KcsA K+ Channel: PDB 1J95

This structure of  the intramembrane portion (residues 1-125) of the KcsA K+ channel from Streptomyces lividans was solved by J. H. Morais Cabral, R. MacKinnon, & M. Zhou in 2001. 

Suggested display options:

Use the right mouse button to color chain to distinguish the 4 copies of the protein.
Display as cartoon. Use the left mouse button to drag the image.

Note the tilt of the 2 transmembrane a-helices in each subunit. The complex has been described as like a teepee or cone.

Select hetero-ligand and display as ball and stick with color CPK.
In addition to K+ ions in the selectivity filter, a channel inhibitor will appear in the internal vestibule.
Try to
identify this compound.

Select atom K and display spacefill to visualize more clearly the potassium ions.

The K+ ions are in the region called the selectivity filter, where the lumen exactly corresponds to the diameter of  K+. The selectivity filter consists of residues 75-79, the consensus sequence (T, V, G, Y, G) found in all K+ channels. 

Type into the command line select 75-79 and enter.
Use the mouse to change the display to ball & stick, & color CPK.
To zoom in, left drag while holding down the shift key.
Note the rings of oxygen atoms that define the binding sites for K+.

How many
oxygen atoms of the selectivity filter interact with each K+?
Why are cation-binding sites often rich in oxygen atoms?
is it unlikely that there are actually 4 K+ ions within any individual channel at any time?

Follow the peptide chain in the region of the selectivity filter. You can click on each amino acid to identify it.
Question: Are most of the oxygen atoms that interact with K+ part of the peptide backbone or part of amino acid side-chains (R-groups)?

Also view with the consensus sequence (select 75-79) displayed as spacefill.
the close interaction of K+ with the oxygen atoms.

The channel gate is where the transmembrane a-helices meet at the narrow end (the pointed end of the teepee).
Select protein-protein and change the display to spacefill. Drag to view.
Is there
any opening at the narrow end of the channel?

What anionic residues are located near each mouth of the channel?
might you explain this common feature of cation-selective channels?

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