234 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
discussed previously in reference 25 for the PDB: 1ORS structure. EPR data
gathered by Perozo and co - workers for KvAP show a lipid environment for
arg123, a lipid – water mixed environment for arg126 and a protein (neither
lipid nor aqueous) for arg133 and arg136 [arginine numbering system is that
found for KvAP (PDB: 1ORQ and 1ORS)].^41 These data agree with the X - ray
structural data, indicating that the fi rst two S4 arginine residues are exposed
to lipid when the pore is in the open conformation — as is the case for Kv1.2.
Indications that the Kv channel is open in the Kv1.2 crystal structure (PDB:
2A79) are twofold: (1) The voltage sensors appear to be in an open posi-
tion — that is, the voltage sensors have moved the gating - charges of two argi-
nine residues nearer to the extracellular side of the membrane and all four
arginine residues are above the membrane midpoint; (2) the inner helix bundle
(activation gate) of the ion conduction pore is opened to a diameter of∼ 12 Å.
The researchers (see references 37 and 38) now speculate as to how the pore
might close. In the closed conformation the inner helix bundle might look like
the closed pore of KcsA (PDB: 1BLB, 1K4C, and 1K4D, references 15 and
17). Downward movement of S4 will bring the arginine residues closer to the
intracellular side of the membrane and push down on the S4 – S5 linker helices.
This movement in turn compresses the inner helices (S5 – S6) and closes the
pore. Qualitatively, the positively charged arginine residues are pushed out
when the membrane is positive inside (opening the pore) and pulled in when
the membrane is negative inside (closing the pore). This explanation is con-
sistent with the biotinylation experiments described above in discussing KvAP
structures PDB: 1ORQ and 1ORS (reference 25), and it indicates that a
segment of the S4 helix can move up to 15 Å through the membrane on pore
opening or closing. It is also consistent with observations of antiparallel align-
ment of S3b and S4 in KvAP and the same alignment between S3 and S4 in
Kv1.2. These observations lead to the hypothesis that S3 and S4 move together
as a voltage sensor paddle unit with S4 closer to the membrane ’ s intracellular
side and S3 on top of S4 allowing S3 to remain closer to the membrane ’ s
extracellular side.
Conclusions stated by the authors for the reference 38 studies include the
following: (1) Voltage sensors are independent self - contained domains, except
for the S4 – S5 linkage to the inner pore helices, allowing them to perform
mechanical work opening and closing the pore; (2) voltage sensor domains
(helices S1 – S4) are membrane - spanning with arginine residues on the S4 helix
exposed to lipid on one face and salt - bridging with acidic residues from S1
and S2 on its other face; (3) the voltage sensor domain ’ s position with respect
to the pore may vary among different Kv channels; and (4) motions of the
Kv1.2 helix are transmitted to the inner helix bundle (activation gate) via the
S4 – S5 linker helices affecting the movements of the S5 and S6 helices to open
or close the pore. The most important conclusion involves the positions of and
interactions between the voltage sensor S1 – S4 helices as they affect opening
and closing of the pore. In the Kv1.2 structure, with the pore in the open con-
formation, S4 helix residues arg294 and arg297 appear to be on the lipid -