Colicin E1 in planar lipid bilayers

The channel formed by the C-terminal domain of colicin E1 in planar lipid bilayers has proven to be more complex than one might have guessed for such a simple system. The protein undergoes a pH-dependent rearrangement which transforms it from a water soluble form to a much different membrane bound form. There are at least two bound states which don't form a channel. The process by which the channel opens and closes is regulated by the pH and the transmembrane voltage. The voltage is probably sensed by at least 3 (and more likely 4 or more) lysine residues which must be driven through the field to open the channel. The process appears to be hindered by particular carboxyl groups when they are in the unprotonated state. The open channel has several substates and several superstates. Very large positive voltage catalyzes a transition of the open channel to an inactivated state, and may be able to drive the channel-forming region of the protein across the membrane. Little is known about the structure of any of these states, but the open channel is large enough to allow NAD to traverse the membrane and appears to be formed by one colicin molecule. This single polypeptide mimics many of the properties found in channels of mammalian cell membranes, but it may prove more relevant as a model for the transport of proteins across membranes. The comparative ease with which the protein can be manipulated chemically and genetically, along with the complexity of its behavior, promises to keep several laboratories busy for some time.