Gating of a voltage-dependent channel (colicin E1) in planar lipid bilayers: translocation of regions outside the channel-forming domain

C-terminal fragments of colicin E1, ranging in mol wt from 14.5 to 20kD, form channels with voltage dependence and ion selectivity qualitatively similar to those of whole E1, placing an upper limit on the channel-forming domain. Under certain conditions, however, the gating kinetics and ion selectivity of channels formed by these different E1 peptides can be distinguished. The differences in channel behavior appear to be correlated with peptide length. Enzymatic digestion with trypsin of membrane-bound E1 peptides converts channel behavior of longer peptides to that characteristic of channels formed by shorter fragments. Apparently trypsin removes segments of protein N-terminal to the channel-forming region, since gating behavior of the shortest fragment is little affected by the enzyme. The success of this conversion depends on the side of the membrane to which trypsin is added and on the state, open or closed, of the channel. Trypsin modifies only closed channels from the cis side (the side to which protein has been added) and only open channels from the trans side. These results suggest that regions outside the channel-forming domain affect ion selectivity and gating, and they also provide evidence that large protein segments outside the channel-forming domain are translocated across the membrane with channel gating.