POTASSIUM-DEPENDENT MOLECULES 215
It is known that all voltage - dependent ion channels contain six hydrophobic
segments per subunit, referred to as S1 – S6. In K + channels four identical S1 – S6
subunits surround the central ion - conduction pore (the pore is structurally
detailed for the KscA K + channel in reference 15 as discussed earlier). Seg-
ments S5 – S6 line the pore and determine ion selectivity while S1 – S4 form the
voltage sensors. Charged amino acids, particularly the fi rst four arginines in S4
(arg117, arg120, arg123, and arg126 in KvAP), account for most of the gating
charge. The reference 18 researchers have studied the mechanism of voltage -
dependent gating using biochemical, X - ray crystallographic, and electrophysi-
ological methods with special attention to KvAP whose structure is described
in the 2003Nature publication. It should be said that the KvAP amino acid
sequence and its electrophysical properties are closely related to those of other
K+ channels. Reference 18 describes the KvAP structure as well as two con-
formations of the important voltage sensor.
First, the researchers found that the voltage sensor region was very diffi cult
to crystallize, leading to the idea that it might be a mobile structure within the
K+ channel assembly. A more stable (and therefore less mobile) voltage sensor
domain might be found for the thermophilic KvAP species; however, even in
this case monoclonal antibodies (Fab) raised against the K + channel were
necessary to “ hold ” the channel in an orientation to foster crystallization. This
approach proved successful for two structures: (1) the full - length channel in
complex with Fab determined at a resolution of 3.2 Å (PDB: 1ORQ); and
(2) the isolated voltage sensor domain in complex with Fab determined at a
resolution of 1.9 Å (PDB: 1ORS).
In the 1ORQ structure, the ion conduction pore with its selectivity fi lter
(conserved sequence TVGYG) looks virtually identical to that of KcsA (PDB:
1BL8) discussed above. In the hydrophobic core surrounding the selectivity
fi lter, some deviation in amino acid sequence from KcsA is found, but the
secondary structure overlap between KvAP and KscA is essentially complete.
These fi ndings are not surprising in that the fi lter serves the same purpose in
both proteins — the selective and rapid conduction of potassium ions. The
structures of KvAP (PDB: 1ORQ) and KscA (PDB: 1BL8) begin to deviate
in the area of the S6 helices (inner helices), diverging most notably at a point
referred to as the glycine - gating hinge. The gating hinge appears to affect the
openness of the conduction pore, as can be seen when comparing the struc-
tures of different pores. These authors observe that the “ inner helices seem to
open like the aperture of a camera by bending at a glycine residue. ” When
comparing the KcsA (closed K + channel) with that of MthK (an opened K +
channel), the researchers fi nd that the KvAP channel is opened wider than
the closed KscA channel and is nearly as wide open as the MthK channel.
MthK is a calcium - gated potassium channel whose structure was determined
by the MacKinnon group in 2002.^23 The S6 (inner relative to the pore) and S5
(outer) helices are in an antiparallel conformation to one another and proba-
bly move together as a single unit. This is important in that the voltage sensor
domain (to be discussed next) is directly attached to the outer S5 helix so that