Molecular Properties of Voltagedependent Potassium Channels

The DNA coding for various other voltage-activated channels, including voltage-activated potassium channels, has also been analyzed to reveal the sequence of amino acids making up those proteins. It is interesting that these voltage-activated channels all have similar (though, of course, not identical) amino-acid sequences, especially in segment S4, which seems to impart the voltage sensitivity. Thus, voltage-activated channels of various kinds represent a family of proteins coded by related genes that probably arose during the course of evolution from a single ancestral ion-channel gene that existed eons ago. Potassium channel genes, however, encode proteins that are much smaller than sodium channels. In fact, the protein encoded by potassium channel genes seems to correspond to a single one of the four domains present in the voltage-activated sodium channel (Figure 6-11). It is thought that functional potassium channels are formed by the aggregation of four of these individual protein subunits, so that the whole channel has an arrangement similar to that of the sodium channel shown in Figure 6-11b. In the sodium channel, however, the four domains are combined together into one large, continuous protein molecule, while in potassium channels each domain consists of a separate protein subunit.

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