POTASSIUM-DEPENDENT MOLECULES 233
sensor paddles in Kv1.2 are in a slightly different position with respect to
helices S1 and S2, understandable if the voltage sensor paddle is mobile; (4)
S4 and S3 are nearly two helical turns longer in Kv1.2 than in KvAP, meaning
that the Kv1.2 paddle will project further into the extracellular solution and
possibly react differently with molecules that interact with the voltage sensor
from outside the cell (see experiments on biotinylation described in discussion
of reference 25 above).
The S4 – S5 linker helix is an amphipathic (containing both polar and non-
polar amino acid residues)α - helix that runs parallel to the membrane plane
inside the cell. The helix ’ s hydrophobic surface faces the membrane, and the
polar side faces the cytoplasm. The S4 – S5 linker helix rests against the curved
part of helix S6 (after the Pro – X – Pro sequence) in a parallel conformation,
and this interaction appears to be essential for coupling of voltage sensor
movements to pore open or closed positions (see Figure 3C and 3D of refer-
ence 38). Mutations to the Pro – X – Pro sequence have large effects on K +
channel gating that appear to uncouple the voltage sensor from the pore. In
experiments conducted by Lu and co - workers, transfer of the entire S1 – S4
voltage sensor, the S4 – S5 linker, and the C - terminal end of S6 were all required
to engineer voltage dependence into KcsA, an otherwise voltage - independent
(or weakly voltage - dependent) K + channel.^40 The required S6 segment exactly
corresponded to that contacting the S4 – S5 linker in Kv1.2, indicating that
these domains are necessary and suffi cient to construct a functioning voltage
sensor. Further description of the voltage sensor arrangements in the tetra-
meric K + channel structure benefi ts greatly from access to Figure 4 of refer-
ence 38. As this fi gure illustrates for the overall structure, and in detail for the
voltage sensor domain, the voltage sensors appear to be fl oating as separate
domains at the corners of a square surrounding the ion conduction pore. In
addition, the S4 – S5 linker runs across to a neighboring subunit with the S4
helix adjacent to the S5 helix of the neighboring subunit. Experimental mea-
surements have placed S4 arginine residues from one subunit at distances
(between C α carbons) of 45 Å to the adjacent subunit and 64 Å diagonally
across the pore, reinforcing the idea that S4 helices occupy the outer corners
of the ion conduction pore helical structure. Other than the contacts discussed
here, contacts between the S1 – S4 voltage sensor helices and S5 – S6 pore helices
are found to be insubstantial, reinforcing the idea of mobile voltage sensor
domains that undergo voltage - dependent conformational changes within the
membrane.
The gating - charge arginine residues on the voltage sensor subunits (arg294,
297, 300, 303 in Kv1.2) reside near the N - terminal end of the S4 helix. These
arginine residues are the most conserved among all Kv channel proteins and
account for most of the gating charge. In Kv1.2, arg294 and arg297 are located
on the voltage sensor ’ s lipid facing surface, and arg294 may actually extend
into the phospholipid headgroup layer. Meanwhile, arg300 and arg303 face
helices S1 and S2 and make salt bridges with acidic amino acid residues on
those helices. Similar salt bridges were found for the KvAP voltage sensor