POTASSIUM-DEPENDENT MOLECULES 221
and changes the kinetics of current activation. The authors believe that incom-
plete biotinylation of interior residues is responsible for this behavior with this
and some other mutated residues.
Now the reference 25 researchers conducted the cysteine mutant – biotin –
avidin test on selected residues of the voltage - sensor paddle. Residues gly101,
leu103, ala104, gly108, leu110, gly112, leu113, leu115, and val119 (red stick form
in Figure 5.8 ) include all of S3b, the tip of the paddle (S3 – S4 loop) and the
fi rst helical turn into S4. They behave in the following manner: (1) At negative
voltages — channel closed — none of these positions test positively — that is,
they lie at least 10 Å from the membrane ’ s edge internally and externally; (2)
at positive membrane voltage — channel open — these residues become acces-
sible to external avidin — that is, they are within 10 Å of the membrane surface.
Residues leu125 and leu127 (royal blue in Figure 5.8 ) in the S4 segment of the
voltage - sensor paddle behave as follows: (1) At negative voltages — channel
closed — these residues have a positive avidin test only internally — that is, they
lie within 10 Å of the internal membrane surface; (2) at positive membrane
voltage — channel open — these residues test negatively — that is, they are more
than 10 Å from the internal and external membrane surface. Residues leu121
and leu122 (yellow in Figure 5.8 ), near the center of segment S4, react posi-
tively to internal avidin at negative voltages and positively to external avidin
at positive voltages — that is, they are within 10 Å of the internal membrane at
negative voltages and within 10 Å of the external membrane when depolariza-
tion takes place. Figures 5a,b of reference 25 and Figure 5.8 indicate the
changes graphically, showing that under conditions of depolarization the tip
(S3b – S4 connector loop) of the voltage - sensor paddle is actually pulled up out
of the membrane into the external solution, whereas under negative voltage
conditions the voltage - sensor paddle lies deep within the membrane and por-
tions of S4 actually reach through the internal membrane surface into the cell
cytoplasm. The authors estimate that the voltage - sensor paddle ’ s center of
mass translates approximately 20 Å through the ∼ 30 - to 35 - Å - thick membrane
from inside to out, while the orientation with respect to the membrane changes
from nearly horizontal — pore closed — to nearly vertical — pore open.
The authors ’ conclusions from references 18 and 25 are the following: (1)
Gating charges (principally on residues arg117, arg120, arg123, arg126) are
carried on the voltage - sensor paddles; (2) voltage - sensor paddles are helix –
turn – helix structures comprised of helix S3b, loop S3b – S4, and the N - terminal
half of helix S4; (3) voltage - sensor paddles are attached to the K + channel
through fl exible S3 loops and S4 – S5 loop linkers; (4) the paddles are located
at the K + channel ’ s outer perimeter and can move through the lipid membrane;
(5) S2 helices lie beside the K + channel pore and contain acidic amino acid
residues, glu53, asp62, and asp72, that could help stabilize positive arginine
residues as they traverse the membrane; (6) voltage - sensor paddles move
approximately 20 Å across the membrane and perpendicular to it; and (7) the
large displacement of the voltage - sensor paddles could open the pore by
pulling on the S4 – S5 linker.