Structural models of Na+, Ca2+, and K+ channels.
The structure determination of voltage-gated channels by the combination of molecular modeling and mutagenesis experiments is a long term, iterative process. As such, the models should be considered as a work in progress, with changes expected as more data becomes available. The primary role of the models is that they assimilate the known data and provide ideas for further experiments to elucidate the real structures. Although the models presented here have already gone through two or three cycles of development and testing, many aspects remain tentative. Perhaps the most significant result so far is that the P segment was experimentally confirmed to form the ion-selective part of the channel. In a subsequent cycle of testing and modeling, the specific residues responsibility for Na+ and Ca2+ selectivity have been identified and the selectivity filter of K+ channels is now predicted to be formed by the side chains, but rather by the carbonyl oxygens of the conserved Gly-Tyr-Gly sequence backbone. As another example, the 9p residue of the P segment of K+ channels was originally modeled as either being buried in the protein or accessible channels was originally modeled as either being buried in the protein or accessible from inside the cell only. However, once mutation of this residue to histidine was found to affect blockade by extracellular TEA, protons, Zn2+ and histidine reagents (DeBiasi et al., 1993), the models were updated to have this and the hydrophilic residues in the first part of P form a helix that comprises part of the extracellular, outer vestibular of the pore. While this motif was used also for Na+ and Ca2+ pore models (see Fig. 2) where the putative helices are amphipathic, it remains to be verified. Modeling of the size and shape of the outer vestibule of K+ channels was also aided by the data for the binding of CTX in the extracellular entrance to the pore. Similarly, experiments with peptide toxins such as mu and omega conotoxins may prove useful in modeling the outer vestibules of the Na+ and/or Ca2+ channels. While important advances have been made, it is important to realize that these approaches are still very new. In the future we are likely to see improvements on both the theoretical and experimental sides which will greatly advance the process.(ABSTRACT TRUNCATED AT 400 WORDS)
Guy
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HR
Laboratory of Mathematical Biology, DCBDC, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Durell
S R
SR
D000818:Animals; D001665:Binding Sites; D015220:Calcium Channels / Q000502:physiology*; D017136:Ion Transport; D008958:Models, Molecular; D015221:Potassium Channels / Q000502:physiology*; D015222:Sodium Channels / Q000502:physiology*
D015220:Calcium Channels; D015221:Potassium Channels; D015222:Sodium Channels