Membrane biophysical studies of the colicin E1 channel

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Colicins are toxic proteins produced by certain strains of E. coli during bacterial cell growth and they are often produced in response to a host of metabolic challenges, including DNA damage, anaerobiosis, catabolite repression and nutrient depletion. Colicin E1 (colE1) is a member of the channel-forming colicins, which also includes colicins A, B, Ia, Ib, N and K. The three-dimensional structures of colicins reveal the presence of three domains: receptor binding, translocation and channel domains. The receptor-binding domain initiates toxin entry into target cells by binding with the vitamin B12 receptor on the bacterial outer membrane. Next, the translocation domain associates with the trimeric ß-barrel TolC outer membrane receptor, through which the colicin is transported into the periplasm of the target bacterium. In a series of steps, colE1 sequentially unfolds, binds and spontaneously inserts into the membrane to form a closed channel state. The channel opens in the presence of a trans-negative membrane potential and causes depolarization of the host cell cytoplasmic membrane through the escape of cellular ions such as Na+, K+ and H+.