214 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
Inactivation peptide interaction with the assembly was used to fi nd answers
to this question. It was known that mutation of basic (positively charged)
amino acids to neutral or hydrophobic residues on the inactivation peptide
infl uenced the extent of inactivation. Therefore, the researchers looked for
areas of acidic (negatively charged) amino acids at the known site of inactiva-
tion peptide binding using another voltage - dependent K + channel protein,
Kv 1.4. They found a cluster of acidic residues (glu273, asp274, and glu275 for
Kv 1.4) at the T1 – S1 linker region — the area where the C - terminal end of the
T1 domain interfaces with connectors to the membrane spanning helices
(S helices) that surround the ion conduction pore. (See black asterisks on
Figure 5.6 .)
When these acidic residues where mutated to alanine (neutral) or lysine
(positive), rates of inactivation were affected. The largest effect took place
with the positive amino acid mutation. The authors found that mutations in
the inactivation peptide are tied to those in the T1 – S1 linker near the T1
domain; in other words, the inactivation peptide must be near the T1 – S1 linker
when it inactivates the pore. The conclusion reached is that since the inactiva-
tion peptide cannot fi t through the center of the T1 4 β 4 complex, it must reach
the ion conduction pore through lateral openings above the T1 tetramer.
From their work reported in reference 16, the research group reached the
following conclusions: (1) The T1 domain forms a docking platform for the
necessaryβ subunit through loops at the N - terminal end of the T1 domain and
amino acid residues on the T1 - facing surface of the β subunit; (2) lateral open-
ings between the T1 domain and the cell membrane - spanning region of the
α subunit form a conduit for an inactivation peptide (and probably K + ions
as well); and (3) the central question of why K + channels contain an oxido -
reductase enzyme subunit (the β subunit) remains unanswered, although
the authors suggest that interactions betweenα and β subunits could allow
cellular redox regulation of the channel.
The MacKinnon group continued its study of K + channels and presented
the structure of KvAP, the voltage - dependent K + channel from the thermo-
philic archaebacteriaAeropyrum pernix in 2003.^18 In this publication, the
process known as gating is explained. The control of pore opening or closing
is related to the membrane voltage — that is, we have a voltage - dependent
channel. First, it is known that the cell membrane can undergo transient
changes in permeability to its selected ion and that these changes depend on
membrane voltage. As this article puts it: “ Selective permeability to ions deter-
mines the membrane voltage and the voltage determines the permeability. ”
The question then is, By what means does the voltage - dependent channel
know the membrane voltage and open (or close) its gate as a function of the
voltage value? It is believed that charged amino acids called “ gating charges ”
move through the membrane electric fi eld and that the transient electric cur-
rents generated in this manner affect pore opening and closing. K + channels
have a large gating charge, giving rise to an “ open ” probability as a function
of membrane voltage.