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⁺.

Questions:
How many oxygen atoms of the selectivity filter interact with each K⁺?
Why are cation-binding sites often rich in oxygen atoms?
Why 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.
Note 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.
Questions:
Is there any opening at the narrow end of the channel?

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

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