230 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
∼ 30 Å in length across the membrane. The T1 domain and β subunit are each
∼ 40 Å in length along the fourfold axis, and the T1 domain is offset from the
intracellular pore opening by∼ 15 – 20 Å. The α - helical linker T1 – S1 functions
as a spacer between the transmembrane pore and the intracellular regions.
The ion conduction pore is made up of the α - helices S5, pore helix, and S6,
these being related to the voltage sensorα - helices of the adjacent subunit by
wrapping around them. (See Figure 2C on p 899 of reference 37.) In a given
subunit, the S4 – S5 linker that runs parallel to the intracellular membrane
surface connects the conduction pore and the voltage sensor. The S4 – S5 linker
lies at the level of the inner - helix bundle crossing, defi ned as the right - handed
bundle of four inner helices, one from each of the tetramer subunits. In Kv1.2,
the S6 helices line the ion conduction pore just on the intracellular side of the
selectivity fi lter. The inner - helix bundle crossing is important because it forms
an expandable constriction (the crossing) for opening and closing the pore.
This construction is known as the activation gate.
The ion conduction pore structures of four different prokaryotic K + chan-
nels, including KcsA, MthK, KirBac, and KvAP, have been determined by X -
ray crystallography as discussed above and described in Table 5.3. These four
structures indicate a high level of conserved amino acid sequence and struc-
ture for the K + selectivity fi lter, while the inner pore (between the selectivity
fi lter and intracellular solution) varies in sequence and conformation. Inner
pore conformational changes open and close the pore. Some of the mentioned
structures, those for KcsA and KirBac, have the pore in the closed position
while MthK and KvAP pores appear to be open. These determinations are
made by considering the pore ’ s dimension at its narrowest point — the inner -
helix bundle or bundle crossing point. Comparison to the current structure
under discussion, Kv1.2, PDB: 2A79, indicates that this pore is open and has
a diameter of approximately 12 Å. Other experiments have indicated that
when the Shaker K + channel is closed, not even a small Ag + ion will pass the
bundle crossing. When it is open, however, large organic quaternary ammo-
nium cations, and even hydrophobic polypeptides, can enter and plug Kv
channels. The inner helices (S6) of Kv1.2 contain the pro – val – pro sequence
(pro405, val406, and pro407 in PDB: 2A79), highly conserved among Shaker
family Kv channels, and is responsible for curving or bending the S6 inner
helices so that they are almost parallel to the membrane near the intracellular
membrane surface. In KvAP (PDB: 1ORQ, 1ORS) a glycine residue is respon-
sible for a similar bend. The α - helical linkers connecting the T1 domain through
S1 radiate out to create a wide space between T1 and the pore. This serves
two purposes: (1) It allows space for the inner helices to undergo large gating
movements to open the pore and (2) provides diffusion pathways for ions and
inactivation peptides between the cytoplasm and the pore entryway. These
so - called side portals have been described here previously in discussing the
T1 4 β 4 complex of Kv channel of PDB: 1EXB as described in reference 16. The
four side portals above the T1 domain each have a diameter of 15 – 20 Å and
are lined with negatively charged amino acids (glu128, asp129, and glu130 for