212 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS
Conclusions from the 1998 PDB: 1BL8 structural study are as follows: (1)
The K + channel pore is constructed of an inverted teepee with the selectivity
fi lter at the wide (extracellular - facing) end; (2) the narrow selectivity fi lter is
12 Å long (limiting the distance of strong K + attraction), while the remainder
of the pore is wider and lined with hydrophobic residues; (3) a large water -
fi lled central cavity and helix dipole orientation help overcome a high electro-
static energy barrier facing cations in the hydrophobic environment of the
membrane center; (4) the K + selectivity fi lter is lined with carbonyl oxygen
atoms spaced for proper coordination of potassium ions but too far apart to
coordinate smaller sodium ions; and (5) repulsion between two K + ions close
to each other in the selectivity fi lter overcomes the strong attraction between
the potassium ions and carbonyl oxygens of fi lter amino acid residues, allowing
rapid ion conduction.
The MacKinnon group next published the X - ray crystallographic structure
(at 2.1 - Å resolution) of a eukaryotic (rat) voltage - dependent K + channel β
subunit: T1 cytoplasmic (cell interior) assembly attached to the pore subunit
described in reference 16 (PDB: 1EXB). First, it is important to describe the
two subunits of the K + channel. The α subunit includes the K + channel ion
conduction pore residing in the cell membrane and the so - called T1 domain
that resides in the cell interior. The pore and T1 domain are connected by the
voltage - sensing region and other connectors of unknown structure at the time
of this research. The β subunit is a tetramer of oxidoreductase proteins with
fourfold rotational symmetry as was found for the K + channel ion conduction
pore discussed above. Each β subunit contains an active site with an NADPH
(nicotinamide adenine dinucleotide phosphate) cofactor, but its substrate and
biological function were unknown. It was known that the α and β subunits
assemble in the endoplasmic reticulum and remain together throughout their
active life. The α subunit ’ s T1 domain, consisting of approximately 100 amino
acids, resides between the membrane - situated ion conduction pore and the β
subunit. It forms a tetrameric ring with a narrow positively charged central
core. Figure 5 of reference 16 (Figure 5.6 ) shows an illustrative composite
model of the entire assembly. The model of the pore region is modeled using
the PDB: 1BL8 structure.
It was known that large organic cations such as the tetraethylammonium
(TEA) ion and large inactivation peptides (part of theβ subunit — T1 assem-
bly) enter the transmembrane pore and the question arises as to how that
would take place given the T1 domain ’ s narrow central core. These researchers
answer the question in reference 16 through their analysis of the structure and
function at the cytoplasmic interface (the connection region between the α
andβ subunits).
First, the researchers confi rmed that the β subunit ’ s association with the α
subunit is disrupted if the T1 domain is removed. The X - ray crystallographic
structure of the assembled T1 4 β 4 complex showed why this is the case. The
structure shows that large fl at surfaces of the β subunit interact with four
prominent loops — called contact loops — that extend from the T1 tetramer ’ s