216 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
the voltage sensor could pull on the outer helix, thereby affecting the inner S6
helix and opening the pore.
The voltage sensor, S1 – S4 helices, are attached to the pore by forming
concentric helices around it. Looking at Figure 3a of reference 18, one sees
that S5 and S6 are closest to the pore while S1 and S2 form another concentric
layer of helices outside S5, and helices S3 and S4 are located at the K + chan-
nel ’ s outer perimeter. Additionally, helix S3 appears to be two individual
helices, designated S3a and S3b. Figure 3c of reference 18 and Figure 5.7 show
that the pore helix is close to the external (extracellular) side of the membrane
while helices S3b and S4 straddle the membrane at its internal side facing the
cytoplasm.
Helices 3b and the N - terminal end of the S4 helix pack tightly against
each other and form a mostly hydrophobic helix – turn – helix structure called
the “ voltage sensor paddle. ” In contrast to its mostly hydrophobic amino acid
composition, the S4 helix contains four important, neatly spaced, basic arginine
residues (arg117, arg120, arg123, and arg126) that are highly conserved among
voltage - dependent K + channels. These four arginine residues are believed to
account for most of the gating charge responsible for opening and closing the
pore. Finding the S4 helix arranged perpendicular to the pore helix at the
intracellular side of the membrane was an unexpected fi nding because it con-
tradicts many previously established electrophysiological measurements. The
previous measurements show that toxins and thiol - reactive compounds react
with the N - terminal portion of S4 at the membrane ’ s extracellular side but not
from the intracellular side.
To address these contradictions the researchers crystallized only the S1 – S4
portion of the protein (PDB: 1ORS) in the presence of a Fab different from
that used in the 1ORQ structure. The X - ray crystallographic structure of the
voltage sensor portion only (PDB: 1ORS), at a resolution of 1.9 Å , has a similar
conformation to that in 1ORQ and behaves as a normal voltage sensor.
However, the atomic resolution shows more detail of the interactions among
the helices in the voltage sensor paddle. The fi rst four arginine residues (117,
120, 123, and 126) on S4 are exposed to solvent in the crystal. The fi fth arginine
residue (arg133) makes a salt bridge with asp62 in S2. This has the effect of
bringing the S2 and S4 helices closer together right beside the S3 loop con-
necting S3a and S3b. Further salt bridges form between arg76 in S2 with asp72
(also in S2) and glu93 in S3a bridging S3a to the C - terminal end of S2. These
residues are all highly conserved in voltage - dependent K + channels. The salt
bridges found in the isolated S1 – S4 voltage - sensor paddle structure (1ORS)
are not found in the full - length channel structure (1ORQ). Other differences
found between the two voltage sensor paddle structures are as follows: (1) S1
is folded back; and (2) S4 is straight in the isolated domain and bent in the
full channel structure, the bend occurring just after the voltage - sensor paddle.
In the full channel structure the S4 bends at a glycine residue to redirect
S4 into S5 while S4 can continue straight in the isolated domain (S5 is
not present). Signifi cantly, the N - terminal half of S4 has residues that are