Anatomy of a Typical Ion Channel

In the early 20th century, ion channels as we know them were considered nothing more than "holes in the membrane'' that allowed ions to pass through them. We now know that ion channels are more complex than simple holes. In particular, there are two properties that distinguish them from simple holes: They exhibit selectivity for certain ions, and they open and close in response to stimuli.

All ion channels display many of the same basic properties. All ion channels are integral membrane proteins that form a pore in the lipid bilayer (Fig. 4). Like other membrane proteins, ion channels contain stretches of hydrophobic amino acids called transmembrane segments that anchor the protein within the lipid bilayer. These transmembrane segments pack against each other to stabilize the basic pore structure. Part of the pore is narrow and forms a barrier beyond which impermeant ions cannot cross. This region is lined with amino acid sidechains or backbone atoms that interact with the ion briefly as it crosses the pore. A flexible region of the ion channel forms a gate that acts to open and close access to the pore. The opening and closing of the ion channel is referred to as gating. The position of the gate can be influenced by regions that are modified by enzymes such as kinases or phosphatases. Finally, many ion channels extend processes that dock onto intracellular scaffolding proteins that are part of the cellular architecture. These interactions help direct those ion channels to specific locations within the cell such as the synapse or an intracellular organelle.



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