Crystallographic Analyses of Ion Channels: Lessons and Challenges

Membrane proteins fascinate at many levels, from their central functional roles in transport, energy transduction, and signal transduction processes to structural questions concerning how they fold and operate in the exotic environments of the membrane bilayer and the water-bilayer interface and to methodological issues associated with studying membrane proteins either in situ or extracted from the membrane. This interplay is beautifully exemplified by ion channels, a collection of integral membrane proteins that mediate the transmembrane passage of ions down their electrochemical potential gradient (for general reviews, see Refs. 1.Hille B. Ionic Channels of Excitable Membranes. 2nd Ed. Sinauer Associates, Sunderland, MA1992Google Scholar and 2.Aidley D.J. Stanfield P.R. Ion Channels. Cambridge University Press, Cambridge, UK1996Google Scholar). Ion channels are key elements of signaling and sensing pathways, including nerve cell conduction, hormone response, and mechanosensation. The characteristic properties of ion channels reflect their conductance, ion selectivity, and gating. Ion channels are often specific for a particular type of ion (such as potassium or chloride) or a class of ions (such as anions) and are typically regulated by conformational switching of the protein structure between “open” and “closed” states. This conformational switching may be gated in response to changes in membrane potential, ligand binding, or application of mechanical forces. Detailed functional characterizations of channels and their gating mechanisms have been achieved, reflecting exquisite methodological advances such as patch clamp methods that can monitor the activities of individual channels (3.Neher E. Sakmann B. Nature. 1976; 260: 799-802Crossref PubMed Scopus (1515) Google Scholar). Until recently, corresponding information about the three-dimensional structures of channels was not available, reflecting difficulties in obtaining sufficient quantities of membrane proteins for crystallization trials. Happily, this situation has started to change with the structure determinations of the Streptomyces lividans K+channel (KcsA (4.Doyle D.A. Cabral J.M. Pfuetzner R.A. Kuo A. Gulbis J.M. Cohen S.L. Chait B.T. MacKinnon R. Science. 1998; 280: 69-77Crossref PubMed Scopus (5701) Google Scholar)) and the Mycobacterium tuberculosismechanosensitive channel (MscL (5.Chang G. Spencer R.H. Lee A.T. Barclay M.T. Rees D.C. Science. 1998; 282: 2220-2226Crossref PubMed Scopus (852) Google Scholar)). A variety of reviews (6.Clapham D.E. Cell. 1999; 97: 547-550Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar, 7.Gouaux E. Structure. 1998; 6: 1221-1226Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar, 8.Yellen G. Curr. Opin. Neurobiol. 1999; 9: 267-273Crossref PubMed Scopus (23) Google Scholar, 9.Batiza A.F. Rayment I. Kung C. Struct. Folding Design. 1999; 7: R99-R103Abstract Full Text Full Text PDF Scopus (37) Google Scholar, 10.Spencer R. Chang G. Rees D.C. Curr. Opin. Struct. Biol. 1999; 9: 448-454Crossref PubMed Scopus (40) Google Scholar, 11.Heginbotham L. Nat. Struct. Biol. 1999; 6: 811-814Crossref PubMed Scopus (6) Google Scholar, 12.Oakley A.J. Martinac B. Wilce M.C.J. Protein Sci. 1999; 8: 1915-1921Crossref PubMed Scopus (43) Google Scholar) have appeared recently that discuss functional implications of these channel structures. This review discusses these developments from a complementary perspective, by considering the implications of these structures from within the larger framework of membrane protein structure and function. Because of space restrictions, this review necessarily emphasizes membrane proteins that are composed primarily of α-helical bundles, such as KcsA and MscL, rather than β-barrel proteins, such as porins, typically found in bacterial outer membranes.