222 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
The preceding conclusions were presented as a new theory about channel
pore opening and closing. The tilted positions of the voltage sensor paddles
and the position of S4 exposed to the lipid bilayer at the periphery of the K +
channel ion conduction pore contradicted many other experimental results. In
fact, the proposal as to the position of the S4 helix ignited controversy among
scientists in the fi eld that continues to the current time. Earlier theories had
been put forward and backed by large bodies of experimental data from many
different sources. For instance, Bezanilla and co - workers and Isacoff had used
fl uorescence resonance energy transfer (FRET) experiments to show much
smaller movements of S4 across the membrane (3 Å rather than 15 – 20 Å ) and
voltage sensors in more upright positions than indicated by the MacKinnon ’ s
group work.^27
One theory that differs from the MacKinnon proposals places the S4 helix
in a more upright position, at the edge of the K + channel pore in a so - called
canaliculi, a canal or aqueous gating pore, through which helix S4 (or the
charges themselves) could move in response to changes in the electric fi eld of
the membrane. A protein wall consisting of helices S1, S2, and S3 would sur-
round the positively charged arginine gating charges of S4 (and the rest of this
helix), shielding it from the lipid membrane. In early 2004, Laine, Papazian,
and Roux published a minireview inFEBS Letters that compared their K +
channel voltage sensor domain structural theories with those of the MacKin-
non group.^28 These researchers proposed a model for the Shaker K + channel
in its activated state.^29 (see Figures 9 and 10 of reference 29 or Figure 2 of
reference 28). The model shows the voltage sensor domain S1 – S4 segments as
transmembraneα - helices. In the tetrameric complex of S1 – S6 helices, the S4
segment is located near a groove formed in the interface between the ion
conduction pores of adjacent subunits. The S4 helices of the tetramer appear
at its four corners interior to the S1, S2, and S3 helices and almost halfway
between neighboring S5 helices. The overall organization of the domains is
similar to that seen in the KvAP crystal structure (PDB: 1ORQ, 1ORS);
however, the position of the S4 helix, in particular, differs.
The reference 28 authors continue to detail experimental observations that
place voltage sensor helices in positions within the membrane. Miller and
coworkers conducted site - directed mutagenesis for all residues of helices S1 –
S3.^30 In these experiments, tryptophan (trp) residues were substituted for each
amino acid in turn to determine which residues would be trp - tolerant. These
experiments confi rmed α - helical conformations for S1 and S2 and showed that
K+ channel function was altered when trp residues were placed in some
(labeled non - trp - tolerant), but not all, positions. The same treatment for helix
S3 yielded complex results. At S3 ’ s N - terminal end the distribution of trp -
tolerant positions were consistent with anα - helical structure, however, this
was not the case at S3 ’ s C - terminal end. Other tests indicated that S3 might
be helical for its entire length and that the N - terminal end interfaces with both
lipid and protein while the C - terminal end interfaces with water. Comparisons
of trp - tolerant or trp - intolerant residues over several different Kv channel