3D Structures by Professor Roderick MacKinnon,
Rockefeller University

The gene for potassium channels from the bacterium Streptomyces was cloned and expressed as a protein. The protein was crystallized (the hard part!), analyzed by x-ray crystallography, and its structure was reconstructed in three dimensions. In 2005, the same laboratory (headed by Rod MacKinnon, who received the Nobel Prize in 2003) published a structure for the Shaker K channel, a much more complex voltage-dependent channel found in higher animals.

A complete K channel is a tetramer of 4 identical proteins that together form a pore for potassium ions. For the bacterial channel, each protein has two helices that span the membrane, plus a loop that lines the pore. The voltage-dependent Shaker channel has six transmembrane helices, a loop that lines the pore, and two large additional regions (the T1 domain and Beta subunit) that project into the cytoplasm. The four additional transmembrane helices add the functions of a voltage sensor to control an activation gate. The T1 domain or Beta subunit is the likely location of the Shaker channel's inactivation gate.

The channel is selective for potassium ions because of a narrow "selectivity filter" at the extracellular end (top) of the pore. The filter is just the right size to surround potassium ions with a ring of carbonyl oxygens, which take the place of the water molecules that are normally bound to an ion. This allows a "naked" ion to slip through the filter. Smaller ions like Na+ are too small to fit tightly in the "oxygen cage," so they do not lose their surrounding water molecules and cannot pass through the filter.

References:
Doyle, D.A. et al. (1998) The structure of the potassium channel: molecular basis of K+ conduction and selectivity. Science 280: 69-77.
MacKinnon, R. (2003) Potassium channels. Febs Letters 555: 62-65.
Long, S.B., Campbell, E.B. and MacKinnon, R. (2005) Crystal structure of a mammalian voltage-dependent Shaker family K+ channel. Science 309: 897-903.
Long, S.B., Xiao, T., Campbell, E.B. and MacKinnon, R. (2007) Atomic structure of a voltage-dependent K+ channel in a lipid membrane-like environment. Nature 450: 376-382