POTASSIUM-DEPENDENT MOLECULES 209
Shaker potassium channel), vertebrate inward rectifi er K + channels (K ir ), Ca 2+ -
activated K + channels, and other potassium ion channels. It was also common
knowledge that there are two closely related varieties of K + channels, with one
type containing two membrane - spanning segments per subunit and the other
containing six. In all cases, the functional K + channel is a tetramer of four
usually identical subunits. KcsA is a prokaryotic two - membrane - spanning K +
channel, although its amino acid sequence is closer to that of the eukaryotic
six - membrane K + channel proteins. Comparisons of amino acid sequences for
various species indicate that in the potassium ion pore — the region where the
K+ channel crosses the cellular membrane — the K + channel signature sequence
(TVGYG) is identical to that found for Shaker proteins and for vertebrate
voltage - gated K + channels. Additional research has shown that the KscA
potassium ion pore structure is nearly identical for all K + channel proteins. K +
channels are classifi ed as “ long pore channels ” because multiple ions can
queue inside the long narrow pore in single fi le. The pores of all K + channels
can be blocked by tetraethylammonium (TEA) ions.
KcsA crystals suitable for X - ray crystallographic analysis using synchrotron
radiation were obtained and the data collected and analyzed for multiple
crystals and six different data sets as described in the 1998Science publication
(reference 15). The fi nal KcsA pore structure, including amino acid residues
23 to 119 of the K + channel, refi ned to 3.2 Å. The X - ray data were deposited
in the Protein Data Bank with the accession number 1BL8.
A top - down view of the tetrameric pore helices is shown in Figure 5.5A.
The tetramer of four identical subunits created an inverted teepee, or cone,
shape with the important ion selectivity fi lter in its extracellular facing end as
shown in Figure 5.5B. The four chains of the tetramer are shown in different
colors. The colors emphasize that each chain contains an outer helix and an
inner helix. Three potassium ions (space fi ll gray) and one water molecule (red
small sphere) are shown in the selectivity fi lter. The potassium ions in the
selectivity fi lter are surrounded by the backbone oxygen atoms of the residues
gly77, tyr78, and gly79, shown in Figure 5.5A and B in ball - and - stick form. The
entire ion conduction pore in the PDB: 1BL8 structure was found to be 45 Å
long with a variable diameter, while the selectivity fi lter region had a length
of 12 Å. The membrane thickness is ∼ 34 Å. Note that aromatic, mostly hydro-
phobic, amino acid residues — tryptophan, phenylalanine, and tyrosine — collect
near the membrane surfaces. The selectivity fi lter region separating the rest of
the pore from the extracellular solution was found to be so narrow that K +
ions need to shed their hydration shell to enter it. Following the selectivity
fi lter, potassium ions enter the internal pore and cavity that are lined with
predominantly hydrophobic residues, a general feature of K + channels. These
sections of the pore have diameters that will accommodate hydrated K + ions.
More details are found below.
Two aspects of potassium ion conduction across membranes were of
particular interest to these researchers and to many other biophysicists and
biochemists working in this area: (1) What is the chemical basis for the ion