Eukaryotic Ion Channels at High Resolution Divide and Conquer

Bacterial channels have provided insight into the guts of ion channel permeation machineries, revealing the intimate details of permeation pathways that are likely to be conserved and recapitulated in their larger eukaryotic cousins. In contrast to prokaryotic membrane proteins (which are difficult to obtain in their own right), eukaryotic membrane proteins are currently extremely difficult to obtain in the quantities required for high-resolution study. Eukaryotic channels often contain a host of extramembranous regulatory domains and subunits that are essential for their activity, signal sensing, and gating. These domains have proven to be a tractable entry point for the study of eukaryotic ion channel structure and function.

A number of groups have successfully "liberated" extramembranous domains from the membrane-spanning part of a variety of ion channels so that they can be expressed, purified, crystallized, and treated like soluble proteins.

Figure 6 Opening mechanisms of ion channels. (Left) Simplified diagram of the opening mechanism of the nAChR. Acetylcholine (Ach) binds to the extracellular domain of the receptor initiates a rotation that causes the closest approach of the ring of pore-lining helices to widen creating a pathway for the ions. (Right) Schematic model for the opening of a bacterial calcium-gated potassium channel. Calcium binding to the cytoplasmic domains causes a conformational change that is propagated to the inner pore-lining helices. This opens the pathway for the ions to enter the channel. (Adapted from Unwin, N., J. Struct. Biol., 121, 181-190, 1998; Schumacher, M. A. and Adelman, J. P., Nature, 417, 501-502, 2002.)

Figure 6 Opening mechanisms of ion channels. (Left) Simplified diagram of the opening mechanism of the nAChR. Acetylcholine (Ach) binds to the extracellular domain of the receptor initiates a rotation that causes the closest approach of the ring of pore-lining helices to widen creating a pathway for the ions. (Right) Schematic model for the opening of a bacterial calcium-gated potassium channel. Calcium binding to the cytoplasmic domains causes a conformational change that is propagated to the inner pore-lining helices. This opens the pathway for the ions to enter the channel. (Adapted from Unwin, N., J. Struct. Biol., 121, 181-190, 1998; Schumacher, M. A. and Adelman, J. P., Nature, 417, 501-502, 2002.)

This divide-and-conquer approach has proven particularly powerful for illuminating channel-gating mechanisms when the high-resolution information about these domains is incorporated into structure-function studies of the intact channel. For example, studies of an assembly domain from eukaryotic voltage-gated potassium channels led to the discovery of a new role for this domain in channel gating [14,15]. The structure of a soluble homolog of the extracellular domain of the nAchR found in snail glial cells has provided molecular landmarks for interpreting decades of study by chemical modification, mutagenesis, and electron microscopy of the intact receptor [16]. Similarly, structures of the ligand binding domains of glutamate receptors [17] and calmodulin-activated potassium channels [18] have led to detailed models of channel gating and ligand recognition. This divide-and-conquer approach is likely to remain a fruitful endeavor in the near future while better methods for purifying ion channels from native sources and new means for expressing full-length ion channels are developed.

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