226 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
and S3, N - terminal ends of S2 and S4, and S1 – S2 and S3 – S4 loops are accessi-
ble from the extracellular side of the membrane in all conformations, whereas
the same regions in the PDB: 1ORQ structure appear to be on or near the
internal side of the membrane; (3) in Shaker channels, residues immediately
preceding helix S1 are located in the cytoplasm, whereas in PDB: 1ORQ these
are located in the transmembrane region; and (4) except for the S3 – S4 hairpin
turn, the tertiary structure of the voltage - sensing domain of PDB: 1ORQ devi-
ates substantially from that of the isolated domain (PDB: 1ORS).
The model put forward by the reference 36 researchers is a more traditional
one in which each of the S1 – S6 segments traverses the entire transmembrane
region and in which the S4 helix moves through the membrane via the “ helical
screw mechanism. ” The S4 helical screw model is believed to be more energeti-
cally favorable than that put forth by the MacKinnon group because the posi-
tively charged S4 groups in the transmembrane region are always close to a
negatively charge residue on S1, S2, or S3. To build the model they advocate,
these authors began with the KvAP crystal structures PDB: 1ORQ (full - length
channel) and PDB: 1ORS (voltage - sensor domain only), combined these using
the pore - forming domain of PDB: 1ORQ and the voltage - sensor domain of
PDB: 1ORS, and imposed constraints based on experimental results and phys-
iochemical principles. In their model the S4 helix and S4 – S5 linker, with their
hydrophilic residues, are positioned to interact with other residues in the
hydrophilic core of the voltage - sensor domain, with polar lipid headgroups (at
or near the membrane surface), or with water at the membrane – water inter-
face. Also predicted from past studies are hydrophilic cavities or crevasses that
face toward the pore - forming domain and isolate these structures from the
hydrophobic membrane surroundings.
Next, models were made for the voltage - sensor domain in open, transition,
and resting (closed) confi gurations. Viewing the movies supplemental to the
Guy publication (reference 36) will help greatly in visualizing these models.
Go to theBiophysical Journal website at http://www.biophysj.org , enter the
volume number (87) and fi rst - page number (2255) for this article, choose the
full text option, and then click on supplemental materials. There are two
movies, and they are the last two items on the supplemental materials list. In
the open (activated) conformation, most of the positively charged S4 arginine
residues are on the extracellular side, as expected from many experimental
results. (See discussion of references 34 and 35b.) In order to close the ion
conduction pore, the positive charges on S4 must cross the membrane while
remaining charged. This model asserts that negatively charged amino acids
existing on the S1, S2, and S3 segments in the voltage sensor ’ s core domain
facilitate this process. In KvAP, the residues are glu28 ( E1a in the terminology
of Guy, reference 36) and glu45 ( E1b ) in S1, asp62 ( D2a ) and asp72 ( D2b ) in
S2, glu93 ( E3a ) in S3a, and glu107 ( E3b ) in S3b. In the Shaker protein, the
comparable residues would be glu247 (KvAP glu45) in the S1 – S2 linker,
glu283 (KvAP asp62) and glu293 (KvAP asp72) in S2, and asp316 (KvAP
glu93) in S3. Because of their positions in this model, glu45, asp62, and glu107