|MoBio||Ion Channels||Chapter 2|
Ion channels are a special class of proteins conducting small ions such as Na+, K+, Ca2+ or Cl-. Some channels are sensitive to membrane voltage, whereas others are activated by the binding of specific molecules (the ligands). The Na+ and K+ channels involved in the generation of nerve impulses belong to the voltage-sensitive ion channels. During synaptic transmission, the neurotransmitters act on the ligand-activated ion channels.
There are many types of potassium channels. The one involved in the generation of nerve impulses is composed of four subunits, each is homologous to the Shaker protein (Figure 2-F-3). The hydrophobicity profile indicates that it contains six hydrophobic segments, designated as S1 - S6. These segments are likely to be the transmembrane domains (Figure 2-F-4). The selectivity filter is located in the pore-forming region (P-region) which contains the K+ channel signature "GYG". Figure 2-F-5 shows the arrangement of subunits in a K+ channel.
Figure 2-F-3. The amino acid sequence of the Shaker protein.
Figure 2-F-4. The domain structure of the Shaker protein.
The open state of the Shaker K+ channel has been determined by x-ray crystallography (Long et al., 2005). Its selectivity filter is formed by the backbone oxygen atoms which facilitate the passage of K+ ions.