232 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
for the fact that when aShaker K + channel opens, the equivalent of 12 – 14
positive charges cross the membrane electric fi eld from inside to outside and
most of this charge is carried by four S4 arginine residues (arg294, arg297,
arg300, and arg303 in Kv1.2) on each channel tetramer subunit.
Existent voltage sensor models postulate that the positively charged S4
helix (that must be protected from the membrane ’ s lipid environment) rests
in a groove at the interface between adjacent subunits of the K + channel tet-
ramer so that poreα - helices S5 and S6 form a wall on one side of S4 and
voltage sensorα - helices S1 and S3 form a wall on the other side. The groove
or canal forms a gating channel and allows S4 to move its charged arginine
residues across the membrane without exposing them to the lipid environ-
ment. How far the S4 helix moves has been a matter for much debate with
hypotheses of large translations ( ∼ 15 Å ) and experimentally determined (by
fl uorescence resonance energy transfer, FRET) translations of less than 3 Å.
All models have two features in common: (1) The S4 helix must be sequestered
from the lipid membrane, and (2) voltage sensor helices S1 – S4 are packed
tightly against the S5 and S6 helices surrounding the pore. The MacKinnon
research team believes that the voltage sensor may be an almost independent
domain, and site evidence that indicates this. One piece of evidence comes
from the study of KvAP and the PDB: 1ORQ and 1ORS structures discussed
above in references 18 and 25. The PDB: 1ORS structure is that of the isolated
voltage sensor domain itself — expressed without the pore — arguing that this
may be an independent domain. The full - length channel structure, PDB:
1ORQ, showed the voltage sensor domain in a strange position, pulled toward
the cytoplasmic side of the pore. Other evidence, cited above in the discussion
of reference 18 and 25, indicated large movement of the helix – turn – helix
voltage sensor paddle through the membrane and the possibility that some
arginine residues were exposed to the hydrophobic membrane. However,
there were major weaknesses in the KvAP study directly related to the voltage
sensor ’ s mobility: (1) Distortions arising from extracting the protein from its
membrane left many aspects of the structure uncertain; and (2) the connec-
tions between the voltage sensors and the ion conduction pore were disrupted,
leading to a possible incorrect placement of the voltage sensor within the
membrane.
The voltage sensor domain structural problems are further discussed in
reference 38 by using the knowledge gained from the reference 18 KvAP
structural study (PDB: 1ORQ, 1ORS) in concert with another KvAP structure
(PDB: 2A0L) published more recently.^39 In the current structural study of
Kv1.2 (PDB: 2A79), the T1 domain and its connection to the S1 helix (part of
the voltage sensor assembly) help to hold the voltage sensor in its native con-
formation. Similarities and differences between Kv1.2 (PDB: 2A79) and the
KvAP (PDB: 1ORQ, 1ORS) structures are as follows: (1) Helices S3 and S4,
called the voltage sensor paddle, are in an antiparallel arrangement for both
Kv1.2 and KvAP; (2) in KvAP, helix S3 appears to have two subunits, S3a and
S3b, whereas in Kv1.2 S3 appears to be a single helix with a bend; (3) voltage