Transmembrane structure of voltage-gated ion channels

The direct flowof ions across the cell membrane
is regulated by ion channels. The transmembrane
proteins, composed of several domains,
are arranged so that they form pores that can be
opened and closed. The simplest model is the
potassium channel (1). This membrane-bound
polypeptide contains six transmembrane
domains. The amino and the carboxy ends of
the protein lie within the cell. Changes in cell
membrane potential or voltage cause the channel
to open (or close) in order to initiate (or terminate)
a brief flow of ions. Domain 4, which is
composed of polar amino acids, is crucial for the
flow of ions. Sodium and calcium ion channels
consist of four subunits (2) of similar structure,
each resembling a potassium channel. The similarity
is due to the common evolutionary origin
of their genes. The four subunits of the sodium
channel (3) are positioned to form a very narrow
porelike passage, much narrower than a
potassium channel, through the plasma membrane.
Ion transport is brought about by membrane
depolarization