The colicins are a family of antimicrobial proteins that are secreted by Escherichia coli strains under environmental stress, due to nutrient depletion or overcrowding, and these proteins often target-sensitive bacterial strains (1). The lethal action of colicins against their target cells is manifested in a number of different modes that include the following: (i) formation of depolarizing ion channels in the cytoplasmic membrane, (ii) inhibition of protein and peptidoglycan synthesis, and (iii) degradation of cellular nucleic acids (1-7). In this context, the bacterial machinery responsible for colicin biological activity features important mechanisms that are fundamental to various biological processes. These mechanisms include protein receptor binding, membrane translocation, membrane binding and protein unfolding, membrane insertion, voltage-gated ion channel formation, catalysis, and inhibition of enzymes.Colicin E1 is a member of the channel-forming subfamily of colicins and is secreted by E. coli that possesses the naturally occurring colE1 plasmid; the holoprotein consists of three functional segments, the translocation, receptor-binding, and channel-forming domains. Initially, the receptor-binding domain (8) interacts with the vitamin B 12 receptor of target cells. Following recognition, the translocation domain associates with the tolA gene product, which permits the translocation of colicin E1 across the outer membrane and into the periplasm (9). In the periplasm, the channel domain undergoes a conformational change to an insertion-competent state and then inserts spontaneously into the cytoplasmic membrane of the host cell, forming an ion channel. The channel allows the passage of monovalent ions, resulting in the dissipation of the cationic gradients (H ϩ , K ϩ , and Na ϩ ) of the target cell, causing depolarization of the cytoplasmic membrane. In an effort to compensate for the membrane depolarization effected by the colicin E1 channel, Na ϩ /K ϩ -ATPase activity is increased in the host cell, resulting in the consumption of ATP reserves, without concomitant replenishment (4). The final outcome is host cell death.In addition to structural similarities between colicin E1 and several membrane-targeting proteins, an intriguing characteristic shared by colicin E1 and many membrane-active proteins is the observed requirement of an acidic environment for activation of the soluble structure in order to bind and insert into target cell membranes. The important effect of an acidic environment in conferring an optimum activity in vitro, and perhaps in vivo, for colicin E1 has been attributed to an onset of acid-induced protein unfolding events, resulting in an increased structural mobility/flexibility that may potentiate the complex sequence of structural changes of the channel peptide observed at the surface of a membrane (10 -12).An important contribution to our understanding of the mechanism of colicin E1 low pH activation was conducted by Merrill et al. (13) in which a precise pH-sensitive region with...