most experimental measurements appear to support the notion that the S4 backbone at the level of R1 is bounded by the midpoint of the membrane bilayer. It is unclear whether the remaining differences can be attributed to the experimental methods used or to evolutionary variations Rutecarpine within the channels themselves.

Many conformational states are accessible to the VSD, and their relative populations could be highly sensitive to small perturbations. For instance, extreme hyperpolarization of the squid giant axon results in delayed activation and onset of K+ currents, consistent with the existence of deeper resting conformational states (Cole and Moore, 1960). The possibility of multiple resting states is further supported by the temperature-dependent conformational transition in Shaker, which exhibits a decrease in entropy upon activation (Rodríguez and Bezanilla, 1996 and Rodríguez et al., 1998). Therefore, although we may speak of a single activated state conformation, it is conceivable that the “resting state” of the VSD actually comprises a population of conformations.