Reconstituted channels, structural form

The reconstitution of voltage-dependent channels from nerve and muscle in planar bilayers will very likely be an early accomplishment of the eighties, and this will enable us to begin to test the ideas of channel structure and function developed from the study of channel-forming antibodies in the seventies. 

The Streptomyces lividans K+ channel (KcsA) is a 160-residue protein that forms homotetrameric channels closely related to the pore domain of larger voltage-dependent channels (Schrempf et al., 1995). When purified and reconstituted in lipid bilayers, KcsA catalyzes single-channel activities with selectivity properties identical to those of other eukaryotic K+ channels (Cuello et al., 1998 Heginbotham et al., 1999 Meuser et al., 1999). The fact that KcsA is easily expressed in Escherichia coli at milligram levels made this protein an ideal target for structural analysis. 

The fundamental problems regarding reconstitution of connexin channels have been overcome connexin forms channels in unilamellar liposomes and planar bilayers. Size, permeability, and gating behavior are consistent with conducting units that are single hemichannels—the structures that span a single cell membrane and form one-half of the junctional channel. Connexin can be obtained by affinity purification under nondenaturing conditions. Thus, channels formed by a single connexin can be studied in a well-defined and accessible system. 

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