Page 46 - YORAM RUDY BOOK FINAL

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Upon depolarization, the voltage sensor underwent an initial fast ~ 9Å upward Z movement,

followed by a slower ~ 2Å movement. The rate of pore opening was slow compared to the S4
translation. S4 movement preceded ionic current (Figure 2.29, black trace), as in experiment 123c .
Practically all gating charge displacement (Figure 2.29, blue traces) was carried by S4 during its

fast, large movement at negative V (the Q traces saturate at positive V ).
m m

The two distinct voltage–sensor movements can be explained based on the interactions
between S4 and KCNE1. For the physiological stoichiometry of 4KCNQ1:2KCNE1, there are two S4
positions relative to KCNE1, proximal and distal (Figure 2.25 and Figure 2.26). Proximal S4 charged

residues interact strongly with charged residues at the bottom of KCNE1. In contrast, the distal S4
charged residues are only weakly influenced by KCNE1. This difference results in two types of
voltage-sensor Z movement of proximal S4 and distal S4, respectively. Energy computations show

stabilization of distal S4 at Z position 1.44Å and 2.88Å. In addition, proximal S4 shows strong
stabilization at -6.45Å and -4.44Å and increasing stabilization between 0.33Å and 2.88Å. These
different stabilization profiles suggest that distal S4 undergoes single fast Z translation to its
stabilized high Z position, whereas proximal S4 experiences two movements – a slower Z translation
to about -2.78Å, followed by a faster upward Z movement to ~ 3Å. It follows that distal S4 contributes

mostly to the initial fast and large Z translation (to ~ 1Å), and proximal S4 is responsible for the
additional slower S4 Z movement (to ~ 3Å) at positive V , resulting in additional conformations
m
transitioning to high SC clusters.

Sequential Gating

Whether gating of I is concerted (cooperative; all 4 voltage sensors must be in a high
Ks
positions for the channel to open) or sequential (the current increases with increasing number of

moving S4 segments) is a subject of debate. In Figure 2.30, increasing number of S4 were
immobilized during activation. If gating were concerted, then immobilization of a single S4 would
have resulted in zero current. Instead, the current increases linearly with increasing number of

moving S4 segments. Small and intermediate SC conformations are accessible at low S4 Z with
medium-to-large pore. However, high SC conformations are only accessible when all four S4 move
to a high Z position with a large pore. Thus, structurally the model supports the sequential gating
hypothesis. However, functionally significant current is generated only when all S4 reach relatively
high Z positions.

Effects of the ß-Subunit, KCNE1, and the S4-S5 Linker, S4S5L, on Gating of I
Ks

KCNE1 modulates I gating in two important ways: it slows activation and increases the
Ks
macroscopic current compared to KCNQ1. In a separate study , we developed a different approach
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from that used in the previous section to overcome the computational challenge. In this approach,
we dissected the protein into structural elements and computed the energy of each element using