The structure of the selectivity filter exhibits
two essential features. First, the main
chain atoms create a stack of sequential
oxygen rings and thus afford numerous
closely spaced sites of suitable dimensions
for coordinating a dehydrated K1 ion. The
K1 ion thus has only a very small distance
to diffuse from one site to the next within
the selectivity filter. The second important
structural feature of the selectivity filter is
the protein packing around it. The Val and
Tyr side chains from the V-G-Y-G sequence
point away from the pore and make
specific interactions with amino acids from
the tilted pore helix. Together with the
pore helix Trp residues, the four Tyr side
chains form a massive sheet of aromatic
amino acids, twelve in total, that is positioned
like a cuff around the selectivity
filter (Fig. 8C). The hydrogen bonding, for
example between the Tyr hydroxyls and
Trp nitrogens, and the extensive van der
Waals contacts within the sheet, offer the
immediate impression that this structure
behaves like a layer of springs stretched
radially outward to hold the pore open at its
proper diameter.