Trends in Biochemical Sciences
OpinionIon channels: does each subunit do something on its own?
Section snippets
Models of ion-channel activity
In 1968, Eigen [16] proposed the general allosteric model (Fig. 1) in which each subunit of a protein can bind ligand and undergo an activating conformational change. Eigen pointed out that the concerted model of Monod, Wyman and Changeux [17] (Fig. 1, vertical boxes) and the sequential model of Koshland, Nemethy and Filmer [18] (Fig. 1, diagonal box) were both limiting cases of the general allosteric model. These two simpler models were proposed in the mid-1960s as alternative explanations for
Subconductance states are difficult to explain with concerted models
Shortly after the first recordings of the activity of single channels, several types of channel were seen to open with less than the maximal conductance, so-called subconductance states or substates. These states are fleeting in some channels and robust in others. Some early observations of stable substates for GABA and glycine-activated chloride channels (both pentameric channels) are shown in Fig. 2a and Fig. 2b [22]. The simple concerted model allows for only one open-state structure and
The general allosteric model accounts for large numbers of kinetically distinguishable states
In addition to CNG channels, two other carefully studied channels, Shaker voltage-gated K+ channels from Drosophila and Ca2+-activated K+ channels, exhibit many more kinetically distinguishable closed and open states than predicted by simple models. In 1994 a substantial dataset, including macroscopic ionic currents, single channel ionic currents and gating currents was obtained using Shaker K+ channels 29., 30.. These data allowed critical evaluation of several classes of models [31]. Ionic
Are the conformational states not explained by concerted models minor events?
Proteins move on a picosecond time scale and not every motion produces a functionally important state. However, the subconductance states and other kinetically distinguishable states that point away from simple allosteric models are stable on the time-scale of 100 μs to several ms. This is the time frame over which changes in cellular excitability take place. It is clear that several of the subconducting behaviors described earlier are crucial to the overall function of those channels: they are
Concluding remarks
A thorough kinetic analysis of functional states is the key to understanding the sophisticated activation mechanism of ion channels and other allosteric proteins. What emerges from a survey of the different classes of ion channels is that significant subunit-based conformational changes occur. These can be functionally important in their own right (i.e. subconductance states) and they can occur as independent or cooperative transitions. In several cases they might also be the trigger for
Acknowledgements
We are indebted to the National Eye Institute for support and thank William Sather and Thomas Rich for helpful comments on the article.
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